<!-- $PostgreSQL: pgsql/doc/src/sgml/libpq.sgml,v 1.274 2008/12/15 10:28:21 mha Exp $ -->

<chapter id="libpq">
 <title><application>libpq</application> - C Library</title>

 <indexterm zone="libpq">
  <primary>libpq</primary>
 </indexterm>

 <indexterm zone="libpq">
  <primary>C</primary>
 </indexterm>

 <para>
  <application>libpq</application> is the <acronym>C</acronym>
  application programmer's interface to <productname>PostgreSQL</>.
  <application>libpq</> is a set of library functions that allow
  client programs to pass queries to the <productname>PostgreSQL</>
  backend server and to receive the results of these queries.
 </para>

 <para>
  <application>libpq</> is also the underlying engine for several
  other <productname>PostgreSQL</> application interfaces, including
  those written for C++, Perl, Python, Tcl and <application>ECPG</>.
  So some aspects of <application>libpq</>'s behavior will be
  important to you if you use one of those packages.  In particular,
  <xref linkend="libpq-envars">,
  <xref linkend="libpq-pgpass"> and
  <xref linkend="libpq-ssl">
  describe behavior that is visible to the user of any application
  that uses <application>libpq</>.
 </para>

 <para>
  Some short programs are included at the end of this chapter (<xref linkend="libpq-example">) to show how
  to write programs that use <application>libpq</application>.  There are also several
  complete examples of <application>libpq</application> applications in the
  directory <filename>src/test/examples</filename> in the source code distribution.
 </para>

 <para>
  Client programs that use <application>libpq</application> must
  include the header file
  <filename>libpq-fe.h</filename><indexterm><primary>libpq-fe.h</></>
  and must link with the <application>libpq</application> library.
 </para>

 <sect1 id="libpq-connect">
  <title>Database Connection Control Functions</title>

  <para>
   The following functions deal with making a connection to a
   <productname>PostgreSQL</productname> backend server.  An
   application program can have several backend connections open at
   one time.  (One reason to do that is to access more than one
   database.)  Each connection is represented by a
   <structname>PGconn</><indexterm><primary>PGconn</></> object, which
   is obtained from the function <function>PQconnectdb</> or
   <function>PQsetdbLogin</>.  Note that these functions will always
   return a non-null object pointer, unless perhaps there is too
   little memory even to allocate the <structname>PGconn</> object.
   The <function>PQstatus</> function should be called to check
   whether a connection was successfully made before queries are sent
   via the connection object.

   <variablelist>
    <varlistentry>
     <term><function>PQconnectdb</function><indexterm><primary>PQconnectdb</></></term>
     <listitem>
      <para>
       Makes a new connection to the database server.

       <synopsis>
        PGconn *PQconnectdb(const char *conninfo);
       </synopsis>
      </para>

      <para>
       This function opens a new database connection using the parameters taken
       from the string <literal>conninfo</literal>.  Unlike <function>PQsetdbLogin</> below,
       the parameter set can be extended without changing the function signature,
       so use of this function (or its nonblocking analogues <function>PQconnectStart</>
       and <function>PQconnectPoll</function>) is preferred for new application programming.
      </para>

      <para>
       The passed string
       can be empty to use all default parameters, or it can contain one or more
       parameter settings separated by whitespace.
       Each parameter setting is in the form <literal>keyword = value</literal>.
       Spaces around the equal sign are optional.
       To write an empty value or a value containing
       spaces, surround it with single quotes, e.g.,
       <literal>keyword = 'a value'</literal>.
       Single quotes and backslashes within the value must be escaped with a
       backslash, i.e., <literal>\'</literal> and <literal>\\</literal>.
      </para>

      <para>
       The currently recognized parameter key words are:

       <variablelist>
        <varlistentry>
         <term><literal>host</literal></term>
         <listitem>
          <para>
           Name of host to connect to.<indexterm><primary>host name</></>
           If this begins with a slash, it specifies Unix-domain
           communication rather than TCP/IP communication; the value is the
           name of the directory in which the socket file is stored.  The
           default behavior when <literal>host</literal> is not specified
           is to connect to a Unix-domain
           socket<indexterm><primary>Unix domain socket</></> in
           <filename>/tmp</filename> (or whatever socket directory was specified
           when <productname>PostgreSQL</> was built). On machines without
           Unix-domain sockets, the default is to connect to <literal>localhost</>.
          </para>
         </listitem>
        </varlistentry>

        <varlistentry>
         <term><literal>hostaddr</literal></term>
         <listitem>
          <para>
           Numeric IP address of host to connect to.  This should be in the
           standard IPv4 address format, e.g., <literal>172.28.40.9</>.  If
           your machine supports IPv6, you can also use those addresses.
           TCP/IP communication is
           always used when a nonempty string is specified for this parameter.
          </para>

          <para>
           Using <literal>hostaddr</> instead of <literal>host</> allows the
           application to avoid a host name look-up, which might be important in
           applications with time constraints. However, Kerberos and GSSAPI authentication
           requires the host name. The following therefore applies: If
           <literal>host</> is specified without <literal>hostaddr</>, a host name
           lookup occurs. If <literal>hostaddr</> is specified without
           <literal>host</>, the value for <literal>hostaddr</> gives the remote
           address. When Kerberos is used, a reverse name query occurs to obtain
           the host name for Kerberos. If both
           <literal>host</> and <literal>hostaddr</> are specified, the value for
           <literal>hostaddr</> gives the remote address; the value for
           <literal>host</> is ignored, unless Kerberos is used, in which case that
           value is used for Kerberos authentication. (Note that authentication is
           likely to fail if <application>libpq</application> is passed a host name
           that is not the name of the machine at <literal>hostaddr</>.)  Also,
           <literal>host</> rather than <literal>hostaddr</> is used to identify
           the connection in <filename>~/.pgpass</> (see
           <xref linkend="libpq-pgpass">).
          </para>

          <para>
           Without either a host name or host address,
           <application>libpq</application> will connect using a
           local Unix-domain socket; or on machines without Unix-domain
           sockets, it will attempt to connect to <literal>localhost</>.
          </para>
          </listitem>
         </varlistentry>

         <varlistentry>
          <term><literal>port</literal></term>
          <listitem>
          <para>
           Port number to connect to at the server host, or socket file
           name extension for Unix-domain
           connections.<indexterm><primary>port</></>
          </para>
         </listitem>
        </varlistentry>

        <varlistentry>
         <term><literal>dbname</literal></term>
         <listitem>
         <para>
          The database name.  Defaults to be the same as the user name.
         </para>
         </listitem>
        </varlistentry>

        <varlistentry>
         <term><literal>user</literal></term>
         <listitem>
         <para>
          <productname>PostgreSQL</productname> user name to connect as.
          Defaults to be the same as the operating system name of the user
          running the application.
         </para>
         </listitem>
        </varlistentry>

        <varlistentry>
         <term><literal>password</literal></term>
         <listitem>
         <para>
          Password to be used if the server demands password authentication.
         </para>
         </listitem>
        </varlistentry>

        <varlistentry>
         <term><literal>connect_timeout</literal></term>
         <listitem>
         <para>
          Maximum wait for connection, in seconds (write as a decimal integer
          string). Zero or not specified means wait indefinitely.  It is not
          recommended to use a timeout of less than 2 seconds.
         </para>
         </listitem>
        </varlistentry>

        <varlistentry>
         <term><literal>options</literal></term>
         <listitem>
          <para>
           Command-line options to be sent to the server.
          </para>
         </listitem>
        </varlistentry>

        <varlistentry>
         <term><literal>tty</literal></term>
         <listitem>
         <para>
          Ignored (formerly, this specified where to send server debug output).
         </para>
         </listitem>
        </varlistentry>

        <varlistentry>
         <term><literal>sslmode</literal></term>
         <listitem>
          <para>
           This option determines whether or with what priority a
           <acronym>SSL</> TCP/IP connection will be negotiated with the
           server. There are four modes: <literal>disable</> will attempt
           only an unencrypted <acronym>SSL</> connection;
           <literal>allow</> will negotiate, trying first a
           non-<acronym>SSL</> connection, then if that fails, trying an
           <acronym>SSL</> connection; <literal>prefer</> (the default)
           will negotiate, trying first an <acronym>SSL</> connection,
           then if that fails, trying a regular non-<acronym>SSL</>
           connection; <literal>require</> will try only an
           <acronym>SSL</> connection.  <literal>sslmode</> is ignored
           for Unix domain socket communication.
          </para>

          <para>
           If <productname>PostgreSQL</> is compiled without SSL support,
           using option <literal>require</> will cause an error, while
           options <literal>allow</> and <literal>prefer</> will be
           accepted but <application>libpq</> will not in fact attempt
           an <acronym>SSL</>
           connection.<indexterm><primary>SSL</><secondary
           sortas="libpq">with libpq</></indexterm>
          </para>
         </listitem>
        </varlistentry>

        <varlistentry>
         <term><literal>sslverify</literal></term>
         <listitem>
          <para>
           This option controls how libpq verifies the certificate on the
           server when performing an <acronym>SSL</> connection. There are
           three options: <literal>none</> disables verification completely
           (not recommended!); <literal>cert</> enables verification that
           the certificate chains to a known CA only; <literal>cn</> will
           both verify that the certificate chains to a known CA and that
           the <literal>cn</> attribute of the certificate matches the
           hostname the connection is being made to (default).
          </para>

          <para>
           It is always recommended to use the <literal>cn</> value for
           this parameter, since this is the only option that prevents
           man-in-the-middle attacks. Note that this requires the server
           name on the certificate to match exactly with the host name
           used for the connection, and therefore does not support connections
           to aliased names. It can be used with pure IP address connections
           only if the certificate also has just the IP address in the
           <literal>cn</> field.
          </para>

          <para>
           If the <literal>cn</> attribute in the certificate sent by the
           server starts with an asterisk (<literal>*</>), it will be treated
           as a wildcard. This wildcard can only be present at the start of
           the value, and will match all characters <emphasis>except</> a
           dot (<literal>.</>). This means the certificate will not match
           subdomains.
          </para>
         </listitem>
        </varlistentry>

        <varlistentry>
         <term><literal>requiressl</literal></term>
         <listitem>
          <para>
           This option is deprecated in favor of the <literal>sslmode</>
           setting.
          </para>

          <para>
           If set to 1, an <acronym>SSL</acronym> connection to the server
           is required (this is equivalent to <literal>sslmode</>
           <literal>require</>).  <application>libpq</> will then refuse
           to connect if the server does not accept an
           <acronym>SSL</acronym> connection.  If set to 0 (default),
           <application>libpq</> will negotiate the connection type with
           the server (equivalent to <literal>sslmode</>
           <literal>prefer</>).  This option is only available if
           <productname>PostgreSQL</> is compiled with SSL support.
          </para>
         </listitem>
        </varlistentry>

        <varlistentry>
         <term><literal>sslcert</literal></term>
         <listitem>
          <para>
           This parameter specifies the file name of the client SSL
           certificate.
          </para>
         </listitem>
        </varlistentry>

        <varlistentry>
         <term><literal>sslkey</literal></term>
         <listitem>
          <para>
           This parameter specifies the location for the secret key
           used for the client certificate. It can either specify a filename
           that will be used instead of the default
           <filename>~/.postgresql/postgresql.key</>, or can specify an external
           engine (engines are <productname>OpenSSL</> loadable modules). The
           external engine specification should consist of a colon-separated
           engine name and an engine-specific key identifier.
          </para>
         </listitem>
        </varlistentry>

        <varlistentry>
         <term><literal>sslrootcert</literal></term>
         <listitem>
          <para>
           This parameter specifies the file name of the root SSL certificate.
          </para>
         </listitem>
        </varlistentry>

        <varlistentry>
         <term><literal>sslcrl</literal></term>
         <listitem>
          <para>
           This parameter specifies the file name of the SSL certificate
           revocation list (CRL).
          </para>
         </listitem>
        </varlistentry>

        <varlistentry>
         <term><literal>krbsrvname</literal></term>
         <listitem>
          <para>
           Kerberos service name to use when authenticating with Kerberos 5
           or GSSAPI.
           This must match the service name specified in the server
           configuration for Kerberos authentication to succeed. (See also
           <xref linkend="kerberos-auth"> and <xref linkend="gssapi-auth">.)
          </para>
         </listitem>
        </varlistentry>

        <varlistentry>
         <term><literal>gsslib</literal></term>
         <listitem>
          <para>
           GSS library to use for GSSAPI authentication. Only used on Windows.
           Set to <literal>gssapi</literal> to force libpq to use the GSSAPI
           library for authentication instead of the default SSPI.
          </para>
         </listitem>
        </varlistentry>

        <varlistentry>
         <term><literal>service</literal></term>
         <listitem>
          <para>
           Service name to use for additional parameters.  It specifies a service
           name in <filename>pg_service.conf</filename> that holds additional connection parameters.
           This allows applications to specify only a service name so connection parameters
           can be centrally maintained. See <xref linkend="libpq-pgservice">.
          </para>
         </listitem>
        </varlistentry>
       </variablelist>

       If  any  parameter is unspecified, then the corresponding
       environment variable (see <xref linkend="libpq-envars">)
       is checked. If the  environment  variable is not set either,
       then the indicated built-in defaults are used.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><function>PQsetdbLogin</function><indexterm><primary>PQsetdbLogin</></></term>
     <listitem>
      <para>
       Makes a new connection to the database server.
<synopsis>
PGconn *PQsetdbLogin(const char *pghost,
                     const char *pgport,
                     const char *pgoptions,
                     const char *pgtty,
                     const char *dbName,
                     const char *login,
                     const char *pwd);
</synopsis>
       </para>

       <para>
        This is the predecessor of <function>PQconnectdb</function> with a fixed
        set of parameters.  It has the same functionality except that the
        missing parameters will always take on default values.  Write <symbol>NULL</symbol> or an
        empty string for any one of the fixed parameters that is to be defaulted.
      </para>

      <para>
        If the <parameter>dbName</parameter> contains an <symbol>=</symbol> sign, it
        is taken as a <parameter>conninfo</parameter> string in exactly the same way as
        if it had been passed to <function>PQconnectdb</function>, and the remaining
        parameters are then applied as above.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><function>PQsetdb</function><indexterm><primary>PQsetdb</></></term>
     <listitem>
      <para>
   Makes a new connection to the database server.
<synopsis>
PGconn *PQsetdb(char *pghost,
                char *pgport,
                char *pgoptions,
                char *pgtty,
                char *dbName);
</synopsis>
     </para>

     <para>
      This is a macro that calls <function>PQsetdbLogin</function> with null pointers
      for the <parameter>login</> and <parameter>pwd</> parameters.  It is provided
      for backward compatibility with very old programs.
     </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><function>PQconnectStart</function><indexterm><primary>PQconnectStart</></></term>
     <term><function>PQconnectPoll</function><indexterm><primary>PQconnectPoll</></></term>
     <listitem>
      <para>
       <indexterm><primary>nonblocking connection</primary></indexterm>
       Make a connection to the database server in a nonblocking manner.

       <synopsis>
        PGconn *PQconnectStart(const char *conninfo);
       </synopsis>

       <synopsis>
        PostgresPollingStatusType PQconnectPoll(PGconn *conn);
       </synopsis>
      </para>

      <para>
       These two functions are used to open a connection to a database server such
       that your application's thread of execution is not blocked on remote I/O
       whilst doing so.
       The point of this approach is that the waits for I/O to complete can occur
       in the application's main loop, rather than down inside
       <function>PQconnectdb</>, and so the application can manage this
       operation in parallel with other activities.
      </para>

      <para>
       The database connection is made using the parameters taken from the string
       <literal>conninfo</literal>, passed to <function>PQconnectStart</function>. This string is in
       the same format as described above for <function>PQconnectdb</function>.
      </para>
      <para>
       Neither <function>PQconnectStart</function> nor <function>PQconnectPoll</function> will block, so long as a number of
       restrictions are met:
       <itemizedlist>
        <listitem>
         <para>
          The <literal>hostaddr</> and <literal>host</> parameters are used appropriately to ensure that
          name and reverse name queries are not made. See the documentation of
          these parameters under <function>PQconnectdb</function> above for details.
         </para>
        </listitem>

        <listitem>
         <para>
          If you call <function>PQtrace</function>, ensure that the stream object
          into which you trace will not block.
         </para>
        </listitem>

        <listitem>
         <para>
          You ensure that the socket is in the appropriate state
          before calling <function>PQconnectPoll</function>, as described below.
         </para>
        </listitem>
       </itemizedlist>
      </para>

      <para>
       To begin a nonblocking connection request, call <literal>conn = PQconnectStart("<replaceable>connection_info_string</>")</literal>.
       If <varname>conn</varname> is null, then <application>libpq</> has been unable to allocate a new <structname>PGconn</>
       structure. Otherwise, a valid <structname>PGconn</> pointer is returned (though not yet
       representing a valid connection to the database). On return from
       <function>PQconnectStart</function>, call <literal>status = PQstatus(conn)</literal>. If <varname>status</varname> equals
       <symbol>CONNECTION_BAD</symbol>, <function>PQconnectStart</function> has failed.
      </para>

      <para>
       If <function>PQconnectStart</> succeeds, the next stage is to poll
       <application>libpq</> so that it can proceed with the connection sequence.
       Use <function>PQsocket(conn)</function> to obtain the descriptor of the
       socket underlying the database connection.
       Loop thus: If <function>PQconnectPoll(conn)</function> last returned
       <symbol>PGRES_POLLING_READING</symbol>, wait until the socket is ready to
       read (as indicated by <function>select()</>, <function>poll()</>, or
       similar system function).
       Then call <function>PQconnectPoll(conn)</function> again.
       Conversely, if <function>PQconnectPoll(conn)</function> last returned
       <symbol>PGRES_POLLING_WRITING</symbol>, wait until the socket is ready
       to write, then call <function>PQconnectPoll(conn)</function> again.
       If you have yet to call
       <function>PQconnectPoll</function>, i.e., just after the call to
       <function>PQconnectStart</function>, behave as if it last returned
       <symbol>PGRES_POLLING_WRITING</symbol>.  Continue this loop until
       <function>PQconnectPoll(conn)</function> returns
       <symbol>PGRES_POLLING_FAILED</symbol>, indicating the connection procedure
       has failed, or <symbol>PGRES_POLLING_OK</symbol>, indicating the connection
       has been successfully made.
      </para>

      <para>
       At any time during connection, the status of the connection can be
       checked by calling <function>PQstatus</>. If this gives <symbol>CONNECTION_BAD</>, then the
       connection procedure has failed; if it gives <function>CONNECTION_OK</>, then the
       connection is ready.  Both of these states are equally detectable
       from the return value of <function>PQconnectPoll</>, described above. Other states might also occur
       during (and only during) an asynchronous connection procedure. These
       indicate the current stage of the connection procedure and might be useful
       to provide feedback to the user for example. These statuses are:

       <variablelist>
        <varlistentry>
         <term><symbol>CONNECTION_STARTED</symbol></term>
         <listitem>
          <para>
           Waiting for connection to be made.
          </para>
         </listitem>
        </varlistentry>

        <varlistentry>
         <term><symbol>CONNECTION_MADE</symbol></term>
         <listitem>
          <para>
           Connection OK; waiting to send.
          </para>
         </listitem>
        </varlistentry>

        <varlistentry>
         <term><symbol>CONNECTION_AWAITING_RESPONSE</symbol></term>
         <listitem>
          <para>
           Waiting for a response from the server.
          </para>
         </listitem>
        </varlistentry>

        <varlistentry>
         <term><symbol>CONNECTION_AUTH_OK</symbol></term>
         <listitem>
          <para>
           Received authentication; waiting for backend start-up to finish.
          </para>
         </listitem>
        </varlistentry>

        <varlistentry>
         <term><symbol>CONNECTION_SSL_STARTUP</symbol></term>
         <listitem>
          <para>
           Negotiating SSL encryption.
          </para>
         </listitem>
        </varlistentry>

        <varlistentry>
         <term><symbol>CONNECTION_SETENV</symbol></term>
         <listitem>
          <para>
           Negotiating environment-driven parameter settings.
          </para>
         </listitem>
        </varlistentry>
       </variablelist>

       Note that, although these constants will remain (in order to maintain
       compatibility), an application should never rely upon these occurring in a
       particular order, or at all, or on the status always being one of these
       documented values. An application might do something like this:
<programlisting>
switch(PQstatus(conn))
{
        case CONNECTION_STARTED:
            feedback = "Connecting...";
            break;

        case CONNECTION_MADE:
            feedback = "Connected to server...";
            break;
.
.
.
        default:
            feedback = "Connecting...";
}
</programlisting>
      </para>

      <para>
       The <literal>connect_timeout</literal> connection parameter is ignored
       when using <function>PQconnectPoll</function>; it is the application's
       responsibility to decide whether an excessive amount of time has elapsed.
       Otherwise, <function>PQconnectStart</function> followed by a
       <function>PQconnectPoll</function> loop is equivalent to
       <function>PQconnectdb</function>.
      </para>

      <para>
       Note that if <function>PQconnectStart</function> returns a non-null pointer, you must call
       <function>PQfinish</function> when you are finished with it, in order to dispose of
       the structure and any associated memory blocks. This must be done even if
       the connection attempt fails or is abandoned.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><function>PQconndefaults</function><indexterm><primary>PQconndefaults</></></term>
     <listitem>
      <para>
       Returns the default connection options.
<synopsis>
PQconninfoOption *PQconndefaults(void);

typedef struct
{
    char   *keyword;   /* The keyword of the option */
    char   *envvar;    /* Fallback environment variable name */
    char   *compiled;  /* Fallback compiled in default value */
    char   *val;       /* Option's current value, or NULL */
    char   *label;     /* Label for field in connect dialog */
    char   *dispchar;  /* Indicates how to display this field
                          in a connect dialog. Values are:
                          ""        Display entered value as is
                          "*"       Password field - hide value
                          "D"       Debug option - don't show by default */
    int     dispsize;  /* Field size in characters for dialog */
} PQconninfoOption;
</synopsis>
      </para>

      <para>
       Returns a connection options array.  This can be used to determine
       all possible <function>PQconnectdb</function> options and their
       current default values.  The return value points to an array of
       <structname>PQconninfoOption</structname> structures, which ends
       with an entry having a null <structfield>keyword</> pointer.  The
       null pointer is returned if memory could not be allocated. Note that
       the current default values (<structfield>val</structfield> fields)
       will depend on environment variables and other context.  Callers
       must treat the connection options data as read-only.
      </para>

      <para>
       After processing the options array, free it by passing it to
       <function>PQconninfoFree</function>.  If this is not done, a small amount of memory
       is leaked for each call to <function>PQconndefaults</function>.
      </para>

     </listitem>
    </varlistentry>

    <varlistentry>
     <term><function>PQconninfoParse</function><indexterm><primary>PQconninfoParse</></></term>
     <listitem>
      <para>
       Returns parsed connection options from the provided connection string.

<synopsis>
PQconninfoOption *PQconninfoParse(const char *conninfo, char **errmsg);
</synopsis>
      </para>

      <para>
       Parses a connection string and returns the resulting options as an
       array; or returns NULL if there is a problem with the connection
       string.  This can be used to determine
       the <function>PQconnectdb</function> options in the provided
       connection string.  The return value points to an array of
       <structname>PQconninfoOption</structname> structures, which ends
       with an entry having a null <structfield>keyword</> pointer.
      </para>

      <para>
       Note that only options explicitly specified in the string will have
       values set in the result array; no defaults are inserted.
      </para>

      <para>
       If <literal>errmsg</> is not NULL, then <literal>*errmsg</> is set
       to NULL on success, else to a malloc'd error string explaining
       the problem.  (It is also possible for <literal>*errmsg</> to be
       set to NULL even when NULL is returned; this indicates an out-of-memory
       situation.)
      </para>

      <para>
       After processing the options array, free it by passing it to
       <function>PQconninfoFree</function>.  If this is not done, some memory
       is leaked for each call to <function>PQconninfoParse</function>.
       Conversely, if an error occurs and <literal>errmsg</> is not NULL,
       be sure to free the error string using <function>PQfreemem</>.
      </para>

   </listitem>
    </varlistentry>

    <varlistentry>
     <term><function>PQfinish</function><indexterm><primary>PQfinish</></></term>
     <listitem>
      <para>
       Closes  the  connection to the server.  Also frees
       memory used by the <structname>PGconn</structname> object.
       <synopsis>
        void PQfinish(PGconn *conn);
       </synopsis>
      </para>

      <para>
       Note that even if the server connection attempt fails (as
       indicated by <function>PQstatus</function>), the application should call <function>PQfinish</function>
       to free the memory used by the <structname>PGconn</structname> object.
       The <structname>PGconn</> pointer must not be used again after
       <function>PQfinish</function> has been called.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><function>PQreset</function><indexterm><primary>PQreset</></></term>
     <listitem>
      <para>
       Resets the communication channel to the server.
       <synopsis>
        void PQreset(PGconn *conn);
       </synopsis>
      </para>

      <para>
       This function will close the connection
       to the server and attempt to  reestablish  a  new
       connection to the same server, using all the same
       parameters previously used.  This might be useful for
       error recovery if a working connection is lost.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><function>PQresetStart</function><indexterm><primary>PQresetStart</></></term>
     <term><function>PQresetPoll</function><indexterm><primary>PQresetPoll</></></term>
     <listitem>
      <para>
       Reset the communication channel to the server, in a nonblocking manner.

       <synopsis>
        int PQresetStart(PGconn *conn);
       </synopsis>
       <synopsis>
        PostgresPollingStatusType PQresetPoll(PGconn *conn);
       </synopsis>
      </para>

      <para>
       These functions will close the connection to the server and attempt to
       reestablish a new connection to the same server, using all the same
       parameters previously used. This can be useful for error recovery if a
       working connection is lost. They differ from <function>PQreset</function> (above) in that they
       act in a nonblocking manner. These functions suffer from the same
       restrictions as <function>PQconnectStart</> and <function>PQconnectPoll</>.
      </para>

      <para>
       To initiate a connection reset, call
       <function>PQresetStart</function>. If it returns 0, the reset has
       failed. If it returns 1, poll the reset using
       <function>PQresetPoll</function> in exactly the same way as you
       would create the connection using <function>PQconnectPoll</function>.
      </para>
     </listitem>
    </varlistentry>

   </variablelist>
  </para>
 </sect1>

 <sect1 id="libpq-status">
  <title>Connection Status Functions</title>

  <para>
   These functions can be used to interrogate the status
   of an existing database connection object.
  </para>

  <tip>
   <para>
    <indexterm><primary>libpq-fe.h</></>
    <indexterm><primary>libpq-int.h</></>
    <application>libpq</application> application programmers should be careful to
    maintain the <structname>PGconn</structname> abstraction.  Use the accessor
    functions described below to get at the contents of <structname>PGconn</structname>.
    Reference to internal <structname>PGconn</structname> fields using
    <filename>libpq-int.h</> is not recommended because they are subject to change
    in the future.
   </para>
  </tip>

  <para>
   The following functions return parameter values established at connection.
   These values are fixed for the life of the <structname>PGconn</> object.

   <variablelist>
    <varlistentry>
     <term>
      <function>PQdb</function>
      <indexterm>
       <primary>PQdb</primary>
      </indexterm>
     </term>

     <listitem>
      <para>
       Returns the database name of the connection.
       <synopsis>
        char *PQdb(const PGconn *conn);
       </synopsis>
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term>
      <function>PQuser</function>
      <indexterm>
       <primary>PQuser</primary>
      </indexterm>
     </term>

     <listitem>
      <para>
       Returns the user name of the connection.
       <synopsis>
        char *PQuser(const PGconn *conn);
       </synopsis>
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term>
      <function>PQpass</function>
      <indexterm>
       <primary>PQpass</primary>
      </indexterm>
     </term>

     <listitem>
      <para>
       Returns the password of the connection.
       <synopsis>
        char *PQpass(const PGconn *conn);
       </synopsis>
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term>
      <function>PQhost</function>
      <indexterm>
       <primary>PQhost</primary>
      </indexterm>
     </term>

     <listitem>
      <para>
       Returns the server host name of the connection.
       <synopsis>
        char *PQhost(const PGconn *conn);
       </synopsis>
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term>
      <function>PQport</function>
      <indexterm>
       <primary>PQport</primary>
      </indexterm>
     </term>

     <listitem>
      <para>
       Returns the port of the connection.

       <synopsis>
        char *PQport(const PGconn *conn);
       </synopsis>
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term>
      <function>PQtty</function>
      <indexterm>
       <primary>PQtty</primary>
      </indexterm>
     </term>

     <listitem>
      <para>
       Returns the debug <acronym>TTY</acronym> of the connection.
       (This is obsolete, since the server no longer pays attention
       to the <acronym>TTY</acronym> setting, but the function remains
       for backwards compatibility.)

       <synopsis>
        char *PQtty(const PGconn *conn);
       </synopsis>
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term>
      <function>PQoptions</function>
      <indexterm>
       <primary>PQoptions</primary>
      </indexterm>
     </term>

     <listitem>
      <para>
       Returns the command-line options passed in the connection request.
       <synopsis>
        char *PQoptions(const PGconn *conn);
       </synopsis>
      </para>
     </listitem>
    </varlistentry>
   </variablelist>
  </para>

  <para>
   The following functions return status data that can change as operations
   are executed on the <structname>PGconn</> object.

   <variablelist>
    <varlistentry>
     <term>
      <function>PQstatus</function>
      <indexterm>
       <primary>PQstatus</primary>
      </indexterm>
     </term>

     <listitem>
      <para>
       Returns the status of the connection.
       <synopsis>
        ConnStatusType PQstatus(const PGconn *conn);
       </synopsis>
      </para>

      <para>
       The status can be one of a number of values.  However, only two of
       these are seen outside of an asynchronous connection procedure:
       <literal>CONNECTION_OK</literal> and
       <literal>CONNECTION_BAD</literal>. A good connection to the database
       has the status <literal>CONNECTION_OK</literal>.  A failed
       connection attempt is signaled by status
       <literal>CONNECTION_BAD</literal>.  Ordinarily, an OK status will
       remain so until <function>PQfinish</function>, but a communications
       failure might result in the status changing to
       <literal>CONNECTION_BAD</literal> prematurely.  In that case the
       application could try to recover by calling
       <function>PQreset</function>.
      </para>

      <para>
       See the entry for <function>PQconnectStart</> and <function>PQconnectPoll</> with regards
       to other status codes
       that might be seen.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term>
      <function>PQtransactionStatus</function>
      <indexterm>
       <primary>PQtransactionStatus</primary>
      </indexterm>
     </term>

     <listitem>
      <para>
       Returns the current in-transaction status of the server.

       <synopsis>
        PGTransactionStatusType PQtransactionStatus(const PGconn *conn);
       </synopsis>

       The status can be <literal>PQTRANS_IDLE</literal> (currently idle),
       <literal>PQTRANS_ACTIVE</literal> (a command is in progress),
       <literal>PQTRANS_INTRANS</literal> (idle, in a valid transaction block),
       or <literal>PQTRANS_INERROR</literal> (idle, in a failed transaction block).
       <literal>PQTRANS_UNKNOWN</literal> is reported if the connection is bad.
       <literal>PQTRANS_ACTIVE</literal> is reported only when a query
       has been sent to the server and not yet completed.
      </para>

      <caution>
       <para>
        <function>PQtransactionStatus</> will give incorrect results when using
        a <productname>PostgreSQL</> 7.3 server that has the parameter <literal>autocommit</>
        set to off.  The server-side autocommit feature has been
        deprecated and does not exist in later server versions.
       </para>
      </caution>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term>
      <function>PQparameterStatus</function>
      <indexterm>
       <primary>PQparameterStatus</primary>
      </indexterm>
     </term>

     <listitem>
      <para>
       Looks up a current parameter setting of the server.

       <synopsis>
        const char *PQparameterStatus(const PGconn *conn, const char *paramName);
       </synopsis>

       Certain parameter values are reported by the server automatically at
       connection startup or whenever their values change.
       <function>PQparameterStatus</> can be used to interrogate these settings.
       It returns the current value of a parameter if known, or <symbol>NULL</symbol>
       if the parameter is not known.
      </para>

      <para>
       Parameters reported as of the current release include
       <literal>server_version</>,
       <literal>server_encoding</>,
       <literal>client_encoding</>,
       <literal>is_superuser</>,
       <literal>session_authorization</>,
       <literal>DateStyle</>,
       <literal>IntervalStyle</>,
       <literal>TimeZone</>,
       <literal>integer_datetimes</>, and
       <literal>standard_conforming_strings</>.
       (<literal>server_encoding</>, <literal>TimeZone</>, and
       <literal>integer_datetimes</> were not reported by releases before 8.0;
       <literal>standard_conforming_strings</> was not reported by releases
       before 8.1; <literal>IntervalStyle</> was not reported by releases
       before 8.4.)
       Note that
       <literal>server_version</>,
       <literal>server_encoding</> and
       <literal>integer_datetimes</>
       cannot change after startup.
      </para>

      <para>
       Pre-3.0-protocol servers do not report parameter settings, but
       <application>libpq</> includes logic to obtain values for
       <literal>server_version</> and <literal>client_encoding</> anyway.
       Applications are encouraged to use <function>PQparameterStatus</>
       rather than <foreignphrase>ad hoc</> code to determine these values.
       (Beware however that on a pre-3.0 connection, changing
       <literal>client_encoding</> via <command>SET</> after connection
       startup will not be reflected by <function>PQparameterStatus</>.)
       For <literal>server_version</>, see also
       <function>PQserverVersion</>, which returns the information in a
       numeric form that is much easier to compare against.
      </para>

      <para>
       If no value for <literal>standard_conforming_strings</> is reported,
       applications can assume it is <literal>off</>, that is, backslashes
       are treated as escapes in string literals.  Also, the presence of
       this parameter can be taken as an indication that the escape string
       syntax (<literal>E'...'</>) is accepted.
      </para>

      <para>
       Although the returned pointer is declared <literal>const</>, it in fact
       points to mutable storage associated with the <literal>PGconn</> structure.
       It is unwise to assume the pointer will remain valid across queries.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term>
      <function>PQprotocolVersion</function>
      <indexterm>
       <primary>PQprotocolVersion</primary>
      </indexterm>
     </term>

     <listitem>
      <para>
       Interrogates the frontend/backend protocol being used.
       <synopsis>
        int PQprotocolVersion(const PGconn *conn);
       </synopsis>
       Applications might wish to use this to determine whether certain
       features are supported.  Currently, the possible values are 2 (2.0
       protocol), 3 (3.0 protocol), or zero (connection bad).  This will
       not change after connection startup is complete, but it could
       theoretically change during a connection reset.  The 3.0 protocol
       will normally be used when communicating with
       <productname>PostgreSQL</> 7.4 or later servers; pre-7.4 servers
       support only protocol 2.0.  (Protocol 1.0 is obsolete and not
       supported by <application>libpq</application>.)
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term>
      <function>PQserverVersion</function>
      <indexterm>
       <primary>PQserverVersion</primary>
      </indexterm>
     </term>

     <listitem>
      <para>
       Returns an integer representing the backend version.
       <synopsis>
        int PQserverVersion(const PGconn *conn);
       </synopsis>
       Applications might use this to determine the version of the database
       server they are connected to. The number is formed by converting
       the major, minor, and revision numbers into two-decimal-digit
       numbers and appending them together. For example, version 8.1.5
       will be returned as 80105, and version 8.2 will be returned as
       80200 (leading zeroes are not shown).  Zero is returned if the
       connection is bad.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term>
      <function>PQerrorMessage</function>
      <indexterm>
       <primary>PQerrorMessage</primary>
      </indexterm>
     </term>

     <listitem>
      <para>
       <indexterm><primary>error message</></> Returns the error message
       most recently generated by an operation on the connection.

       <synopsis>
        char *PQerrorMessage(const PGconn *conn);
       </synopsis>

      </para>

      <para>
       Nearly all <application>libpq</> functions will set a message for
       <function>PQerrorMessage</function> if they fail.  Note that by
       <application>libpq</application> convention, a nonempty
       <function>PQerrorMessage</function> result can be multiple lines,
       and will include a trailing newline. The caller should not free
       the result directly. It will be freed when the associated
       <structname>PGconn</> handle is passed to
       <function>PQfinish</function>.  The result string should not be
       expected to remain the same across operations on the
       <literal>PGconn</> structure.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><function>PQsocket</function><indexterm><primary>PQsocket</></></term>
     <listitem>
      <para>
       Obtains the file descriptor number of the connection socket to
       the server.  A valid descriptor will be greater than or equal
       to 0; a result of -1 indicates that no server connection is
       currently open.  (This will not change during normal operation,
       but could change during connection setup or reset.)

       <synopsis>
        int PQsocket(const PGconn *conn);
       </synopsis>

      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><function>PQbackendPID</function><indexterm><primary>PQbackendPID</></></term>
     <listitem>
      <para>
       Returns the process <acronym>ID</acronym>
       (PID)<indexterm><primary>PID</><secondary>determining PID of
       server process</><tertiary>in libpq</></> of the backend server
       process handling this connection.

       <synopsis>
        int PQbackendPID(const PGconn *conn);
       </synopsis>
      </para>

      <para>
       The backend <acronym>PID</acronym> is useful for debugging
       purposes and for comparison to <command>NOTIFY</command>
       messages (which include the <acronym>PID</acronym> of the
       notifying backend process).  Note that the
       <acronym>PID</acronym> belongs to a process executing on the
       database server host, not the local host!
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><function>PQconnectionNeedsPassword</function><indexterm><primary>PQconnectionNeedsPassword</></></term>
     <listitem>
      <para>
       Returns true (1) if the connection authentication method
       required a password, but none was available.
       Returns false (0) if not.

       <synopsis>
        int PQconnectionNeedsPassword(const PGconn *conn);
       </synopsis>
      </para>

      <para>
       This function can be applied after a failed connection attempt
       to decide whether to prompt the user for a password.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><function>PQconnectionUsedPassword</function><indexterm><primary>PQconnectionUsedPassword</></></term>
     <listitem>
      <para>
       Returns true (1) if the connection authentication method
       used a password. Returns false (0) if not.

       <synopsis>
        int PQconnectionUsedPassword(const PGconn *conn);
       </synopsis>
      </para>

      <para>
       This function can be applied after either a failed or successful
       connection attempt to detect whether the server demanded a password.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><function>PQgetssl</function><indexterm><primary>PQgetssl</></></term>
     <listitem>
      <para>
       <indexterm><primary>SSL</><secondary sortas="libpq">in libpq</secondary></indexterm>
       Returns the SSL structure used in the connection, or null
       if SSL is not in use.

       <synopsis>
        SSL *PQgetssl(const PGconn *conn);
       </synopsis>
      </para>

      <para>
       This structure can be used to verify encryption levels, check server
       certificates, and more. Refer to the <productname>OpenSSL</>
       documentation for information about this structure.
      </para>

      <para>
       You must define <symbol>USE_SSL</symbol> in order to get the
       correct prototype for this function. Doing this will also
       automatically include <filename>ssl.h</filename> from <productname>OpenSSL</productname>.
      </para>
     </listitem>
    </varlistentry>

   </variablelist>
  </para>

 </sect1>

 <sect1 id="libpq-exec">
  <title>Command Execution Functions</title>

  <para>
   Once a connection to a database server has been successfully
   established, the functions described here are used to perform
   SQL queries and commands.
  </para>

  <sect2 id="libpq-exec-main">
   <title>Main Functions</title>

   <para>
    <variablelist>
     <varlistentry>
      <term>
       <function>PQexec</function>
       <indexterm>
        <primary>PQexec</primary>
       </indexterm>
      </term>

      <listitem>
       <para>
        Submits a command to the server and waits for the result.

        <synopsis>
         PGresult *PQexec(PGconn *conn, const char *command);
        </synopsis>
       </para>

       <para>
        Returns a <structname>PGresult</structname> pointer or possibly a null
        pointer.  A non-null pointer will generally be returned except in
        out-of-memory conditions or serious errors such as inability to send
        the command to the server.  If a null pointer is returned, it should
        be treated like a <symbol>PGRES_FATAL_ERROR</symbol> result.  Use
        <function>PQerrorMessage</function> to get more information about such
        errors.
       </para>
      </listitem>
     </varlistentry>
    </variablelist>

    It is allowed to include multiple SQL commands (separated by semicolons)
    in the command string.  Multiple queries sent in a single
    <function>PQexec</> call are processed in a single transaction, unless
    there are explicit <command>BEGIN</command>/<command>COMMIT</command>
    commands included in the query string to divide it into multiple
    transactions.  Note however that the returned
    <structname>PGresult</structname> structure describes only the result
    of the last command executed from the string.  Should one of the
    commands fail, processing of the string stops with it and the returned
    <structname>PGresult</structname> describes the error condition.
   </para>

   <para>
    <variablelist>
     <varlistentry>
      <term>
       <function>PQexecParams</function>
       <indexterm>
        <primary>PQexecParams</primary>
       </indexterm>
      </term>

      <listitem>
       <para>
        Submits a command to the server and waits for the result,
        with the ability to pass parameters separately from the SQL
        command text.

<synopsis>
PGresult *PQexecParams(PGconn *conn,
                       const char *command,
                       int nParams,
                       const Oid *paramTypes,
                       const char * const *paramValues,
                       const int *paramLengths,
                       const int *paramFormats,
                       int resultFormat);
</synopsis>
       </para>

       <para>
        <function>PQexecParams</> is like <function>PQexec</>, but offers additional
        functionality: parameter values can be specified separately from the command
        string proper, and query results can be requested in either text or binary
        format.  <function>PQexecParams</> is supported only in protocol 3.0 and later
        connections; it will fail when using protocol 2.0.
       </para>

       <para>
        The function arguments are:

        <variablelist>
         <varlistentry>
          <term><parameter>conn</parameter></term>

          <listitem>
           <para>
            The connection object to send the command through.
           </para>
          </listitem>
         </varlistentry>

         <varlistentry>
          <term><parameter>command</parameter></term>
          <listitem>
           <para>
            The SQL command string to be executed. If parameters are used,
            they are referred to in the command string as <literal>$1</>,
            <literal>$2</>, etc.
           </para>
          </listitem>
         </varlistentry>

         <varlistentry>
          <term><parameter>nParams</parameter></term>
          <listitem>
           <para>
            The number of parameters supplied; it is the length of the arrays
            <parameter>paramTypes[]</>, <parameter>paramValues[]</>,
            <parameter>paramLengths[]</>, and <parameter>paramFormats[]</>. (The
            array pointers can be <symbol>NULL</symbol> when <parameter>nParams</>
            is zero.)
           </para>
          </listitem>
         </varlistentry>

         <varlistentry>
          <term><parameter>paramTypes[]</parameter></term>
          <listitem>
           <para>
            Specifies, by OID, the data types to be assigned to the
            parameter symbols.  If <parameter>paramTypes</> is
            <symbol>NULL</symbol>, or any particular element in the array
            is zero, the server infers a data type for the parameter symbol
            in the same way it would do for an untyped literal string.
           </para>
          </listitem>
         </varlistentry>

         <varlistentry>
          <term><parameter>paramValues[]</parameter></term>
          <listitem>
           <para>
            Specifies the actual values of the parameters.  A null pointer
            in this array means the corresponding parameter is null;
            otherwise the pointer points to a zero-terminated text string
            (for text format) or binary data in the format expected by the
            server (for binary format).
           </para>
          </listitem>
         </varlistentry>

         <varlistentry>
          <term><parameter>paramLengths[]</parameter></term>
          <listitem>
           <para>
            Specifies the actual data lengths of binary-format parameters.
            It is ignored for null parameters and text-format parameters.
            The array pointer can be null when there are no binary parameters.
           </para>
          </listitem>
         </varlistentry>

         <varlistentry>
          <term><parameter>paramFormats[]</parameter></term>
          <listitem>
           <para>
            Specifies whether parameters are text (put a zero in the
            array entry for the corresponding parameter) or binary (put
            a one in the array entry for the corresponding parameter).
            If the array pointer is null then all parameters are presumed
            to be text strings.
           </para>
           <para>
            Values passed in binary format require knowlege of
            the internal representation expected by the backend.
            For example, integers must be passed in network byte
            order.  Passing <type>numeric</> values requires
            knowledge of the server storage format, as implemented
            in
            <filename>src/backend/utils/adt/numeric.c::numeric_send()</> and
            <filename>src/backend/utils/adt/numeric.c::numeric_recv()</>.
           </para>
          </listitem>
         </varlistentry>

         <varlistentry>
          <term><parameter>resultFormat</parameter></term>
          <listitem>
           <para>
            Specify zero to obtain results in text format, or one to obtain
            results in binary format.  (There is not currently a provision
            to obtain different result columns in different formats,
            although that is possible in the underlying protocol.)
           </para>
          </listitem>
         </varlistentry>
        </variablelist>
       </para>
      </listitem>
     </varlistentry>
    </variablelist>
   </para>

   <para>
    The primary advantage of <function>PQexecParams</> over
    <function>PQexec</> is that parameter values can be separated from the
    command string, thus avoiding the need for tedious and error-prone
    quoting and escaping.
   </para>

   <para>
    Unlike <function>PQexec</>, <function>PQexecParams</> allows at most
    one SQL command in the given string.  (There can be semicolons in it,
    but not more than one nonempty command.)  This is a limitation of the
    underlying protocol, but has some usefulness as an extra defense against
    SQL-injection attacks.
   </para>

   <tip>
    <para>
     Specifying parameter types via OIDs is tedious, particularly if you prefer
     not to hard-wire particular OID values into your program.  However, you can
     avoid doing so even in cases where the server by itself cannot determine the
     type of the parameter, or chooses a different type than you want.  In the
     SQL command text, attach an explicit cast to the parameter symbol to show what
     data type you will send.  For example:
<programlisting>
SELECT * FROM mytable WHERE x = $1::bigint;
</programlisting>
     This forces parameter <literal>$1</> to be treated as <type>bigint</>, whereas
     by default it would be assigned the same type as <literal>x</>.  Forcing the
     parameter type decision, either this way or by specifying a numeric type OID,
     is strongly recommended when sending parameter values in binary format, because
     binary format has less redundancy than text format and so there is less chance
     that the server will detect a type mismatch mistake for you.
    </para>
   </tip>

   <para>
    <variablelist>
     <varlistentry>
      <term><function>PQprepare</function>
       <indexterm>
        <primary>PQprepare</primary>
       </indexterm>
      </term>

      <listitem>
       <para>
        Submits a request to create a prepared statement with the
        given parameters, and waits for completion.
<synopsis>
PGresult *PQprepare(PGconn *conn,
                    const char *stmtName,
                    const char *query,
                    int nParams,
                    const Oid *paramTypes);
</synopsis>
       </para>

       <para>
        <function>PQprepare</> creates a prepared statement for later
        execution with <function>PQexecPrepared</>.  This feature allows
        commands that will be used repeatedly to be parsed and planned just
        once, rather than each time they are executed.
        <function>PQprepare</> is supported only in protocol 3.0 and later
        connections; it will fail when using protocol 2.0.
       </para>

       <para>
        The function creates a prepared statement named
        <parameter>stmtName</> from the <parameter>query</> string, which
        must contain a single SQL command.  <parameter>stmtName</> can be
        <literal>""</> to create an unnamed statement, in which case any
        pre-existing unnamed statement is automatically replaced; otherwise
        it is an error if the statement name is already defined in the
        current session.  If any parameters are used, they are referred
        to in the query as <literal>$1</>, <literal>$2</>, etc.
        <parameter>nParams</> is the number of parameters for which types
        are pre-specified in the array <parameter>paramTypes[]</>.  (The
        array pointer can be <symbol>NULL</symbol> when
        <parameter>nParams</> is zero.) <parameter>paramTypes[]</>
        specifies, by OID, the data types to be assigned to the parameter
        symbols.  If <parameter>paramTypes</> is <symbol>NULL</symbol>,
        or any particular element in the array is zero, the server assigns
        a data type to the parameter symbol in the same way it would do
        for an untyped literal string.  Also, the query can use parameter
        symbols with numbers higher than <parameter>nParams</>; data types
        will be inferred for these symbols as well.  (See
        <function>PQdescribePrepared</function> for a means to find out
        what data types were inferred.)
       </para>

       <para>
        As with <function>PQexec</>, the result is normally a
        <structname>PGresult</structname> object whose contents indicate
        server-side success or failure.  A null result indicates
        out-of-memory or inability to send the command at all.  Use
        <function>PQerrorMessage</function> to get more information about
        such errors.
       </para>
      </listitem>
     </varlistentry>
    </variablelist>

    Prepared statements for use with <function>PQexecPrepared</> can also
    be created by executing SQL <xref linkend="sql-prepare"
    endterm="sql-prepare-title"> statements.  (But <function>PQprepare</>
    is more flexible since it does not require parameter types to be
    pre-specified.)  Also, although there is no <application>libpq</>
    function for deleting a prepared statement, the SQL <xref
    linkend="sql-deallocate" endterm="sql-deallocate-title"> statement
    can be used for that purpose.
   </para>

   <para>
    <variablelist>
     <varlistentry>
      <term>
       <function>PQexecPrepared</function>
       <indexterm>
        <primary>PQexecPrepared</primary>
       </indexterm>
      </term>

      <listitem>
       <para>
        Sends a request to execute a prepared statement with given
        parameters, and waits for the result.
<synopsis>
PGresult *PQexecPrepared(PGconn *conn,
                         const char *stmtName,
                         int nParams,
                         const char * const *paramValues,
                         const int *paramLengths,
                         const int *paramFormats,
                         int resultFormat);
</synopsis>
       </para>

       <para>
        <function>PQexecPrepared</> is like <function>PQexecParams</>,
        but the command to be executed is specified by naming a
        previously-prepared statement, instead of giving a query string.
        This feature allows commands that will be used repeatedly to be
        parsed and planned just once, rather than each time they are
        executed.  The statement must have been prepared previously in
        the current session.  <function>PQexecPrepared</> is supported
        only in protocol 3.0 and later connections; it will fail when
        using protocol 2.0.
       </para>

       <para>
        The parameters are identical to <function>PQexecParams</>, except that the
        name of a prepared statement is given instead of a query string, and the
        <parameter>paramTypes[]</> parameter is not present (it is not needed since
        the prepared statement's parameter types were determined when it was created).
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>
       <function>PQdescribePrepared</function>
       <indexterm>
        <primary>PQdescribePrepared</primary>
       </indexterm>
      </term>

      <listitem>
       <para>
        Submits a request to obtain information about the specified
        prepared statement, and waits for completion.
<synopsis>
PGresult *PQdescribePrepared(PGconn *conn, const char *stmtName);
</synopsis>
       </para>

       <para>
        <function>PQdescribePrepared</> allows an application to obtain
        information about a previously prepared statement.
        <function>PQdescribePrepared</> is supported only in protocol 3.0
        and later connections; it will fail when using protocol 2.0.
       </para>

       <para>
        <parameter>stmtName</> can be <literal>""</> or NULL to reference
        the unnamed statement, otherwise it must be the name of an existing
        prepared statement.  On success, a <structname>PGresult</> with
        status <literal>PGRES_COMMAND_OK</literal> is returned.  The
        functions <function>PQnparams</function> and
        <function>PQparamtype</function> can be applied to this
        <structname>PGresult</> to obtain information about the parameters
        of the prepared statement, and the functions
        <function>PQnfields</function>, <function>PQfname</function>,
        <function>PQftype</function>, etc provide information about the
        result columns (if any) of the statement.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>
       <function>PQdescribePortal</function>
       <indexterm>
        <primary>PQdescribePortal</primary>
       </indexterm>
      </term>

      <listitem>
       <para>
        Submits a request to obtain information about the specified
        portal, and waits for completion.
<synopsis>
PGresult *PQdescribePortal(PGconn *conn, const char *portalName);
</synopsis>
       </para>

       <para>
        <function>PQdescribePortal</> allows an application to obtain
        information about a previously created portal.
        (<application>libpq</> does not provide any direct access to
        portals, but you can use this function to inspect the properties
        of a cursor created with a <command>DECLARE CURSOR</> SQL command.)
        <function>PQdescribePortal</> is supported only in protocol 3.0
        and later connections; it will fail when using protocol 2.0.
       </para>

       <para>
        <parameter>portalName</> can be <literal>""</> or NULL to reference
        the unnamed portal, otherwise it must be the name of an existing
        portal.  On success, a <structname>PGresult</> with status
        <literal>PGRES_COMMAND_OK</literal> is returned.  The functions
        <function>PQnfields</function>, <function>PQfname</function>,
        <function>PQftype</function>, etc can be applied to the
        <structname>PGresult</> to obtain information about the result
        columns (if any) of the portal.
       </para>
      </listitem>
     </varlistentry>
    </variablelist>
   </para>

   <para>
    The <structname>PGresult</structname><indexterm><primary>PGresult</></>
    structure encapsulates the result returned by the server.
    <application>libpq</application> application programmers should be
    careful to maintain the <structname>PGresult</structname> abstraction.
    Use the accessor functions below to get at the contents of
    <structname>PGresult</structname>.  Avoid directly referencing the
    fields of the <structname>PGresult</structname> structure because they
    are subject to change in the future.

    <variablelist>
     <varlistentry>
      <term>
       <function>PQresultStatus</function>
       <indexterm>
        <primary>PQresultStatus</primary>
       </indexterm>
      </term>

      <listitem>
       <para>
        Returns the result status of the command.
        <synopsis>
         ExecStatusType PQresultStatus(const PGresult *res);
        </synopsis>
       </para>

       <para>
        <function>PQresultStatus</function> can return one of the following values:

        <variablelist>
         <varlistentry>
          <term><literal>PGRES_EMPTY_QUERY</literal></term>
          <listitem>
           <para>
            The string sent to the server was empty.
           </para>
          </listitem>
         </varlistentry>

         <varlistentry>
          <term><literal>PGRES_COMMAND_OK</literal></term>
          <listitem>
           <para>
            Successful completion of a command returning no data.
           </para>
          </listitem>
         </varlistentry>

         <varlistentry>
          <term><literal>PGRES_TUPLES_OK</literal></term>
          <listitem>
           <para>
            Successful completion of a command returning data (such as
            a <command>SELECT</> or <command>SHOW</>).
           </para>
          </listitem>
         </varlistentry>

         <varlistentry>
          <term><literal>PGRES_COPY_OUT</literal></term>
          <listitem>
           <para>
            Copy Out (from server) data transfer started.
           </para>
          </listitem>
         </varlistentry>

         <varlistentry>
          <term><literal>PGRES_COPY_IN</literal></term>
          <listitem>
           <para>
            Copy In (to server) data transfer started.
           </para>
          </listitem>
         </varlistentry>

         <varlistentry>
          <term><literal>PGRES_BAD_RESPONSE</literal></term>
          <listitem>
           <para>
            The server's response was not understood.
           </para>
          </listitem>
         </varlistentry>

         <varlistentry>
          <term><literal>PGRES_NONFATAL_ERROR</literal></term>
          <listitem>
           <para>
            A nonfatal error (a notice or warning) occurred.
           </para>
          </listitem>
         </varlistentry>

         <varlistentry>
          <term><literal>PGRES_FATAL_ERROR</literal></term>
          <listitem>
           <para>
            A fatal error occurred.
           </para>
          </listitem>
         </varlistentry>
        </variablelist>

        If the result status is <literal>PGRES_TUPLES_OK</literal>, then
        the functions described below can be used to retrieve the rows
        returned by the query.  Note that a <command>SELECT</command>
        command that happens to retrieve zero rows still shows
        <literal>PGRES_TUPLES_OK</literal>.
        <literal>PGRES_COMMAND_OK</literal> is for commands that can never
        return rows (<command>INSERT</command>, <command>UPDATE</command>,
        etc.). A response of <literal>PGRES_EMPTY_QUERY</literal> might
        indicate a bug in the client software.
       </para>

       <para>
        A result of status <symbol>PGRES_NONFATAL_ERROR</symbol> will
        never be returned directly by <function>PQexec</function> or other
        query execution functions; results of this kind are instead passed
        to the notice processor (see <xref
        linkend="libpq-notice-processing">).
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>
       <function>PQresStatus</function>
       <indexterm>
        <primary>PQresStatus</primary>
       </indexterm>
      </term>

      <listitem>
       <para>
        Converts the enumerated type returned by
        <function>PQresultStatus</> into a string constant describing the
        status code. The caller should not free the result.

        <synopsis>
         char *PQresStatus(ExecStatusType status);
        </synopsis>
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>
       <function>PQresultErrorMessage</function>
       <indexterm>
        <primary>PQresultErrorMessage</primary>
       </indexterm>
      </term>

      <listitem>
       <para>
        Returns the error message associated with the command, or an empty string
        if there was no error.
        <synopsis>
         char *PQresultErrorMessage(const PGresult *res);
        </synopsis>
        If there was an error, the returned string will include a trailing
        newline.  The caller should not free the result directly. It will
        be freed when the associated <structname>PGresult</> handle is
        passed to <function>PQclear</function>.
       </para>

       <para>
        Immediately following a <function>PQexec</function> or
        <function>PQgetResult</function> call,
        <function>PQerrorMessage</function> (on the connection) will return
        the same string as <function>PQresultErrorMessage</function> (on
        the result).  However, a <structname>PGresult</structname> will
        retain its error message until destroyed, whereas the connection's
        error message will change when subsequent operations are done.
        Use <function>PQresultErrorMessage</function> when you want to
        know the status associated with a particular
        <structname>PGresult</structname>; use
        <function>PQerrorMessage</function> when you want to know the
        status from the latest operation on the connection.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><function>PQresultErrorField</function><indexterm><primary>PQresultErrorField</></></term>
      <listitem>
       <para>
        Returns an individual field of an error report.
        <synopsis>
         char *PQresultErrorField(const PGresult *res, int fieldcode);
        </synopsis>
        <parameter>fieldcode</> is an error field identifier; see the symbols
        listed below.  <symbol>NULL</symbol> is returned if the
        <structname>PGresult</structname> is not an error or warning result,
        or does not include the specified field.  Field values will normally
        not include a trailing newline. The caller should not free the
        result directly. It will be freed when the
        associated <structname>PGresult</> handle is passed to
        <function>PQclear</function>.
       </para>

       <para>
        The following field codes are available:
        <variablelist>
         <varlistentry>
          <term><symbol>PG_DIAG_SEVERITY</></term>
          <listitem>
           <para>
            The severity; the field contents are <literal>ERROR</>,
            <literal>FATAL</>, or <literal>PANIC</> (in an error message),
            or <literal>WARNING</>, <literal>NOTICE</>, <literal>DEBUG</>,
            <literal>INFO</>, or <literal>LOG</> (in a notice message), or
            a localized translation of one of these.  Always present.
           </para>
          </listitem>
         </varlistentry>

         <varlistentry>
          <indexterm>
           <primary>error codes</primary>
           <secondary>libpq</secondary>
          </indexterm>
          <term><symbol>PG_DIAG_SQLSTATE</></term>
          <listitem>
           <para>
            The SQLSTATE code for the error. The SQLSTATE code identifies
            the type of error that has occurred; it can be used by
            front-end applications to perform specific operations (such
            as error handling) in response to a particular database error.
            For a list of the possible SQLSTATE codes, see <xref
            linkend="errcodes-appendix">. This field is not localizable,
            and is always present.
           </para>
          </listitem>
         </varlistentry>

         <varlistentry>
          <term><symbol>PG_DIAG_MESSAGE_PRIMARY</></term>
          <listitem>
           <para>
            The primary human-readable error message (typically one line).
            Always present.
           </para>
          </listitem>
         </varlistentry>

         <varlistentry>
          <term><symbol>PG_DIAG_MESSAGE_DETAIL</></term>
          <listitem>
           <para>
            Detail: an optional secondary error message carrying more
            detail about the problem.  Might run to multiple lines.
           </para>
          </listitem>
         </varlistentry>

         <varlistentry>
          <term><symbol>PG_DIAG_MESSAGE_HINT</></term>
          <listitem>
           <para>
            Hint: an optional suggestion what to do about the problem.
            This is intended to differ from detail in that it offers advice
            (potentially inappropriate) rather than hard facts.  Might
            run to multiple lines.
           </para>
          </listitem>
         </varlistentry>

         <varlistentry>
          <term><symbol>PG_DIAG_STATEMENT_POSITION</></term>
          <listitem>
           <para>
            A string containing a decimal integer indicating an error cursor
            position as an index into the original statement string.  The
            first character has index 1, and positions are measured in
            characters not bytes.
           </para>
          </listitem>
         </varlistentry>

         <varlistentry>
          <term><symbol>PG_DIAG_INTERNAL_POSITION</></term>
          <listitem>
           <para>
            This is defined the same as the
            <symbol>PG_DIAG_STATEMENT_POSITION</> field, but it is used
            when the cursor position refers to an internally generated
            command rather than the one submitted by the client.  The
            <symbol>PG_DIAG_INTERNAL_QUERY</> field will always appear when
            this field appears.
           </para>
          </listitem>
         </varlistentry>

         <varlistentry>
          <term><symbol>PG_DIAG_INTERNAL_QUERY</></term>
          <listitem>
           <para>
            The text of a failed internally-generated command.  This could
            be, for example, a SQL query issued by a PL/pgSQL function.
           </para>
          </listitem>
         </varlistentry>

         <varlistentry>
          <term><symbol>PG_DIAG_CONTEXT</></term>
          <listitem>
           <para>
            An indication of the context in which the error occurred.
            Presently this includes a call stack traceback of active
            procedural language functions and internally-generated queries.
            The trace is one entry per line, most recent first.
           </para>
          </listitem>
         </varlistentry>

         <varlistentry>
          <term><symbol>PG_DIAG_SOURCE_FILE</></term>
          <listitem>
           <para>
            The file name of the source-code location where the error was
            reported.
           </para>
          </listitem>
         </varlistentry>

         <varlistentry>
          <term><symbol>PG_DIAG_SOURCE_LINE</></term>
          <listitem>
           <para>
            The line number of the source-code location where the error
            was reported.
           </para>
          </listitem>
         </varlistentry>

         <varlistentry>
          <term><symbol>PG_DIAG_SOURCE_FUNCTION</></term>
          <listitem>
           <para>
            The name of the source-code function reporting the error.
           </para>
          </listitem>
         </varlistentry>
        </variablelist>
       </para>

       <para>
        The client is responsible for formatting displayed information to meet
        its needs; in particular it should break long lines as needed.
        Newline characters appearing in the error message fields should be
        treated as paragraph breaks, not line breaks.
       </para>

       <para>
        Errors generated internally by <application>libpq</application> will
        have severity and primary message, but typically no other fields.
        Errors returned by a pre-3.0-protocol server will include severity and
        primary message, and sometimes a detail message, but no other fields.
       </para>

       <para>
        Note that error fields are only available from
        <structname>PGresult</structname> objects, not
        <structname>PGconn</structname> objects; there is no
        <function>PQerrorField</function> function.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><function>PQclear</function><indexterm><primary>PQclear</></></term>
      <listitem>
       <para>
        Frees  the  storage  associated with a
        <structname>PGresult</structname>.  Every command result should be
        freed via <function>PQclear</function> when it  is  no  longer
        needed.

        <synopsis>
         void PQclear(PGresult *res);
        </synopsis>
       </para>

       <para>
        You can keep a <structname>PGresult</structname> object around for
        as long as you need it; it does not go away when you issue a new
        command, nor even if you close the connection.  To get rid of it,
        you must call <function>PQclear</function>.  Failure to do this
        will result in memory leaks in your application.
       </para>
      </listitem>
     </varlistentry>
    </variablelist>
   </para>
  </sect2>

  <sect2 id="libpq-exec-select-info">
   <title>Retrieving Query Result Information</title>

   <para>
    These functions are used to extract information from a
    <structname>PGresult</structname> object that represents a successful
    query result (that is, one that has status
    <literal>PGRES_TUPLES_OK</literal>).  They can also be used to extract
    information from a successful Describe operation: a Describe's result
    has all the same column information that actual execution of the query
    would provide, but it has zero rows.  For objects with other status values,
    these functions will act as though the result has zero rows and zero columns.
   </para>

   <variablelist>
    <varlistentry>
     <term>
      <function>PQntuples</function>
      <indexterm>
       <primary>PQntuples</primary>
      </indexterm>
     </term>

     <listitem>
      <para>
       Returns the number of rows (tuples) in the query result.  Because
       it returns an integer result, large result sets might overflow the
       return value on 32-bit operating systems.

       <synopsis>
        int PQntuples(const PGresult *res);
       </synopsis>

      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term>
      <function>PQnfields</function>
      <indexterm>
       <primary>PQnfields</primary>
      </indexterm>
     </term>

     <listitem>
      <para>
       Returns the number of columns (fields) in each row of the query
       result.

       <synopsis>
        int PQnfields(const PGresult *res);
       </synopsis>
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term>
      <function>PQfname</function>
      <indexterm>
       <primary>PQfname</primary>
      </indexterm>
     </term>

     <listitem>
      <para>
       Returns the column name associated with the given column number.
       Column numbers start at 0. The caller should not free the result
       directly. It will be freed when the associated
       <structname>PGresult</> handle is passed to
       <function>PQclear</function>.
       <synopsis>
        char *PQfname(const PGresult *res,
                      int column_number);
       </synopsis>
      </para>

      <para>
       <symbol>NULL</symbol> is returned if the column number is out of range.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term>
      <function>PQfnumber</function>
      <indexterm>
       <primary>PQfnumber</primary>
      </indexterm>
     </term>

     <listitem>
      <para>
       Returns the column number associated with the given column name.
       <synopsis>
        int PQfnumber(const PGresult *res,
                      const char *column_name);
       </synopsis>
      </para>

      <para>
       -1 is returned if the given name does not match any column.
      </para>

      <para>
       The given name is treated like an identifier in an SQL command,
       that is, it is downcased unless double-quoted.  For example, given
       a query result generated from the SQL command:
<programlisting>
SELECT 1 AS FOO, 2 AS "BAR";
</programlisting>
       we would have the results:
<programlisting>
PQfname(res, 0)              <lineannotation>foo</lineannotation>
PQfname(res, 1)              <lineannotation>BAR</lineannotation>
PQfnumber(res, "FOO")        <lineannotation>0</lineannotation>
PQfnumber(res, "foo")        <lineannotation>0</lineannotation>
PQfnumber(res, "BAR")        <lineannotation>-1</lineannotation>
PQfnumber(res, "\"BAR\"")    <lineannotation>1</lineannotation>
</programlisting>
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term>
      <function>PQftable</function>
      <indexterm>
       <primary>PQftable</primary>
      </indexterm>
     </term>

     <listitem>
      <para>
       Returns the OID of the table from which the given column was
       fetched.  Column numbers start at 0.
       <synopsis>
        Oid PQftable(const PGresult *res,
                     int column_number);
       </synopsis>
      </para>

      <para>
       <literal>InvalidOid</> is returned if the column number is out of range,
       or if the specified column is not a simple reference to a table column,
       or when using pre-3.0 protocol.
       You can query the system table <literal>pg_class</literal> to determine
       exactly which table is referenced.
      </para>

      <para>
       The type <type>Oid</type> and the constant
       <literal>InvalidOid</literal> will be defined when you include
       the <application>libpq</application> header file. They will both
       be some integer type.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term>
      <function>PQftablecol</function>
      <indexterm>
       <primary>PQftablecol</primary>
      </indexterm>
     </term>

     <listitem>
      <para>
       Returns the column number (within its table) of the column making
       up the specified query result column.  Query-result column numbers
       start at 0, but table columns have nonzero numbers.
       <synopsis>
       int PQftablecol(const PGresult *res,
                       int column_number);
       </synopsis>
      </para>

      <para>
       Zero is returned if the column number is out of range, or if the
       specified column is not a simple reference to a table column, or
       when using pre-3.0 protocol.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term>
      <function>PQfformat</function>
      <indexterm>
       <primary>PQfformat</primary>
      </indexterm>
     </term>

     <listitem>
      <para>
       Returns the format code indicating the format of the given
       column.  Column numbers start at 0.
       <synopsis>
        int PQfformat(const PGresult *res,
                      int column_number);
       </synopsis>
      </para>

      <para>
       Format code zero indicates textual data representation, while format
       code one indicates binary representation.  (Other codes are reserved
       for future definition.)
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term>
      <function>PQftype</function>
      <indexterm>
       <primary>PQftype</primary>
      </indexterm>
     </term>

     <listitem>
      <para>
       Returns the data type associated with the given  column number.
       The  integer  returned is the internal OID number of the type.
       Column numbers start at 0.
       <synopsis>
        Oid PQftype(const PGresult *res,
                    int column_number);
       </synopsis>
      </para>

      <para>
       You can query the system table <literal>pg_type</literal> to
       obtain the names and properties of the various data types. The
       <acronym>OID</acronym>s of the built-in data types are defined
       in the file <filename>src/include/catalog/pg_type.h</filename>
       in the source tree.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term>
      <function>PQfmod</function>
      <indexterm>
       <primary>PQfmod</primary>
      </indexterm>
     </term>

     <listitem>
      <para>
       Returns  the type modifier of the column associated with the
       given column number.  Column numbers start at 0.
       <synopsis>
        int PQfmod(const PGresult *res,
                   int column_number);
       </synopsis>
      </para>

      <para>
       The interpretation of modifier values is type-specific; they
       typically indicate precision or size limits.  The value -1 is
       used to indicate <quote>no information available</>.  Most data
       types do not use modifiers, in which case the value is always
       -1.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term>
      <function>PQfsize</function>
      <indexterm>
       <primary>PQfsize</primary>
      </indexterm>
     </term>

     <listitem>
      <para>
       Returns  the  size  in bytes of the column associated with the
       given column number.  Column numbers start at 0.
       <synopsis>
        int PQfsize(const PGresult *res,
                    int column_number);
       </synopsis>
      </para>

      <para>
       <function>PQfsize</> returns the space allocated for this column
       in a database row, in other words the size of the server's
       internal representation of the data type.  (Accordingly, it is
       not really very useful to clients.) A negative value indicates
       the data type is variable-length.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term>
      <function>PQbinaryTuples</function>
      <indexterm>
       <primary>PQbinaryTuples</primary>
      </indexterm>
     </term>

     <listitem>
      <para>
       Returns 1 if the <structname>PGresult</> contains binary data
       and 0 if it contains text data.
       <synopsis>
        int PQbinaryTuples(const PGresult *res);
       </synopsis>
      </para>

      <para>
       This function is deprecated (except for its use in connection with
       <command>COPY</>), because it is possible for a single
       <structname>PGresult</> to contain text data in some columns and
       binary data in others.  <function>PQfformat</> is preferred.
       <function>PQbinaryTuples</> returns 1 only if all columns of the
       result are binary (format 1).
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term>
      <function>PQgetvalue</function>
       <indexterm>
        <primary>PQgetvalue</primary>
       </indexterm>
     </term>

     <listitem>
      <para>
       Returns a single field value of one row of a
       <structname>PGresult</structname>.  Row and column numbers start
       at 0.  The caller should not free the result directly.  It will
       be freed when the associated <structname>PGresult</> handle is
       passed to <function>PQclear</function>.
       <synopsis>
        char *PQgetvalue(const PGresult *res,
                         int row_number,
                         int column_number);
       </synopsis>
      </para>

      <para>
       For data in text format, the value returned by
       <function>PQgetvalue</function> is a null-terminated character
       string  representation of the field value.  For data in binary
       format, the value is in the binary representation determined by
       the data type's <function>typsend</> and <function>typreceive</>
       functions.  (The value is actually followed by a zero byte in
       this case too, but that is not ordinarily useful, since the
       value is likely to contain embedded nulls.)
      </para>

      <para>
       An empty string is returned if the field value is null.  See
       <function>PQgetisnull</> to distinguish null values from
       empty-string values.
      </para>

      <para>
       The pointer returned  by  <function>PQgetvalue</function> points
       to storage that is part of the <structname>PGresult</structname>
       structure.  One should not modify the data it points to, and one
       must explicitly copy the data into other storage if it is to be
       used past the lifetime of the  <structname>PGresult</structname>
       structure itself.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term>
      <function>PQgetisnull</function>
      <indexterm>
       <primary>PQgetisnull</primary>
      </indexterm>
      <indexterm>
       <primary>null value</primary>
       <secondary sortas="libpq">in libpq</secondary>
      </indexterm>
     </term>

     <listitem>
      <para>
       Tests a field for a null value.  Row and column numbers start
       at 0.
       <synopsis>
        int PQgetisnull(const PGresult *res,
                        int row_number,
                        int column_number);
       </synopsis>
      </para>

      <para>
       This function returns  1 if the field is null and 0 if it
       contains a non-null value.  (Note that
       <function>PQgetvalue</function> will return an empty string,
       not a null pointer, for a null field.)
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term>
     <function>PQgetlength</function>
     <indexterm>
      <primary>PQgetlength</primary>
     </indexterm></term>

     <listitem>
      <para>
       Returns the actual length of a field value in bytes.  Row and
       column numbers start at 0.
       <synopsis>
        int PQgetlength(const PGresult *res,
                        int row_number,
                        int column_number);
       </synopsis>
      </para>

      <para>
       This is the actual data length for the particular data value,
       that is, the size of the object pointed to by
       <function>PQgetvalue</function>.  For text data format this is
       the same as <function>strlen()</>.  For binary format this is
       essential information.  Note that one should <emphasis>not</>
       rely on <function>PQfsize</function> to obtain the actual data
       length.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term>
      <function>PQnparams</function>
      <indexterm>
       <primary>PQnparams</primary>
      </indexterm>
     </term>

     <listitem>
      <para>
       Returns the number of parameters of a prepared statement.
       <synopsis>
        int PQnparams(const PGresult *res);
       </synopsis>
      </para>

      <para>
       This function is only useful when inspecting the result of
       <function>PQdescribePrepared</>.  For other types of queries it
       will return zero.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term>
      <function>PQparamtype</function>
      <indexterm>
       <primary>PQparamtype</primary>
      </indexterm>
     </term>

     <listitem>
      <para>
       Returns the data type of the indicated statement parameter.
       Parameter numbers start at 0.
       <synopsis>
        Oid PQparamtype(const PGresult *res, int param_number);
       </synopsis>
      </para>

      <para>
       This function is only useful when inspecting the result of
       <function>PQdescribePrepared</>.  For other types of queries it
       will return zero.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term>
      <function>PQprint</function>
      <indexterm>
       <primary>PQprint</primary>
      </indexterm>
     </term>

     <listitem>
      <para>
       Prints out all the rows and,  optionally,  the column names  to
       the specified output stream.
       <synopsis>
void PQprint(FILE *fout,      /* output stream */
             const PGresult *res,
             const PQprintOpt *po);
typedef struct {
  pqbool  header;      /* print output field headings and row count */
  pqbool  align;       /* fill align the fields */
  pqbool  standard;    /* old brain dead format */
  pqbool  html3;       /* output HTML tables */
  pqbool  expanded;    /* expand tables */
  pqbool  pager;       /* use pager for output if needed */
  char    *fieldSep;   /* field separator */
  char    *tableOpt;   /* attributes for HTML table element */
  char    *caption;    /* HTML table caption */
  char    **fieldName; /* null-terminated array of replacement field names */
} PQprintOpt;
       </synopsis>
      </para>

      <para>
       This function was formerly used by <application>psql</application>
       to print query results, but this is no longer the case.  Note
       that it assumes all the data is in text format.
      </para>
     </listitem>
    </varlistentry>
   </variablelist>
  </sect2>

  <sect2 id="libpq-exec-nonselect">
   <title>Retrieving Result Information for Other Commands</title>

   <para>
    These functions are used to extract information from
    <structname>PGresult</structname> objects that are not
    <command>SELECT</> results.
   </para>

   <variablelist>
    <varlistentry>
     <term>
      <function>PQcmdStatus</function>
      <indexterm>
       <primary>PQcmdStatus</primary>
      </indexterm>
     </term>

     <listitem>
      <para>
       Returns the command status tag from the SQL command that generated
       the <structname>PGresult</structname>.
       <synopsis>
        char *PQcmdStatus(PGresult *res);
       </synopsis>
      </para>

      <para>
       Commonly this is just the name of the command, but it might include
       additional data such as the number of rows processed. The caller
       should not free the result directly. It will be freed when the
       associated <structname>PGresult</> handle is passed to
       <function>PQclear</function>.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term>
      <function>PQcmdTuples</function>
      <indexterm>
       <primary>PQcmdTuples</primary>
      </indexterm>
     </term>

     <listitem>
      <para>
       Returns the number of rows affected by the SQL command.
       <synopsis>
        char *PQcmdTuples(PGresult *res);
       </synopsis>
      </para>

      <para>
       This function returns a string containing the number of rows
       affected by the <acronym>SQL</> statement that generated the
       <structname>PGresult</>. This function can only be used following
       the execution of an <command>INSERT</>, <command>UPDATE</>,
       <command>DELETE</>, <command>MOVE</>, <command>FETCH</>, or
       <command>COPY</> statement, or an <command>EXECUTE</> of a
       prepared query that contains an <command>INSERT</>,
       <command>UPDATE</>, or <command>DELETE</> statement.  If the
       command that generated the <structname>PGresult</> was anything
       else, <function>PQcmdTuples</> returns an empty string. The caller
       should not free the return value directly. It will be freed when
       the associated <structname>PGresult</> handle is passed to
       <function>PQclear</function>.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term>
      <function>PQoidValue</function>
      <indexterm>
       <primary>PQoidValue</primary>
      </indexterm>
     </term>

     <listitem>
      <para>
       Returns the OID<indexterm><primary>OID</><secondary>in libpq</></>
       of the inserted row, if the <acronym>SQL</> command was an
       <command>INSERT</> that inserted exactly one row into a table that
       has OIDs, or a <command>EXECUTE</> of a prepared query containing
       a suitable <command>INSERT</> statement.  Otherwise, this function
       returns <literal>InvalidOid</literal>. This function will also
       return <literal>InvalidOid</literal> if the table affected by the
       <command>INSERT</> statement does not contain OIDs.
       <synopsis>
        Oid PQoidValue(const PGresult *res);
       </synopsis>
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term>
      <function>PQoidStatus</function>
      <indexterm>
       <primary>PQoidStatus</primary>
      </indexterm>
     </term>

     <listitem>
      <para>
       Returns a string with the OID of the inserted row, if the
       <acronym>SQL</acronym> command was an <command>INSERT</command>
       that inserted exactly one row, or a <command>EXECUTE</command> of
       a prepared statement consisting of a suitable
       <command>INSERT</command>.  (The string will be <literal>0</> if
       the <command>INSERT</command> did not insert exactly one row, or
       if the target table does not have OIDs.)  If the command was not
       an <command>INSERT</command>, returns an empty string.
       <synopsis>
        char *PQoidStatus(const PGresult *res);
       </synopsis>
      </para>

      <para>
       This function is deprecated in favor of
       <function>PQoidValue</function>.  It is not thread-safe.
      </para>
     </listitem>
    </varlistentry>
   </variablelist>

  </sect2>

  <sect2 id="libpq-exec-escape-string">
   <title>Escaping Strings for Inclusion in SQL Commands</title>

   <indexterm zone="libpq-exec-escape-string">
    <primary>PQescapeStringConn</primary>
   </indexterm>
   <indexterm zone="libpq-exec-escape-string">
    <primary>PQescapeString</primary>
   </indexterm>
   <indexterm zone="libpq-exec-escape-string">
    <primary>escaping strings</primary>
    <secondary>in libpq</secondary>
   </indexterm>

   <para>
    <function>PQescapeStringConn</function> escapes a string for use within an SQL
    command.  This is useful when inserting data values as literal constants
    in SQL commands.  Certain characters (such as quotes and backslashes) must
    be escaped to prevent them from being interpreted specially by the SQL parser.
    <function>PQescapeStringConn</> performs this operation.
   </para>

   <tip>
    <para>
     It is especially important to do proper escaping when handling strings that
     were received from an untrustworthy source.  Otherwise there is a security
     risk: you are vulnerable to <quote>SQL injection</> attacks wherein unwanted
     SQL commands are fed to your database.
    </para>
   </tip>

   <para>
    Note that it is not necessary nor correct to do escaping when a data
    value is passed as a separate parameter in <function>PQexecParams</> or
    its sibling routines.

    <synopsis>
     size_t PQescapeStringConn (PGconn *conn,
                                char *to, const char *from, size_t length,
                                int *error);
    </synopsis>
   </para>

   <para>
    <function>PQescapeStringConn</> writes an escaped version of the
    <parameter>from</> string to the <parameter>to</> buffer, escaping
    special characters so that they cannot cause any harm, and adding a
    terminating zero byte.  The single quotes that must surround
    <productname>PostgreSQL</> string literals are not included in the
    result string; they should be provided in the SQL command that the
    result is inserted into.  The parameter <parameter>from</> points to
    the first character of the string that is to be escaped, and the
    <parameter>length</> parameter gives the number of bytes in this
    string.  A terminating zero byte is not required, and should not be
    counted in <parameter>length</>.  (If a terminating zero byte is found
    before <parameter>length</> bytes are processed,
    <function>PQescapeStringConn</> stops at the zero; the behavior is
    thus rather like <function>strncpy</>.) <parameter>to</> shall point
    to a buffer that is able to hold at least one more byte than twice
    the value of <parameter>length</>, otherwise the behavior is undefined.
    Behavior is likewise undefined if the <parameter>to</> and
    <parameter>from</> strings overlap.
   </para>

   <para>
    If the <parameter>error</> parameter is not NULL, then
    <literal>*error</> is set to zero on success, nonzero on error.
    Presently the only possible error conditions involve invalid multibyte
    encoding in the source string.  The output string is still generated
    on error, but it can be expected that the server will reject it as
    malformed.  On error, a suitable message is stored in the
    <parameter>conn</> object, whether or not <parameter>error</> is NULL.
   </para>

   <para>
    <function>PQescapeStringConn</> returns the number of bytes written
    to <parameter>to</>, not including the terminating zero byte.
   </para>

   <para>
    <synopsis>
     size_t PQescapeString (char *to, const char *from, size_t length);
    </synopsis>
   </para>

   <para>
    <function>PQescapeString</> is an older, deprecated version of
    <function>PQescapeStringConn</>; the difference is that it does
    not take <parameter>conn</> or <parameter>error</> parameters.
    Because of this, it cannot adjust its behavior depending on the
    connection properties (such as character encoding) and therefore
    <emphasis>it might give the wrong results</>.  Also, it has no way
    to report error conditions.
   </para>

   <para>
    <function>PQescapeString</> can be used safely in single-threaded
    client programs that work with only one <productname>PostgreSQL</>
    connection at a time (in this case it can find out what it needs to
    know <quote>behind the scenes</>).  In other contexts it is a security
    hazard and should be avoided in favor of
    <function>PQescapeStringConn</>.
   </para>
  </sect2>


  <sect2 id="libpq-exec-escape-bytea">
   <title>Escaping Binary Strings for Inclusion in SQL Commands</title>

   <indexterm zone="libpq-exec-escape-bytea">
    <primary>bytea</primary>
    <secondary sortas="libpq">in libpq</secondary>
   </indexterm>

   <variablelist>
    <varlistentry>
     <term>
      <function>PQescapeByteaConn</function>
      <indexterm>
       <primary>PQescapeByteaConn</primary>
      </indexterm>
     </term>

     <listitem>
     <para>
       Escapes binary data for use within an SQL command with the type
       <type>bytea</type>.  As with <function>PQescapeStringConn</function>,
       this is only used when inserting data directly into an SQL command string.
       <synopsis>
        unsigned char *PQescapeByteaConn(PGconn *conn,
                                         const unsigned char *from,
                                         size_t from_length,
                                         size_t *to_length);
       </synopsis>
      </para>

      <para>
       Certain byte values <emphasis>must</emphasis> be escaped (but all
       byte values <emphasis>can</emphasis> be escaped) when used as part
       of a <type>bytea</type> literal in an <acronym>SQL</acronym>
       statement. In general, to escape a byte, it is converted into the
       three digit octal number equal to the octet value, and preceded by
       usually two backslashes. The single quote (<literal>'</>) and backslash
       (<literal>\</>) characters have special alternative escape
       sequences. See <xref linkend="datatype-binary"> for more
       information. <function>PQescapeByteaConn</function> performs this
       operation, escaping only the minimally required bytes.
      </para>

      <para>
       The <parameter>from</parameter> parameter points to the first
       byte of the string that is to be escaped, and the
       <parameter>from_length</parameter> parameter gives the number of
       bytes in this binary string.  (A terminating zero byte is
       neither necessary nor counted.)  The <parameter>to_length</parameter>
       parameter points to a variable that will hold the resultant
       escaped string length. This result string length includes the terminating
       zero byte of the result.
      </para>

      <para>
       <function>PQescapeByteaConn</> returns an escaped version of the
       <parameter>from</parameter> parameter binary string in memory
       allocated with <function>malloc()</>.  This memory must be freed using
       <function>PQfreemem()</> when the result is no longer needed.  The
       return string has all special characters replaced so that they can
       be properly processed by the <productname>PostgreSQL</productname>
       string literal parser, and the <type>bytea</type> input function. A
       terminating zero byte is also added.  The single quotes that must
       surround <productname>PostgreSQL</productname> string literals are
       not part of the result string.
      </para>

      <para>
       On error, a NULL pointer is returned, and a suitable error message
       is stored in the <parameter>conn</> object.  Currently, the only
       possible error is insufficient memory for the result string.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term>
      <function>PQescapeBytea</function>
      <indexterm>
       <primary>PQescapeBytea</primary>
      </indexterm>
     </term>

     <listitem>
      <para>
       <function>PQescapeBytea</> is an older, deprecated version of
       <function>PQescapeByteaConn</>.
       <synopsis>
        unsigned char *PQescapeBytea(const unsigned char *from,
                                     size_t from_length,
                                     size_t *to_length);
       </synopsis>
      </para>

      <para>
       The only difference from <function>PQescapeByteaConn</> is that
       <function>PQescapeBytea</> does not take a <structname>PGconn</>
       parameter.  Because of this, it cannot adjust its behavior
       depending on the connection properties (in particular, whether
       standard-conforming strings are enabled) and therefore
       <emphasis>it might give the wrong results</>.  Also, it has no
       way to return an error message on failure.
      </para>

      <para>
       <function>PQescapeBytea</> can be used safely in single-threaded
       client programs that work with only one <productname>PostgreSQL</>
       connection at a time (in this case it can find out what it needs
       to know <quote>behind the scenes</>).  In other contexts it is
       a security hazard and should be avoided in favor of
       <function>PQescapeByteaConn</>.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term>
      <function>PQunescapeBytea</function>
      <indexterm>
       <primary>PQunescapeBytea</primary>
      </indexterm>
     </term>

     <listitem>
      <para>
       Converts a string representation of binary data into binary data
       &mdash; the reverse of <function>PQescapeBytea</function>.  This
       is needed when retrieving <type>bytea</type> data in text format,
       but not when retrieving it in binary format.

       <synopsis>
        unsigned char *PQunescapeBytea(const unsigned char *from, size_t *to_length);
       </synopsis>
      </para>

      <para>
       The <parameter>from</parameter> parameter points to a string
       such as might be returned by <function>PQgetvalue</function> when applied
       to a <type>bytea</type> column. <function>PQunescapeBytea</function>
       converts this string representation into its binary representation.
       It returns a pointer to a buffer allocated with
       <function>malloc()</function>, or null on error, and puts the size of
       the buffer in <parameter>to_length</parameter>. The result must be
       freed using <function>PQfreemem</> when it is no longer needed.
      </para>

      <para>
       This conversion is not exactly the inverse of
       <function>PQescapeBytea</function>, because the string is not expected
       to be <quote>escaped</> when received from <function>PQgetvalue</function>.
       In particular this means there is no need for string quoting considerations,
       and so no need for a <structname>PGconn</> parameter.
      </para>
     </listitem>
    </varlistentry>
   </variablelist>

  </sect2>

 </sect1>

 <sect1 id="libpq-async">
  <title>Asynchronous Command Processing</title>

  <indexterm zone="libpq-async">
   <primary>nonblocking connection</primary>
  </indexterm>

  <para>
   The <function>PQexec</function> function is adequate for submitting
   commands in normal, synchronous applications.  It has a couple of
   deficiencies, however, that can be of importance to some users:

   <itemizedlist>
    <listitem>
     <para>
      <function>PQexec</function> waits for the command to be completed.
      The application might have other work to do (such as maintaining a
      user interface), in which case it won't want to block waiting for
      the response.
     </para>
    </listitem>

    <listitem>
     <para>
      Since the execution of the client application is suspended while it
      waits for the result, it is hard for the application to decide that
      it would like to try to cancel the ongoing command.  (It can be done
      from a signal handler, but not otherwise.)
     </para>
    </listitem>

    <listitem>
     <para>
      <function>PQexec</function> can return only one
      <structname>PGresult</structname> structure.  If the submitted command
      string contains multiple <acronym>SQL</acronym> commands, all but
      the last <structname>PGresult</structname> are discarded by
      <function>PQexec</function>.
     </para>
    </listitem>
   </itemizedlist>
  </para>

  <para>
   Applications that do not like these limitations can instead use the
   underlying functions that <function>PQexec</function> is built from:
   <function>PQsendQuery</function> and <function>PQgetResult</function>.
   There are also
   <function>PQsendQueryParams</function>,
   <function>PQsendPrepare</function>,
   <function>PQsendQueryPrepared</function>,
   <function>PQsendDescribePrepared</function>, and
   <function>PQsendDescribePortal</function>,
   which can be used with <function>PQgetResult</function> to duplicate
   the functionality of
   <function>PQexecParams</function>,
   <function>PQprepare</function>,
   <function>PQexecPrepared</function>,
   <function>PQdescribePrepared</function>, and
   <function>PQdescribePortal</function>
   respectively.

   <variablelist>
    <varlistentry>
     <term>
      <function>PQsendQuery</function>
      <indexterm>
       <primary>PQsendQuery</primary>
      </indexterm>
     </term>

     <listitem>
      <para>
       Submits a command to the server without waiting for the result(s).
       1 is returned if the command was successfully dispatched and 0 if
       not (in which case, use <function>PQerrorMessage</> to get more
       information about the failure).
       <synopsis>
        int PQsendQuery(PGconn *conn, const char *command);
       </synopsis>

       After successfully calling <function>PQsendQuery</function>, call
       <function>PQgetResult</function> one or more times to obtain the
       results.  <function>PQsendQuery</function> cannot be called again
       (on the same connection) until <function>PQgetResult</function>
       has returned a null pointer, indicating that the command is done.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term>
      <function>PQsendQueryParams</function>
      <indexterm>
       <primary>PQsendQueryParams</primary>
      </indexterm>
     </term>

     <listitem>
      <para>
       Submits a command and separate parameters to the server without
       waiting for the result(s).
       <synopsis>
        int PQsendQueryParams(PGconn *conn,
                              const char *command,
                              int nParams,
                              const Oid *paramTypes,
                              const char * const *paramValues,
                              const int *paramLengths,
                              const int *paramFormats,
                              int resultFormat);
       </synopsis>

       This is equivalent to <function>PQsendQuery</function> except that
       query parameters can be specified separately from the query string.
       The function's parameters are handled identically to
       <function>PQexecParams</function>.  Like
       <function>PQexecParams</function>, it will not work on 2.0-protocol
       connections, and it allows only one command in the query string.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term>
      <function>PQsendPrepare</>
      <indexterm>
       <primary>PQsendPrepare</primary>
      </indexterm>
     </term>

     <listitem>
      <para>
       Sends a request to create a prepared statement with the given
       parameters, without waiting for completion.
       <synopsis>
        int PQsendPrepare(PGconn *conn,
                          const char *stmtName,
                          const char *query,
                          int nParams,
                          const Oid *paramTypes);
       </synopsis>

       This is an asynchronous version of <function>PQprepare</>: it
       returns 1 if it was able to dispatch the request, and 0 if not.
       After a successful call, call <function>PQgetResult</function> to
       determine whether the server successfully created the prepared
       statement.  The function's parameters are handled identically to
       <function>PQprepare</function>.  Like
       <function>PQprepare</function>, it will not work on 2.0-protocol
       connections.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term>
      <function>PQsendQueryPrepared</function>
      <indexterm>
       <primary>PQsendQueryPrepared</primary>
      </indexterm>
     </term>

     <listitem>
      <para>
       Sends a request to execute a prepared statement with given
       parameters, without waiting for the result(s).
       <synopsis>
        int PQsendQueryPrepared(PGconn *conn,
                                const char *stmtName,
                                int nParams,
                                const char * const *paramValues,
                                const int *paramLengths,
                                const int *paramFormats,
                                int resultFormat);
       </synopsis>

       This is similar to <function>PQsendQueryParams</function>, but
       the command to be executed is specified by naming a
       previously-prepared statement, instead of giving a query string.
       The function's parameters are handled identically to
       <function>PQexecPrepared</function>.  Like
       <function>PQexecPrepared</function>, it will not work on
       2.0-protocol connections.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term>
      <function>PQsendDescribePrepared</>
      <indexterm>
       <primary>PQsendDescribePrepared</primary>
      </indexterm>
     </term>

     <listitem>
      <para>
       Submits a request to obtain information about the specified
       prepared statement, without waiting for completion.
       <synopsis>
        int PQsendDescribePrepared(PGconn *conn, const char *stmtName);
       </synopsis>

       This is an asynchronous version of <function>PQdescribePrepared</>:
       it returns 1 if it was able to dispatch the request, and 0 if not.
       After a successful call, call <function>PQgetResult</function> to
       obtain the results.  The function's parameters are handled
       identically to <function>PQdescribePrepared</function>.  Like
       <function>PQdescribePrepared</function>, it will not work on
       2.0-protocol connections.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term>
      <function>PQsendDescribePortal</>
      <indexterm>
       <primary>PQsendDescribePortal</primary>
      </indexterm>
     </term>

     <listitem>
      <para>
       Submits a request to obtain information about the specified
       portal, without waiting for completion.
       <synopsis>
        int PQsendDescribePortal(PGconn *conn, const char *portalName);
       </synopsis>

       This is an asynchronous version of <function>PQdescribePortal</>:
       it returns 1 if it was able to dispatch the request, and 0 if not.
       After a successful call, call <function>PQgetResult</function> to
       obtain the results.  The function's parameters are handled
       identically to <function>PQdescribePortal</function>.  Like
       <function>PQdescribePortal</function>, it will not work on
       2.0-protocol connections.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term>
      <function>PQgetResult</function>
      <indexterm>
       <primary>PQgetResult</primary>
      </indexterm>
     </term>

     <listitem>
      <para>
       Waits for the next result from a prior
       <function>PQsendQuery</function>,
       <function>PQsendQueryParams</function>,
       <function>PQsendPrepare</function>, or
       <function>PQsendQueryPrepared</function> call, and returns it.
       A null pointer is returned when the command is complete and there
       will be no more results.
       <synopsis>
        PGresult *PQgetResult(PGconn *conn);
       </synopsis>
      </para>

      <para>
       <function>PQgetResult</function> must be called repeatedly until
       it returns a null pointer, indicating that the command is done.
       (If called when no command is active,
       <function>PQgetResult</function> will just return a null pointer
       at once.) Each non-null result from
       <function>PQgetResult</function> should be processed using the
       same <structname>PGresult</> accessor functions previously
       described.  Don't forget to free each result object with
       <function>PQclear</function> when done with it.  Note that
       <function>PQgetResult</function> will block only if a command is
       active and the necessary response data has not yet been read by
       <function>PQconsumeInput</function>.
      </para>
     </listitem>
    </varlistentry>
   </variablelist>
  </para>

  <para>
   Using <function>PQsendQuery</function> and
   <function>PQgetResult</function> solves one of
   <function>PQexec</function>'s problems:  If a command string contains
   multiple <acronym>SQL</acronym> commands, the results of those commands
   can be obtained individually.  (This allows a simple form of overlapped
   processing, by the way: the client can be handling the results of one
   command while the server is still working on later queries in the same
   command string.)  However, calling <function>PQgetResult</function>
   will still cause the client to block until the server completes the
   next <acronym>SQL</acronym> command.  This can be avoided by proper
   use of two more functions:

   <variablelist>
    <varlistentry>
     <term>
      <function>PQconsumeInput</function>
      <indexterm>
       <primary>PQconsumeInput</primary>
      </indexterm>
     </term>

     <listitem>
      <para>
       If input is available from the server, consume it.
       <synopsis>
        int PQconsumeInput(PGconn *conn);
       </synopsis>
      </para>

      <para>
       <function>PQconsumeInput</function> normally returns 1 indicating
       <quote>no error</quote>, but returns 0 if there was some kind of
       trouble (in which case <function>PQerrorMessage</function> can be
       consulted).  Note that the result does not say whether any input
       data was actually collected. After calling
       <function>PQconsumeInput</function>, the application can check
       <function>PQisBusy</function> and/or
       <function>PQnotifies</function> to see if their state has changed.
      </para>

      <para>
       <function>PQconsumeInput</function> can be called even if the
       application is not prepared to deal with a result or notification
       just yet.  The function will read available data and save it in
       a buffer, thereby causing a <function>select()</function>
       read-ready indication to go away.  The application can thus use
       <function>PQconsumeInput</function> to clear the
       <function>select()</function> condition immediately, and then
       examine the results at leisure.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term>
      <function>PQisBusy</function>
      <indexterm>
       <primary>PQisBusy</primary>
      </indexterm>
     </term>

     <listitem>
      <para>
       Returns 1 if a command is busy, that is,
       <function>PQgetResult</function> would block waiting for input.
       A 0 return indicates that <function>PQgetResult</function> can be
       called with assurance of not blocking.
       <synopsis>
        int PQisBusy(PGconn *conn);
       </synopsis>
      </para>

      <para>
       <function>PQisBusy</function> will not itself attempt to read data
       from the server; therefore <function>PQconsumeInput</function>
       must be invoked first, or the busy state will never end.
      </para>
     </listitem>
    </varlistentry>
   </variablelist>
  </para>

  <para>
   A typical application using these functions will have a main loop that
   uses <function>select()</function> or <function>poll()</> to wait for
   all the conditions that it must respond to.  One of the conditions
   will be input available from the server, which in terms of
   <function>select()</function> means readable data on the file
   descriptor identified by <function>PQsocket</function>.  When the main
   loop detects input ready, it should call
   <function>PQconsumeInput</function> to read the input.  It can then
   call <function>PQisBusy</function>, followed by
   <function>PQgetResult</function> if <function>PQisBusy</function>
   returns false (0).  It can also call <function>PQnotifies</function>
   to detect <command>NOTIFY</> messages (see <xref
   linkend="libpq-notify">).
  </para>

  <para>
   A client that uses
   <function>PQsendQuery</function>/<function>PQgetResult</function>
   can also attempt to cancel a command that is still being processed
   by the server; see <xref linkend="libpq-cancel">.  But regardless of
   the return value of <function>PQcancel</function>, the application
   must continue with the normal result-reading sequence using
   <function>PQgetResult</function>.  A successful cancellation will
   simply cause the command to terminate sooner than it would have
   otherwise.
  </para>

  <para>
   By using the functions described above, it is possible to avoid
   blocking while waiting for input from the database server.  However,
   it is still possible that the application will block waiting to send
   output to the server.  This is relatively uncommon but can happen if
   very long SQL commands or data values are sent.  (It is much more
   probable if the application sends data via <command>COPY IN</command>,
   however.)  To prevent this possibility and achieve completely
   nonblocking database operation, the following additional functions
   can be used.

   <variablelist>
    <varlistentry>
     <term>
      <function>PQsetnonblocking</function>
      <indexterm>
       <primary>PQsetnonblocking</primary>
      </indexterm>
     </term>

     <listitem>
      <para>
       Sets the nonblocking status of the connection.
       <synopsis>
        int PQsetnonblocking(PGconn *conn, int arg);
       </synopsis>
      </para>

      <para>
       Sets the state of the connection to nonblocking if
       <parameter>arg</parameter> is 1, or blocking if
       <parameter>arg</parameter> is 0.  Returns 0 if OK, -1 if error.
      </para>

      <para>
       In the nonblocking state, calls to
       <function>PQsendQuery</function>, <function>PQputline</function>,
       <function>PQputnbytes</function>, and
       <function>PQendcopy</function> will not block but instead return
       an error if they need to be called again.
      </para>

      <para>
       Note that <function>PQexec</function> does not honor nonblocking
       mode; if it is called, it will act in blocking fashion anyway.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term>
      <function>PQisnonblocking</function>
      <indexterm>
       <primary>PQisnonblocking</primary>
      </indexterm>
     </term>

     <listitem>
      <para>
       Returns the blocking status of the database connection.
       <synopsis>
        int PQisnonblocking(const PGconn *conn);
       </synopsis>
      </para>

      <para>
       Returns 1 if the connection is set to nonblocking mode and 0 if
       blocking.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term>
      <function>PQflush</function>
       <indexterm>
        <primary>PQflush</primary>
       </indexterm>
      </term>

      <listitem>
       <para>
       Attempts to flush any queued output data to the server.  Returns
       0 if successful (or if the send queue is empty), -1 if it failed
       for some reason, or 1 if it was unable to send all the data in
       the send queue yet (this case can only occur if the connection
       is nonblocking).
       <synopsis>
        int PQflush(PGconn *conn);
       </synopsis>
      </para>
     </listitem>
    </varlistentry>
   </variablelist>
  </para>

  <para>
   After sending any command or data on a nonblocking connection, call
   <function>PQflush</function>.  If it returns 1, wait for the socket
   to be write-ready and call it again; repeat until it returns 0.  Once
   <function>PQflush</function> returns 0, wait for the socket to be
   read-ready and then read the response as described above.
  </para>

 </sect1>

 <sect1 id="libpq-cancel">
  <title>Cancelling Queries in Progress</title>

  <indexterm zone="libpq-cancel">
   <primary>canceling</primary>
   <secondary>SQL command</secondary>
  </indexterm>

  <para>
   A client application can request cancellation of a command that is
   still being processed by the server, using the functions described in
   this section.

   <variablelist>
    <varlistentry>
     <term>
      <function>PQgetCancel</function>
      <indexterm>
       <primary>PQgetCancel</primary>
      </indexterm>
     </term>

     <listitem>
      <para>
       Creates a data structure containing the information needed to cancel
       a command issued through a particular database connection.
       <synopsis>
        PGcancel *PQgetCancel(PGconn *conn);
       </synopsis>
      </para>

      <para>
       <function>PQgetCancel</function> creates a
       <structname>PGcancel</><indexterm><primary>PGcancel</></> object
       given a <structname>PGconn</> connection object.  It will return
       NULL if the given <parameter>conn</> is NULL or an invalid
       connection.  The <structname>PGcancel</> object is an opaque
       structure that is not meant to be accessed directly by the
       application; it can only be passed to <function>PQcancel</function>
       or <function>PQfreeCancel</function>.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term>
      <function>PQfreeCancel</function>
      <indexterm>
       <primary>PQfreeCancel</primary>
      </indexterm>
     </term>

     <listitem>
      <para>
       Frees a data structure created by <function>PQgetCancel</function>.
       <synopsis>
        void PQfreeCancel(PGcancel *cancel);
       </synopsis>
      </para>

      <para>
       <function>PQfreeCancel</function> frees a data object previously created
       by <function>PQgetCancel</function>.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term>
      <function>PQcancel</function>
      <indexterm>
       <primary>PQcancel</primary>
      </indexterm>
     </term>

     <listitem>
      <para>
       Requests that the server abandon processing of the current command.
       <synopsis>
        int PQcancel(PGcancel *cancel, char *errbuf, int errbufsize);
       </synopsis>
      </para>

      <para>
       The return value is 1 if the cancel request was successfully
       dispatched and 0 if not.  If not, <parameter>errbuf</> is filled
       with an error message explaining why not.  <parameter>errbuf</>
       must be a char array of size <parameter>errbufsize</> (the
       recommended size is 256 bytes).
      </para>

      <para>
       Successful dispatch is no guarantee that the request will have
       any effect, however.  If the cancellation is effective, the current
       command will terminate early and return an error result.  If the
       cancellation fails (say, because the server was already done
       processing the command), then there will be no visible result at
       all.
      </para>

      <para>
       <function>PQcancel</function> can safely be invoked from a signal
       handler, if the <parameter>errbuf</> is a local variable in the
       signal handler.  The <structname>PGcancel</> object is read-only
       as far as <function>PQcancel</function> is concerned, so it can
       also be invoked from a thread that is separate from the one
       manipulating the <structname>PGconn</> object.
      </para>
     </listitem>
    </varlistentry>
   </variablelist>

   <variablelist>
    <varlistentry>
     <term>
      <function>PQrequestCancel</function>
      <indexterm>
       <primary>PQrequestCancel</primary>
      </indexterm>
     </term>

     <listitem>
      <para>
       Requests that the server abandon processing of the current
       command.
       <synopsis>
        int PQrequestCancel(PGconn *conn);
       </synopsis>
      </para>

      <para>
       <function>PQrequestCancel</function> is a deprecated variant of
       <function>PQcancel</function>.  It operates directly on the
       <structname>PGconn</> object, and in case of failure stores the
       error message in the <structname>PGconn</> object (whence it can
       be retrieved by <function>PQerrorMessage</function>).  Although
       the functionality is the same, this approach creates hazards for
       multiple-thread programs and signal handlers, since it is possible
       that overwriting the <structname>PGconn</>'s error message will
       mess up the operation currently in progress on the connection.
      </para>
     </listitem>
    </varlistentry>
   </variablelist>
  </para>

 </sect1>

 <sect1 id="libpq-fastpath">
  <title>The Fast-Path Interface</title>

  <indexterm zone="libpq-fastpath">
   <primary>fast path</primary>
  </indexterm>

  <para>
   <productname>PostgreSQL</productname> provides a fast-path interface
   to send simple function calls to the server.
  </para>

  <tip>
   <para>
    This interface is somewhat obsolete, as one can achieve similar
    performance and greater functionality by setting up a prepared
    statement to define the function call.  Then, executing the statement
    with binary transmission of parameters and results substitutes for a
    fast-path function call.
   </para>
  </tip>

  <para>
   The function <function>PQfn</function><indexterm><primary>PQfn</></>
   requests execution of a server function via the fast-path interface:
   <synopsis>
    PGresult *PQfn(PGconn *conn,
                   int fnid,
                   int *result_buf,
                   int *result_len,
                   int result_is_int,
                   const PQArgBlock *args,
                   int nargs);

    typedef struct {
        int len;
        int isint;
        union {
            int *ptr;
            int integer;
        } u;
    } PQArgBlock;
   </synopsis>
  </para>

  <para>
   The <parameter>fnid</> argument is the OID of the function to be
   executed.  <parameter>args</> and <parameter>nargs</> define the
   parameters to be passed to the function; they must match the declared
   function argument list.  When the <parameter>isint</> field of a
   parameter structure is true, the <parameter>u.integer</> value is sent
   to the server as an integer of the indicated length (this must be 1,
   2, or 4 bytes); proper byte-swapping occurs.  When <parameter>isint</>
   is false, the indicated number of bytes at <parameter>*u.ptr</> are
   sent with no processing; the data must be in the format expected by
   the server for binary transmission of the function's argument data
   type.  <parameter>result_buf</parameter> is the buffer in which to
   place the return value.  The caller must  have  allocated sufficient
   space to store the return value.  (There is no check!) The actual result
   length will be returned in the integer pointed to  by
   <parameter>result_len</parameter>.  If a 1, 2, or 4-byte integer result
   is expected, set <parameter>result_is_int</parameter> to 1, otherwise
   set it to 0.  Setting <parameter>result_is_int</parameter> to 1 causes
   <application>libpq</> to byte-swap the value if necessary, so that it
   is delivered as a proper <type>int</type> value for the client machine.
   When <parameter>result_is_int</> is 0, the binary-format byte string
   sent by the server is returned unmodified.
  </para>

  <para>
   <function>PQfn</function> always returns a valid
   <structname>PGresult</structname> pointer. The result status should be
   checked before the result is used.   The caller is responsible for
   freeing  the  <structname>PGresult</structname>  with
   <function>PQclear</function> when it is no longer needed.
  </para>

  <para>
   Note that it is not possible to handle null arguments, null results,
   nor set-valued results when using this interface.
  </para>

 </sect1>

 <sect1 id="libpq-notify">
  <title>Asynchronous Notification</title>

  <indexterm zone="libpq-notify">
   <primary>NOTIFY</primary>
   <secondary>in libpq</secondary>
  </indexterm>

  <para>
   <productname>PostgreSQL</productname> offers asynchronous notification
   via the <command>LISTEN</command> and <command>NOTIFY</command>
   commands.  A client session registers its interest in a particular
   notification condition with the <command>LISTEN</command> command (and
   can stop listening with the <command>UNLISTEN</command> command).  All
   sessions listening on a particular condition will be notified
   asynchronously when a <command>NOTIFY</command> command with that
   condition name is executed by any session.  No additional information
   is passed from the notifier to the listener.  Thus, typically, any
   actual data that needs to be communicated is transferred through a
   database table.  Commonly, the condition name is the same as the
   associated table, but it is not necessary for there to be any associated
   table.
  </para>

  <para>
   <application>libpq</application> applications submit
   <command>LISTEN</command> and <command>UNLISTEN</command> commands as
   ordinary SQL commands.  The arrival of <command>NOTIFY</command>
   messages can subsequently be detected by calling
   <function>PQnotifies</function>.<indexterm><primary>PQnotifies</></>
  </para>

  <para>
   The function <function>PQnotifies</function>
             returns  the next notification from a list of unhandled
             notification messages received from the server.  It returns a null pointer if
             there are no pending notifications.  Once a notification is
             returned from <function>PQnotifies</>, it is considered handled and will be
             removed from the list of notifications.
   <synopsis>
   PGnotify *PQnotifies(PGconn *conn);

   typedef struct pgNotify {
       char *relname;              /* notification condition name */
       int  be_pid;                /* process ID of notifying server process */
       char *extra;                /* notification parameter */
   } PGnotify;
   </synopsis>
   After processing a <structname>PGnotify</structname> object returned
   by <function>PQnotifies</function>, be sure to free it with
   <function>PQfreemem</function>.  It is sufficient to free the
   <structname>PGnotify</structname> pointer; the
   <structfield>relname</structfield> and <structfield>extra</structfield>
   fields do not represent separate allocations.  (At present, the
   <structfield>extra</structfield> field is unused and will always point
   to an empty string.)
  </para>

  <para>
   <xref linkend="libpq-example-2"> gives a sample program that illustrates
   the use of asynchronous notification.
  </para>

  <para>
   <function>PQnotifies</function> does not actually read data from the
   server; it just returns messages previously absorbed by another
   <application>libpq</application> function.  In prior releases of
   <application>libpq</application>, the only way to ensure timely receipt
   of <command>NOTIFY</> messages was to constantly submit commands, even
   empty ones, and then check <function>PQnotifies</function> after each
   <function>PQexec</function>.  While this still works, it is deprecated
   as a waste of processing power.
  </para>

  <para>
   A better way to check for <command>NOTIFY</> messages when you have no
   useful commands to execute is to call
   <function>PQconsumeInput</function>, then check
   <function>PQnotifies</function>.  You can use
   <function>select()</function> to wait for data to arrive from the
   server, thereby using no <acronym>CPU</acronym> power unless there is
   something to do.  (See <function>PQsocket</function> to obtain the file
   descriptor number to use with <function>select()</function>.) Note that
   this will work OK whether you submit commands with
   <function>PQsendQuery</function>/<function>PQgetResult</function> or
   simply use <function>PQexec</function>.  You should, however, remember
   to check <function>PQnotifies</function> after each
   <function>PQgetResult</function> or <function>PQexec</function>, to
   see if any notifications came in during the processing of the command.
  </para>

 </sect1>

 <sect1 id="libpq-copy">
  <title>Functions Associated with the <command>COPY</command> Command</title>

  <indexterm zone="libpq-copy">
   <primary>COPY</primary>
   <secondary>with libpq</secondary>
  </indexterm>

  <para>
   The <command>COPY</command> command in
   <productname>PostgreSQL</productname> has options to read from or write
   to the network connection used by <application>libpq</application>.
   The functions described in this section allow applications to take
   advantage of this capability by supplying or consuming copied data.
  </para>

  <para>
   The overall process is that the application first issues the SQL
   <command>COPY</command> command via <function>PQexec</function> or one
   of the equivalent functions.  The response to this (if there is no
   error in the command) will be a <structname>PGresult</> object bearing
   a status code of <literal>PGRES_COPY_OUT</literal> or
   <literal>PGRES_COPY_IN</literal> (depending on the specified copy
   direction).  The application should then use the functions of this
   section to receive or transmit data rows.  When the data transfer is
   complete, another <structname>PGresult</> object is returned to indicate
   success or failure of the transfer.  Its status will be
   <literal>PGRES_COMMAND_OK</literal> for success or
   <literal>PGRES_FATAL_ERROR</literal> if some problem was encountered.
   At this point further SQL commands can be issued via
   <function>PQexec</function>.  (It is not possible to execute other SQL
   commands using the same connection while the <command>COPY</command>
   operation is in progress.)
  </para>

  <para>
   If a <command>COPY</command> command is issued via
   <function>PQexec</function> in a string that could contain additional
   commands, the application must continue fetching results via
   <function>PQgetResult</> after completing the <command>COPY</command>
   sequence.  Only when <function>PQgetResult</> returns
   <symbol>NULL</symbol> is it certain that the <function>PQexec</function>
   command string is done and it is safe to issue more commands.
  </para>

  <para>
   The functions of this section should be executed only after obtaining
   a result status of <literal>PGRES_COPY_OUT</literal> or
   <literal>PGRES_COPY_IN</literal> from <function>PQexec</function> or
   <function>PQgetResult</function>.
  </para>

  <para>
   A <structname>PGresult</> object bearing one of these status values
   carries some additional data about the <command>COPY</command> operation
   that is starting.  This additional data is available using functions
   that are also used in connection with query results:

   <variablelist>
    <varlistentry>
     <term>
      <function>PQnfields</function>
      <indexterm>
       <primary>PQnfields</primary>
       <secondary>with COPY</secondary>
      </indexterm>
     </term>

     <listitem>
      <para>
       Returns the number of columns (fields) to be copied.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term>
      <function>PQbinaryTuples</function>
      <indexterm>
       <primary>PQbinaryTuples</primary>
       <secondary>with COPY</secondary>
      </indexterm>
     </term>

     <listitem>
      <para>
       0 indicates the overall copy format is textual (rows separated by
       newlines, columns separated by separator characters, etc).  1
       indicates the overall copy format is binary.  See <xref
       linkend="sql-copy" endterm="sql-copy-title"> for more information.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term>
      <function>PQfformat</function>
      <indexterm>
       <primary>PQfformat</primary>
       <secondary>with COPY</secondary>
      </indexterm>
     </term>

     <listitem>
      <para>
       Returns the format code (0 for text, 1 for binary) associated with
       each column of the copy operation.  The per-column format codes
       will always be zero when the overall copy format is textual, but
       the binary format can support both text and binary columns.
       (However, as of the current implementation of <command>COPY</>,
       only binary columns appear in a binary copy; so the per-column
       formats always match the overall format at present.)
      </para>
     </listitem>
    </varlistentry>
   </variablelist>
  </para>

  <note>
   <para>
    These additional data values are only available when using protocol
    3.0.  When using protocol 2.0, all these functions will return 0.
   </para>
  </note>

  <sect2 id="libpq-copy-send">
   <title>Functions for Sending <command>COPY</command> Data</title>

   <para>
    These functions are used to send data during <literal>COPY FROM
    STDIN</>.  They will fail if called when the connection is not in
    <literal>COPY_IN</> state.
   </para>

   <variablelist>
    <varlistentry>
     <term>
      <function>PQputCopyData</function>
      <indexterm>
       <primary>PQputCopyData</primary>
      </indexterm>
     </term>

     <listitem>
      <para>
       Sends data to the server during <literal>COPY_IN</> state.
       <synopsis>
        int PQputCopyData(PGconn *conn,
                          const char *buffer,
                          int nbytes);
       </synopsis>
      </para>

      <para>
       Transmits the <command>COPY</command> data in the specified
       <parameter>buffer</>, of length <parameter>nbytes</>, to the server.
       The result is 1 if the data was sent, zero if it was not sent
       because the attempt would block (this case is only possible if the
       connection is in nonblocking mode), or -1 if an error occurred.
       (Use <function>PQerrorMessage</function> to retrieve details if
       the return value is -1.  If the value is zero, wait for write-ready
       and try again.)
      </para>

      <para>
       The application can divide the <command>COPY</command> data stream
       into buffer loads of any convenient size.  Buffer-load boundaries
       have no semantic significance when sending.  The contents of the
       data stream must match the data format expected by the
       <command>COPY</> command; see <xref linkend="sql-copy"
       endterm="sql-copy-title"> for details.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term>
      <function>PQputCopyEnd</function>
      <indexterm>
       <primary>PQputCopyEnd</primary>
      </indexterm>
     </term>

     <listitem>
      <para>
       Sends end-of-data indication to the server during <literal>COPY_IN</> state.
       <synopsis>
        int PQputCopyEnd(PGconn *conn,
                         const char *errormsg);
       </synopsis>
      </para>

      <para>
       Ends the <literal>COPY_IN</> operation successfully if
       <parameter>errormsg</> is <symbol>NULL</symbol>.  If
       <parameter>errormsg</> is not <symbol>NULL</symbol> then the
       <command>COPY</> is forced to fail, with the string pointed to by
       <parameter>errormsg</> used as the error message.  (One should not
       assume that this exact error message will come back from the server,
       however, as the server might have already failed the
       <command>COPY</> for its own reasons.  Also note that the option
       to force failure does not work when using pre-3.0-protocol
       connections.)
      </para>

      <para>
       The result is 1 if the termination data was sent, zero if it was
       not sent because the attempt would block (this case is only possible
       if the connection is in nonblocking mode), or -1 if an error
       occurred.  (Use <function>PQerrorMessage</function> to retrieve
       details if the return value is -1.  If the value is zero, wait for
       write-ready and try again.)
      </para>

      <para>
       After successfully calling <function>PQputCopyEnd</>, call
       <function>PQgetResult</> to obtain the final result status of the
       <command>COPY</> command.  One can wait for this result to be
       available in the usual way.  Then return to normal operation.
      </para>
     </listitem>
    </varlistentry>
   </variablelist>

  </sect2>

  <sect2 id="libpq-copy-receive">
   <title>Functions for Receiving <command>COPY</command> Data</title>

   <para>
    These functions are used to receive data during <literal>COPY TO
    STDOUT</>.  They will fail if called when the connection is not in
    <literal>COPY_OUT</> state.
   </para>

   <variablelist>
    <varlistentry>
     <term>
      <function>PQgetCopyData</function>
      <indexterm>
       <primary>PQgetCopyData</primary>
      </indexterm>
     </term>

     <listitem>
      <para>
       Receives data from the server during <literal>COPY_OUT</> state.
       <synopsis>
        int PQgetCopyData(PGconn *conn,
                          char **buffer,
                          int async);
       </synopsis>
      </para>

      <para>
       Attempts to obtain another row of data from the server during a
       <command>COPY</command>.  Data is always returned one data row at
       a time; if only a partial row is available, it is not returned.
       Successful return of a data row involves allocating a chunk of
       memory to hold the data.  The <parameter>buffer</> parameter must
       be non-<symbol>NULL</symbol>.  <parameter>*buffer</> is set to
       point to the allocated memory, or to <symbol>NULL</symbol> in cases
       where no buffer is returned.  A non-<symbol>NULL</symbol> result
       buffer must be freed using <function>PQfreemem</> when no longer
       needed.
      </para>

      <para>
       When a row is successfully returned, the return value is the number
       of data bytes in the row (this will always be greater than zero).
       The returned string is always null-terminated, though this is
       probably only useful for textual <command>COPY</command>.  A result
       of zero indicates that the <command>COPY</command> is still in
       progress, but no row is yet available (this is only possible when
       <parameter>async</> is true).  A result of -1 indicates that the
       <command>COPY</command> is done.  A result of -2 indicates that an
       error occurred (consult <function>PQerrorMessage</> for the reason).
      </para>

      <para>
       When <parameter>async</> is true (not zero),
       <function>PQgetCopyData</> will not block waiting for input; it
       will return zero if the <command>COPY</command> is still in progress
       but no complete row is available.  (In this case wait for read-ready
       and then call <function>PQconsumeInput</> before calling
       <function>PQgetCopyData</> again.)  When <parameter>async</> is
       false (zero), <function>PQgetCopyData</> will block until data is
       available or the operation completes.
      </para>

      <para>
       After <function>PQgetCopyData</> returns -1, call
       <function>PQgetResult</> to obtain the final result status of the
       <command>COPY</> command.  One can wait for this result to be
       available in the usual way.  Then return to normal operation.
      </para>
     </listitem>
    </varlistentry>
   </variablelist>

  </sect2>

  <sect2 id="libpq-copy-deprecated">
   <title>Obsolete Functions for <command>COPY</command></title>

   <para>
    These functions represent older methods of handling <command>COPY</>.
    Although they still work, they are deprecated due to poor error handling,
    inconvenient methods of detecting end-of-data, and lack of support for binary
    or nonblocking transfers.
   </para>

   <variablelist>
    <varlistentry>
     <term>
      <function>PQgetline</function>
      <indexterm>
       <primary>PQgetline</primary>
      </indexterm>
     </term>

     <listitem>
      <para>
       Reads  a  newline-terminated  line  of  characters (transmitted
       by the server) into a buffer string of size <parameter>length</>.
       <synopsis>
        int PQgetline(PGconn *conn,
                      char *buffer,
                      int length);
       </synopsis>
      </para>

      <para>
       This function copies up to <parameter>length</>-1 characters into
       the buffer and converts the terminating newline into a zero byte.
       <function>PQgetline</function> returns <symbol>EOF</symbol> at the
       end of input, 0 if the entire line has been read, and 1 if the
       buffer is full but the terminating newline has not yet been read.
       </para>
       <para>
       Note that the application must check to see if a new line consists
       of  the  two characters  <literal>\.</literal>, which  indicates
       that the server has finished sending the results  of  the
       <command>COPY</command> command.  If  the  application might receive
       lines that are more than <parameter>length</>-1  characters  long,
       care is needed to be sure it recognizes the <literal>\.</literal>
       line correctly (and does not, for example, mistake the end of a
       long data line for a terminator line).
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term>
      <function>PQgetlineAsync</function>
      <indexterm>
       <primary>PQgetlineAsync</primary>
      </indexterm>
     </term>

     <listitem>
      <para>
       Reads a row of <command>COPY</command> data (transmitted  by the
       server) into a buffer without blocking.
       <synopsis>
        int PQgetlineAsync(PGconn *conn,
                           char *buffer,
                           int bufsize);
       </synopsis>
      </para>

      <para>
       This function is similar to <function>PQgetline</function>, but it can be used
       by applications
       that must read <command>COPY</command> data asynchronously, that is, without blocking.
       Having issued the <command>COPY</command> command and gotten a <literal>PGRES_COPY_OUT</literal>
       response, the
       application should call <function>PQconsumeInput</function> and
       <function>PQgetlineAsync</function> until the
       end-of-data signal is detected.
       </para>
       <para>
       Unlike <function>PQgetline</function>, this function takes
       responsibility for detecting end-of-data.
      </para>

      <para>
       On each call, <function>PQgetlineAsync</function> will return data if a
       complete data row is available in <application>libpq</>'s input buffer.
       Otherwise, no data is returned until the rest of the row arrives.
       The function returns -1 if the end-of-copy-data marker has been recognized,
       or 0 if no data is available, or a positive number giving the number of
       bytes of data returned.  If -1 is returned, the caller must next call
       <function>PQendcopy</function>, and then return to normal processing.
      </para>

      <para>
       The data returned will not extend beyond a data-row boundary.  If possible
       a whole row will be returned at one time.  But if the buffer offered by
       the caller is too small to hold a row sent by the server, then a partial
       data row will be returned.  With textual data this can be detected by testing
       whether the last returned byte is <literal>\n</literal> or not.  (In a binary
       <command>COPY</>, actual parsing of the <command>COPY</> data format will be needed to make the
       equivalent determination.)
       The returned string is not null-terminated.  (If you want to add a
       terminating null, be sure to pass a <parameter>bufsize</parameter> one smaller
       than the room actually available.)
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term>
      <function>PQputline</function>
      <indexterm>
       <primary>PQputline</primary>
      </indexterm>
     </term>

     <listitem>
      <para>
       Sends  a  null-terminated  string  to  the server.  Returns 0 if
       OK and <symbol>EOF</symbol> if unable to send the string.
       <synopsis>
        int PQputline(PGconn *conn,
                      const char *string);
       </synopsis>
      </para>

      <para>
       The <command>COPY</command> data stream sent by a series of calls
       to <function>PQputline</function> has the same format as that
       returned by <function>PQgetlineAsync</function>, except that
       applications are not obliged to send exactly one data row per
       <function>PQputline</function> call; it is okay to send a partial
       line or multiple lines per call.
      </para>

      <note>
       <para>
        Before <productname>PostgreSQL</productname> protocol 3.0, it was necessary
        for the application to explicitly send the two characters
        <literal>\.</literal> as a final line to indicate to the server that it had
        finished sending <command>COPY</> data.  While this still works, it is deprecated and the
        special meaning of <literal>\.</literal> can be expected to be removed in a
        future release.  It is sufficient to call <function>PQendcopy</function> after
        having sent the actual data.
       </para>
      </note>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term>
      <function>PQputnbytes</function>
      <indexterm>
       <primary>PQputnbytes</primary>
      </indexterm>
     </term>

     <listitem>
      <para>
       Sends  a  non-null-terminated  string  to  the server.  Returns
       0 if OK and <symbol>EOF</symbol> if unable to send the string.
       <synopsis>
        int PQputnbytes(PGconn *conn,
                        const char *buffer,
                        int nbytes);
       </synopsis>
      </para>

      <para>
       This is exactly like <function>PQputline</function>, except that the data
       buffer need not be null-terminated since the number of bytes to send is
       specified directly.  Use this procedure when sending binary data.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term>
      <function>PQendcopy</function>
      <indexterm>
       <primary>PQendcopy</primary>
      </indexterm>
     </term>

     <listitem>
      <para>
       Synchronizes with the server.
       <synopsis>
        int PQendcopy(PGconn *conn);
       </synopsis>
       This function waits until the  server  has  finished  the copying.
       It should either be issued when the  last  string  has  been sent
       to  the  server using <function>PQputline</function> or when the
       last string has been  received  from  the  server using
       <function>PGgetline</function>.  It must be issued or the server
       will get <quote>out of sync</quote> with  the client.   Upon return
       from this function, the server is ready to receive the next SQL
       command.  The return value is 0  on  successful  completion,
       nonzero otherwise.  (Use <function>PQerrorMessage</function> to
       retrieve details if the return value is nonzero.)
      </para>

      <para>
       When using <function>PQgetResult</function>, the application should
       respond to a <literal>PGRES_COPY_OUT</literal> result by executing
       <function>PQgetline</function> repeatedly, followed by
       <function>PQendcopy</function> after the terminator line is seen.
       It should then return to the <function>PQgetResult</function> loop
       until <function>PQgetResult</function> returns a null pointer.
       Similarly a <literal>PGRES_COPY_IN</literal> result is processed
       by a series of <function>PQputline</function> calls followed by
       <function>PQendcopy</function>, then return to the
       <function>PQgetResult</function> loop.  This arrangement will
       ensure that a <command>COPY</command> command embedded in a series
       of <acronym>SQL</acronym> commands will be executed correctly.
      </para>

      <para>
       Older applications are likely to submit a <command>COPY</command>
       via <function>PQexec</function> and assume that the transaction
       is done after <function>PQendcopy</function>.  This will work
       correctly only if the <command>COPY</command> is the only
       <acronym>SQL</acronym> command in the command string.
      </para>
     </listitem>
    </varlistentry>
   </variablelist>

  </sect2>

 </sect1>

 <sect1 id="libpq-control">
  <title>Control Functions</title>

  <para>
   These functions control miscellaneous details of <application>libpq</>'s
   behavior.
  </para>

  <variablelist>
   <varlistentry>
    <term>
     <function>PQclientEncoding</function>
     <indexterm>
      <primary>PQclientEncoding</primary>
     </indexterm>
    </term>

    <listitem>
     <para>
      Returns the client encoding.
      <synopsis>
      int PQclientEncoding(const PGconn *<replaceable>conn</replaceable>);
      </synopsis>

      Note that it returns the encoding ID, not a symbolic string
      such as <literal>EUC_JP</literal>. To convert an encoding ID to an encoding name, you
      can use:

<synopsis>
char *pg_encoding_to_char(int <replaceable>encoding_id</replaceable>);
</synopsis>
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term>
     <function>PQsetClientEncoding</function>
     <indexterm>
      <primary>PQsetClientEncoding</primary>
     </indexterm>
    </term>

    <listitem>
     <para>
      Sets the client encoding.
      <synopsis>
      int PQsetClientEncoding(PGconn *<replaceable>conn</replaceable>, const char *<replaceable>encoding</replaceable>);
      </synopsis>

      <replaceable>conn</replaceable> is a connection to the server,
      and <replaceable>encoding</replaceable> is the encoding you want to
      use. If the function successfully sets the encoding, it returns 0,
      otherwise -1. The current encoding for this connection can be
      determined by using <function>PQclientEncoding</>.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term>
     <function>PQsetErrorVerbosity</function>
     <indexterm>
      <primary>PQsetErrorVerbosity</primary>
     </indexterm>
    </term>

    <listitem>
     <para>
      Determines the verbosity of messages returned by
      <function>PQerrorMessage</> and <function>PQresultErrorMessage</>.
      <synopsis>
      typedef enum {
          PQERRORS_TERSE,
          PQERRORS_DEFAULT,
          PQERRORS_VERBOSE
      } PGVerbosity;

      PGVerbosity PQsetErrorVerbosity(PGconn *conn, PGVerbosity verbosity);
      </synopsis>

      <function>PQsetErrorVerbosity</> sets the verbosity mode, returning
      the connection's previous setting.  In <firstterm>TERSE</> mode,
      returned messages include severity, primary text, and position only;
      this will normally fit on a single line.  The default mode produces
      messages that include the above plus any detail, hint, or context
      fields (these might span multiple lines).  The <firstterm>VERBOSE</>
      mode includes all available fields.  Changing the verbosity does not
      affect the messages available from already-existing
      <structname>PGresult</> objects, only subsequently-created ones.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term>
     <function>PQtrace</function>
     <indexterm>
      <primary>PQtrace</primary>
     </indexterm>
    </term>

    <listitem>
     <para>
      Enables  tracing of the client/server communication to a debugging file stream.
      <synopsis>
       void PQtrace(PGconn *conn, FILE *stream);
      </synopsis>
     </para>

     <note>
      <para>
       On Windows, if the <application>libpq</> library and an application are
       compiled with different flags, this function call will crash the
       application because the internal representation of the <literal>FILE</>
       pointers differ.  Specifically, multithreaded/single-threaded,
       release/debug, and static/dynamic flags should be the same for the
       library and all applications using that library.
      </para>
     </note>

    </listitem>
   </varlistentry>

   <varlistentry>
    <term>
     <function>PQuntrace</function>
     <indexterm>
      <primary>PQuntrace</primary>
     </indexterm>
    </term>

    <listitem>
     <para>
      Disables tracing started by <function>PQtrace</function>.
      <synopsis>
       void PQuntrace(PGconn *conn);
      </synopsis>
     </para>
    </listitem>
   </varlistentry>
  </variablelist>

 </sect1>

 <sect1 id="libpq-misc">
  <title>Miscellaneous Functions</title>

  <para>
   As always, there are some functions that just don't fit anywhere.
  </para>

  <variablelist>
   <varlistentry>
    <term>
     <function>PQfreemem</function>
     <indexterm>
      <primary>PQfreemem</primary>
     </indexterm>
    </term>

    <listitem>
     <para>
      Frees memory allocated by <application>libpq</>.
      <synopsis>
       void PQfreemem(void *ptr);
      </synopsis>
     </para>

     <para>
      Frees memory allocated by <application>libpq</>, particularly
      <function>PQescapeByteaConn</function>,
      <function>PQescapeBytea</function>,
      <function>PQunescapeBytea</function>,
      and <function>PQnotifies</function>.
      It is particularly important that this function, rather than
      <function>free()</>, be used on Microsoft Windows.  This is because
      allocating memory in a DLL and releasing it in the application works
      only if multithreaded/single-threaded, release/debug, and static/dynamic
      flags are the same for the DLL and the application.  On non-Microsoft
      Windows platforms, this function is the same as the standard library
      function <function>free()</>.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term>
     <function>PQconninfoFree</function>
     <indexterm>
      <primary>PQconninfoFree</primary>
     </indexterm>
    </term>

    <listitem>
     <para>
      Frees the data structures allocated by
      <function>PQconndefaults</> or <function>PQconninfoParse</>.
      <synopsis>
       void PQconninfoFree(PQconninfoOption *connOptions);
      </synopsis>
     </para>

     <para>
      A simple <function>PQfreemem</function> will not do for this, since
      the array contains references to subsidiary strings.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term>
     <function>PQencryptPassword</function>
     <indexterm>
      <primary>PQencryptPassword</primary>
     </indexterm>
    </term>

    <listitem>
     <para>
      Prepares the encrypted form of a <productname>PostgreSQL</> password.
      <synopsis>
       char * PQencryptPassword(const char *passwd, const char *user);
      </synopsis>
      This function is intended to be used by client applications that
      wish to send commands like <literal>ALTER USER joe PASSWORD
      'pwd'</>.  It is good practice not to send the original cleartext
      password in such a command, because it might be exposed in command
      logs, activity displays, and so on.  Instead, use this function to
      convert the password to encrypted form before it is sent.  The
      arguments are the cleartext password, and the SQL name of the user
      it is for.  The return value is a string allocated by
      <function>malloc</function>, or <symbol>NULL</symbol> if out of
      memory.  The caller can assume the string doesn't contain any
      special characters that would require escaping.  Use
      <function>PQfreemem</> to free the result when done with it.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term>
     <function>PQmakeEmptyPGresult</function>
     <indexterm>
      <primary>PQmakeEmptyPGresult</primary>
     </indexterm>
    </term>

    <listitem>
     <para>
      Constructs an empty <structname>PGresult</structname> object with the given status.
      <synopsis>
       PGresult *PQmakeEmptyPGresult(PGconn *conn, ExecStatusType status);
      </synopsis>
     </para>

     <para>
      This is <application>libpq</>'s internal function to allocate and
      initialize an empty <structname>PGresult</structname> object.  This
      function returns NULL if memory could not be allocated. It is
      exported because some applications find it useful to generate result
      objects (particularly objects with error status) themselves.  If
      <parameter>conn</parameter> is not null and <parameter>status</>
      indicates an error, the current error message of the specified
      connection is copied into the <structname>PGresult</structname>.
      Also, if <parameter>conn</parameter> is not null, any event procedures
      registered in the connection are copied into the
      <structname>PGresult</structname>.  (They do not get
      <literal>PGEVT_RESULTCREATE</> calls, but see
      <function>PQfireResultCreateEvents</function>.)
      Note that <function>PQclear</function> should eventually be called
      on the object, just as with a <structname>PGresult</structname>
      returned by <application>libpq</application> itself.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term>
     <function>PQfireResultCreateEvents</function>
     <indexterm>
      <primary>PQfireResultCreateEvents</primary>
     </indexterm>
    </term>
    <listitem>
     <para>
      Fires a <literal>PGEVT_RESULTCREATE</literal> event (see <xref
      linkend="libpq-events">) for each event procedure registered in the
      <structname>PGresult</structname> object.  Returns non-zero for success,
      zero if any event procedure fails.

      <synopsis>
       int PQfireResultCreateEvents(PGconn *conn, PGresult *res);
      </synopsis>
     </para>

     <para>
      The <literal>conn</> argument is passed through to event procedures
      but not used directly.  It can be <literal>NULL</> if the event
      procedures won't use it.
     </para>

     <para>
      Event procedures that have already received a
      <literal>PGEVT_RESULTCREATE</> or <literal>PGEVT_RESULTCOPY</> event
      for this object are not fired again.
     </para>

     <para>
      The main reason that this function is separate from
      <function>PQmakeEmptyPGResult</function> is that it is often appropriate
      to create a <structname>PGresult</structname> and fill it with data
      before invoking the event procedures.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term>
     <function>PQcopyResult</function>
     <indexterm>
      <primary>PQcopyResult</primary>
     </indexterm>
    </term>

    <listitem>
     <para>
      Makes a copy of a <structname>PGresult</structname> object.  The copy is
      not linked to the source result in any way and
      <function>PQclear</function> must be called when the copy is no longer
      needed.  If the function fails, NULL is returned.

      <synopsis>
       PGresult *PQcopyResult(const PGresult *src, int flags);
      </synopsis>
     </para>

     <para>
      This is not intended to make an exact copy.  The returned result is
      always put into <literal>PGRES_TUPLES_OK</literal> status, and does not
      copy any error message in the source.  (It does copy the command status
      string, however.)  The <parameter>flags</parameter> argument determines
      what else is copied.  It is a bitwise OR of several flags.
      <literal>PG_COPYRES_ATTRS</literal> specifies copying the source
      result's attributes (column definitions).
      <literal>PG_COPYRES_TUPLES</literal> specifies copying the source
      result's tuples.  (This implies copying the attributes, too.)
      <literal>PG_COPYRES_NOTICEHOOKS</literal> specifies
      copying the source result's notify hooks.
      <literal>PG_COPYRES_EVENTS</literal> specifies copying the source
      result's events.  (But any instance data associated with the source
      is not copied.)
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term>
     <function>PQsetResultAttrs</function>
     <indexterm>
      <primary>PQsetResultAttrs</primary>
     </indexterm>
    </term>

    <listitem>
     <para>
      Sets the attributes of a <structname>PGresult</structname> object.
      <synopsis>
       int PQsetResultAttrs(PGresult *res, int numAttributes, PGresAttDesc *attDescs);
      </synopsis>
     </para>

     <para>
      The provided <parameter>attDescs</parameter> are copied into the result.
      If the <parameter>attDescs</parameter> pointer is NULL or
      <parameter>numAttributes</parameter> is less than one, the request is
      ignored and the function succeeds.  If <parameter>res</parameter>
      already contains attributes, the function will fail.  If the function
      fails, the return value is zero.  If the function succeeds, the return
      value is non-zero.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term>
     <function>PQsetvalue</function>
     <indexterm>
      <primary>PQsetvalue</primary>
     </indexterm>
    </term>

    <listitem>
     <para>
      Sets a tuple field value of a <structname>PGresult</structname> object.
      <synopsis>
       int PQsetvalue(PGresult *res, int tup_num, int field_num, char *value, int len);
      </synopsis>
     </para>

     <para>
      The function will automatically grow the result's internal tuples array
      as needed.  However, the <parameter>tup_num</parameter> argument must be
      less than or equal to <function>PQntuples</function>, meaning this
      function can only grow the tuples array one tuple at a time.  But any
      field of any existing tuple can be modified in any order.  If a value at
      <parameter>field_num</parameter> already exists, it will be overwritten.
      If <parameter>len</parameter> is <literal>-1</literal> or
      <parameter>value</parameter> is <literal>NULL</literal>, the field value
      will be set to an SQL <literal>NULL</literal>.  The
      <parameter>value</parameter> is copied into the result's private storage,
      thus is no longer needed after the function
      returns.  If the function fails, the return value is zero.  If the
      function succeeds, the return value is non-zero.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term>
     <function>PQresultAlloc</function>
     <indexterm>
      <primary>PQresultAlloc</primary>
     </indexterm>
    </term>

    <listitem>
     <para>
      Allocate subsidiary storage for a <structname>PGresult</structname> object.
      <synopsis>
       void *PQresultAlloc(PGresult *res, size_t nBytes);
      </synopsis>
     </para>

     <para>
      Any memory allocated with this function will be freed when
      <parameter>res</parameter> is cleared.  If the function fails,
      the return value is <literal>NULL</literal>.  The result is
      guaranteed to be adequately aligned for any type of data,
      just as for <function>malloc</>.
     </para>
    </listitem>
   </varlistentry>

  </variablelist>

 </sect1>

 <sect1 id="libpq-notice-processing">
  <title>Notice Processing</title>

  <indexterm zone="libpq-notice-processing">
   <primary>notice processing</primary>
   <secondary>in libpq</secondary>
  </indexterm>

  <para>
   Notice and warning messages generated by the server are not returned
   by the query execution functions, since they do not imply failure of
   the query.  Instead they are passed to a notice handling function, and
   execution continues normally after the handler returns.  The default
   notice handling function prints the message on
   <filename>stderr</filename>, but the application can override this
   behavior by supplying its own handling function.
  </para>

  <para>
   For historical reasons, there are two levels of notice handling, called
   the notice receiver and notice processor.  The default behavior is for
   the notice receiver to format the notice and pass a string to the notice
   processor for printing.  However, an application that chooses to provide
   its own notice receiver will typically ignore the notice processor
   layer and just do all the work in the notice receiver.
  </para>

  <para>
   The function <function>PQsetNoticeReceiver</function>
   <indexterm><primary>notice
   receiver</></><indexterm><primary>PQsetNoticeReceiver</></> sets or
   examines the current notice receiver for a connection object.
   Similarly, <function>PQsetNoticeProcessor</function>
   <indexterm><primary>notice
   processor</></><indexterm><primary>PQsetNoticeProcessor</></> sets or
   examines the current notice processor.

   <synopsis>
    typedef void (*PQnoticeReceiver) (void *arg, const PGresult *res);

    PQnoticeReceiver
    PQsetNoticeReceiver(PGconn *conn,
                        PQnoticeReceiver proc,
                        void *arg);

    typedef void (*PQnoticeProcessor) (void *arg, const char *message);

    PQnoticeProcessor
    PQsetNoticeProcessor(PGconn *conn,
                         PQnoticeProcessor proc,
                         void *arg);
   </synopsis>

   Each of these functions returns the previous notice receiver or
   processor function pointer, and sets the new value.  If you supply a
   null function pointer, no action is taken, but the current pointer is
   returned.
  </para>

  <para>
   When a notice or warning message is received from the server, or
   generated internally by <application>libpq</application>, the notice
   receiver function is called.  It is passed the message in the form of
   a <symbol>PGRES_NONFATAL_ERROR</symbol>
   <structname>PGresult</structname>.  (This allows the receiver to extract
   individual fields using <function>PQresultErrorField</>, or the complete
   preformatted message using <function>PQresultErrorMessage</>.) The same
   void pointer passed to <function>PQsetNoticeReceiver</function> is also
   passed.  (This pointer can be used to access application-specific state
   if needed.)
  </para>

  <para>
   The default notice receiver simply extracts the message (using
   <function>PQresultErrorMessage</>) and passes it to the notice
   processor.
  </para>

  <para>
   The notice processor is responsible for handling a notice or warning
   message given in text form.  It is passed the string text of the message
   (including a trailing newline), plus a void pointer that is the same
   one passed to <function>PQsetNoticeProcessor</function>.  (This pointer
   can be used to access application-specific state if needed.)
  </para>

  <para>
   The default notice processor is simply:
   <programlisting>
static void
defaultNoticeProcessor(void *arg, const char *message)
{
    fprintf(stderr, "%s", message);
}
</programlisting>
  </para>

  <para>
   Once you have set a notice receiver or processor, you should expect
   that that function could be called as long as either the
   <structname>PGconn</> object or <structname>PGresult</> objects made
   from it exist.  At creation of a <structname>PGresult</>, the
   <structname>PGconn</>'s current notice handling pointers are copied
   into the <structname>PGresult</> for possible use by functions like
   <function>PQgetvalue</function>.
  </para>

 </sect1>

 <sect1 id="libpq-events">
  <title>Event System</title>

  <para>
   <application>libpq</application>'s event system is designed to notify
   registered event handlers about interesting
   <application>libpq</application> events, such as the creation or
   destruction of <structname>PGconn</structname> and
   <structname>PGresult</structname> objects.  A principal use case is that
   this allows applications to associate their own data with a
   <structname>PGconn</structname> or <structname>PGresult</structname>
   and ensure that that data is freed at an appropriate time.
  </para>

  <para>
   Each registered event handler is associated with two pieces of data,
   known to <application>libpq</application> only as opaque <literal>void *</>
   pointers.  There is a <firstterm>passthrough</> pointer that is provided
   by the application when the event handler is registered with a
   <structname>PGconn</>.  The passthrough pointer never changes for the
   life of the <structname>PGconn</> and all <structname>PGresult</>s
   generated from it; so if used, it must point to long-lived data.
   In addition there is an <firstterm>instance data</> pointer, which starts
   out NULL in every <structname>PGconn</> and <structname>PGresult</>.
   This pointer can be manipulated using the
   <function>PQinstanceData</function>,
   <function>PQsetInstanceData</function>,
   <function>PQresultInstanceData</function> and
   <function>PQsetResultInstanceData</function> functions.  Note that
   unlike the passthrough pointer, instance data of a <structname>PGconn</>
   is not automatically inherited by <structname>PGresult</>s created from
   it.  <application>libpq</application> does not know what passthrough
   and instance data pointers point to (if anything) and will never attempt
   to free them &mdash; that is the responsibility of the event handler.
  </para>

  <sect2 id="libpq-events-types">
   <title>Event Types</title>

   <para>
    The enum <literal>PGEventId</> names the types of events handled by
    the event system.  All its values have names beginning with
    <literal>PGEVT</literal>.  For each event type, there is a corresponding
    event info structure that carries the parameters passed to the event
    handlers.  The event types are:
   </para>

   <variablelist>
    <varlistentry>
     <term><literal>PGEVT_REGISTER</literal></term>
     <listitem>
      <para>
       The register event occurs when <function>PQregisterEventProc</function>
       is called.  It is the ideal time to initialize any
       <literal>instanceData</literal> an event procedure may need.  Only one
       register event will be fired per event handler per connection.  If the
       event procedure fails, the registration is aborted.

      <synopsis>
typedef struct
{
    PGconn *conn;
} PGEventRegister;
      </synopsis>

       When a <literal>PGEVT_REGISTER</literal> event is received, the
       <parameter>evtInfo</parameter> pointer should be cast to a
       <structname>PGEventRegister *</structname>.  This structure contains a
       <structname>PGconn</structname> that should be in the
       <literal>CONNECTION_OK</literal> status; guaranteed if one calls
       <function>PQregisterEventProc</function> right after obtaining a good
       <structname>PGconn</structname>.  When returning a failure code, all
       cleanup must be performed as no <literal>PGEVT_CONNDESTROY</literal>
       event will be sent.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><literal>PGEVT_CONNRESET</literal></term>
     <listitem>
      <para>
       The connection reset event is fired on completion of
       <function>PQreset</function> or <function>PQresetPoll</function>.  In
       both cases, the event is only fired if the reset was successful.  If
       the event procedure fails, the entire connection reset will fail; the
       <structname>PGconn</structname> is put into
       <literal>CONNECTION_BAD</literal> status and
       <function>PQresetPoll</function> will return
       <literal>PGRES_POLLING_FAILED</literal>.

      <synopsis>
typedef struct
{
    PGconn *conn;
} PGEventConnReset;
      </synopsis>

       When a <literal>PGEVT_CONNRESET</literal> event is received, the
       <parameter>evtInfo</parameter> pointer should be cast to a
       <structname>PGEventConnReset *</structname>.  Although the contained
       <structname>PGconn</structname> was just reset, all event data remains
       unchanged.  This event should be used to reset/reload/requery any
       associated <literal>instanceData</literal>.  Note that even if the
       event procedure fails to process <literal>PGEVT_CONNRESET</>, it will
       still receive a <literal>PGEVT_CONNDESTROY</> event when the connection
       is closed.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><literal>PGEVT_CONNDESTROY</literal></term>
     <listitem>
      <para>
       The connection destroy event is fired in response to
       <function>PQfinish</function>.  It is the event procedure's
       responsibility to properly clean up its event data as libpq has no
       ability to manage this memory.  Failure to clean up will lead
       to memory leaks.

      <synopsis>
typedef struct
{
    PGconn *conn;
} PGEventConnDestroy;
      </synopsis>

       When a <literal>PGEVT_CONNDESTROY</literal> event is received, the
       <parameter>evtInfo</parameter> pointer should be cast to a
       <structname>PGEventConnDestroy *</structname>.  This event is fired
       prior to <function>PQfinish</function> performing any other cleanup.
       The return value of the event procedure is ignored since there is no
       way of indicating a failure from <function>PQfinish</function>.  Also,
       an event procedure failure should not abort the process of cleaning up
       unwanted memory.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><literal>PGEVT_RESULTCREATE</literal></term>
     <listitem>
      <para>
       The result creation event is fired in response to any query execution
       function that generates a result, including
       <function>PQgetResult</function>.  This event will only be fired after
       the result has been created successfully.

      <synopsis>
typedef struct
{
    PGconn *conn;
    PGresult *result;
} PGEventResultCreate;
      </synopsis>

       When a <literal>PGEVT_RESULTCREATE</literal> event is received, the
       <parameter>evtInfo</parameter> pointer should be cast to a
       <structname>PGEventResultCreate *</structname>.  The
       <parameter>conn</parameter> is the connection used to generate the
       result.  This is the ideal place to initialize any
       <literal>instanceData</literal> that needs to be associated with the
       result.  If the event procedure fails, the result will be cleared and
       the failure will be propagated.  The event procedure must not try to
       <function>PQclear</> the result object for itself.  When returning a
       failure code, all cleanup must be performed as no
       <literal>PGEVT_RESULTDESTROY</literal> event will be sent.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><literal>PGEVT_RESULTCOPY</literal></term>
     <listitem>
      <para>
       The result copy event is fired in response to
       <function>PQcopyResult</function>.  This event will only be fired after
       the copy is complete.  Only event procedures that have
       successfully handled the <literal>PGEVT_RESULTCREATE</literal>
       or <literal>PGEVT_RESULTCOPY</literal> event for the source result
       will receive <literal>PGEVT_RESULTCOPY</literal> events.

      <synopsis>
typedef struct
{
    const PGresult *src;
    PGresult *dest;
} PGEventResultCopy;
      </synopsis>

       When a <literal>PGEVT_RESULTCOPY</literal> event is received, the
       <parameter>evtInfo</parameter> pointer should be cast to a
       <structname>PGEventResultCopy *</structname>.  The
       <parameter>src</parameter> result is what was copied while the
       <parameter>dest</parameter> result is the copy destination.  This event
       can be used to provide a deep copy of <literal>instanceData</literal>,
       since <literal>PQcopyResult</literal> cannot do that.  If the event
       procedure fails, the entire copy operation will fail and the
       <parameter>dest</parameter> result will be cleared.   When returning a
       failure code, all cleanup must be performed as no
       <literal>PGEVT_RESULTDESTROY</literal> event will be sent for the
       destination result.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><literal>PGEVT_RESULTDESTROY</literal></term>
     <listitem>
      <para>
       The result destroy event is fired in response to a
       <function>PQclear</function>.  It is the event procedure's
       responsibility to properly clean up its event data as libpq has no
       ability to manage this memory.  Failure to clean up will lead
       to memory leaks.

      <synopsis>
typedef struct
{
    PGresult *result;
} PGEventResultDestroy;
      </synopsis>

       When a <literal>PGEVT_RESULTDESTROY</literal> event is received, the
       <parameter>evtInfo</parameter> pointer should be cast to a
       <structname>PGEventResultDestroy *</structname>.  This event is fired
       prior to <function>PQclear</function> performing any other cleanup.
       The return value of the event procedure is ignored since there is no
       way of indicating a failure from <function>PQclear</function>.  Also,
       an event procedure failure should not abort the process of cleaning up
       unwanted memory.
      </para>
     </listitem>
    </varlistentry>
   </variablelist>
  </sect2>

  <sect2 id="libpq-events-proc">
   <title>Event Callback Procedure</title>

   <variablelist>
    <varlistentry>
     <term>
      <literal>PGEventProc</literal>
      <indexterm>
       <primary>PGEventProc</primary>
      </indexterm>
     </term>

     <listitem>
      <para>
       <literal>PGEventProc</literal> is a typedef for a pointer to an
       event procedure, that is, the user callback function that receives
       events from libpq.  The signature of an event procedure must be

      <synopsis>
int eventproc(PGEventId evtId, void *evtInfo, void *passThrough)
      </synopsis>

       The <parameter>evtId</parameter> parameter indicates which
       <literal>PGEVT</literal> event occurred.  The
       <parameter>evtInfo</parameter> pointer must be cast to the appropriate
       structure type to obtain further information about the event.
       The <parameter>passThrough</parameter> parameter is the pointer
       provided to <function>PQregisterEventProc</function> when the event
       procedure was registered.  The function should return a non-zero value
       if it succeeds and zero if it fails.
      </para>

      <para>
       A particular event procedure can be registered only once in any
       <structname>PGconn</>.  This is because the address of the procedure
       is used as a lookup key to identify the associated instance data.
      </para>

      <caution>
       <para>
        On Windows, functions can have two different addresses: one visible
        from outside a DLL and another visible from inside the DLL.  One
        should be careful that only one of these addresses is used with
        <application>libpq</>'s event-procedure functions, else confusion will
        result.  The simplest rule for writing code that will work is to
        ensure that event procedures are declared <literal>static</>.  If the
        procedure's address must be available outside its own source file,
        expose a separate function to return the address.
       </para>
      </caution>
     </listitem>
    </varlistentry>
   </variablelist>
  </sect2>

  <sect2 id="libpq-events-funcs">
   <title>Event Support Functions</title>

    <variablelist>
    <varlistentry>
     <term>
      <function>PQregisterEventProc</function>
      <indexterm>
       <primary>PQregisterEventProc</primary>
      </indexterm>
     </term>

     <listitem>
      <para>
       Registers an event callback procedure with libpq.

       <synopsis>
        int PQregisterEventProc(PGconn *conn, PGEventProc proc,
                                const char *name, void *passThrough);
       </synopsis>
      </para>

      <para>
       An event procedure must be registered once on each
       <structname>PGconn</> you want to receive events about.  There is no
       limit, other than memory, on the number of event procedures that
       can be registered with a connection.  The function returns a non-zero
       value if it succeeds and zero if it fails.
      </para>

      <para>
       The <parameter>proc</parameter> argument will be called when a libpq
       event is fired.  Its memory address is also used to lookup
       <literal>instanceData</literal>.  The <parameter>name</parameter>
       argument is used to refer to the event procedure in error messages.
       This value cannot be NULL or a zero-length string.  The name string is
       copied into the <structname>PGconn</>, so what is passed need not be
       long-lived.  The <parameter>passThrough</parameter> pointer is passed
       to the <parameter>proc</parameter> whenever an event occurs. This
       argument can be NULL.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term>
      <function>PQsetInstanceData</function>
      <indexterm>
       <primary>PQsetInstanceData</primary>
      </indexterm>
     </term>
     <listitem>
      <para>
       Sets the conn's instanceData for proc to data.  This returns non-zero
       for success and zero for failure.  (Failure is only possible if
       the proc has not been properly registered in the conn.)

       <synopsis>
        int PQsetInstanceData(PGconn *conn, PGEventProc proc, void *data);
       </synopsis>
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term>
      <function>PQinstanceData</function>
      <indexterm>
       <primary>PQinstanceData</primary>
      </indexterm>
     </term>
     <listitem>
      <para>
       Returns the conn's instanceData associated with proc, or NULL
       if there is none.

       <synopsis>
        void *PQinstanceData(const PGconn *conn, PGEventProc proc);
       </synopsis>
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term>
      <function>PQresultSetInstanceData</function>
      <indexterm>
       <primary>PQresultSetInstanceData</primary>
      </indexterm>
     </term>
     <listitem>
      <para>
       Sets the result's instanceData for proc to data.  This returns non-zero
       for success and zero for failure.  (Failure is only possible if the
       proc has not been properly registered in the result.)

       <synopsis>
        int PQresultSetInstanceData(PGresult *res, PGEventProc proc, void *data);
       </synopsis>
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term>
      <function>PQresultInstanceData</function>
      <indexterm>
       <primary>PQresultInstanceData</primary>
      </indexterm>
     </term>
     <listitem>
      <para>
       Returns the result's instanceData associated with proc, or NULL
       if there is none.

       <synopsis>
        void *PQresultInstanceData(const PGresult *res, PGEventProc proc);
       </synopsis>
      </para>
     </listitem>
    </varlistentry>
   </variablelist>
  </sect2>

  <sect2 id="libpq-events-example">
   <title>Event Example</title>

   <para>
    Here is a skeleton example of managing private data associated with
    libpq connections and results.
   </para>

   <programlisting>
<![CDATA[
/* required header for libpq events (note: includes libpq-fe.h) */
#include <libpq-events.h>

/* The instanceData */
typedef struct
{
    int n;
    char *str;
} mydata;

/* PGEventProc */
static int myEventProc(PGEventId evtId, void *evtInfo, void *passThrough);

int
main(void)
{
    mydata *data;
    PGresult *res;
    PGconn *conn = PQconnectdb("dbname = postgres");

    if (PQstatus(conn) != CONNECTION_OK)
    {
        fprintf(stderr, "Connection to database failed: %s",
                PQerrorMessage(conn));
        PQfinish(conn);
        return 1;
    }

    /* called once on any connection that should receive events.
     * Sends a PGEVT_REGISTER to myEventProc.
     */
    if (!PQregisterEventProc(conn, myEventProc, "mydata_proc", NULL))
    {
        fprintf(stderr, "Cannot register PGEventProc\n");
        PQfinish(conn);
        return 1;
    }

    /* conn instanceData is available */
    data = PQinstanceData(conn, myEventProc);

    /* Sends a PGEVT_RESULTCREATE to myEventProc */
    res = PQexec(conn, "SELECT 1 + 1");

    /* result instanceData is available */
    data = PQresultInstanceData(res, myEventProc);

    /* If PG_COPYRES_EVENTS is used, sends a PGEVT_RESULTCOPY to myEventProc */
    res_copy = PQcopyResult(res, PG_COPYRES_TUPLES | PG_COPYRES_EVENTS);

    /* result instanceData is available if PG_COPYRES_EVENTS was
     * used during the PQcopyResult call.
     */
    data = PQresultInstanceData(res_copy, myEventProc);

    /* Both clears send a PGEVT_RESULTDESTROY to myEventProc */
    PQclear(res);
    PQclear(res_copy);

    /* Sends a PGEVT_CONNDESTROY to myEventProc */
    PQfinish(conn);

    return 0;
}

static int
myEventProc(PGEventId evtId, void *evtInfo, void *passThrough)
{
    switch (evtId)
    {
        case PGEVT_REGISTER:
        {
            PGEventRegister *e = (PGEventRegister *)evtInfo;
            mydata *data = get_mydata(e->conn);

            /* associate app specific data with connection */
            PQsetInstanceData(e->conn, myEventProc, data);
            break;
        }

        case PGEVT_CONNRESET:
        {
            PGEventConnReset *e = (PGEventConnReset *)evtInfo;
            mydata *data = PQinstanceData(e->conn, myEventProc);

            if (data)
              memset(data, 0, sizeof(mydata));
            break;
        }

        case PGEVT_CONNDESTROY:
        {
            PGEventConnDestroy *e = (PGEventConnDestroy *)evtInfo;
            mydata *data = PQinstanceData(e->conn, myEventProc);

            /* free instance data because the conn is being destroyed */
            if (data)
              free_mydata(data);
            break;
        }

        case PGEVT_RESULTCREATE:
        {
            PGEventResultCreate *e = (PGEventResultCreate *)evtInfo;
            mydata *conn_data = PQinstanceData(e->conn, myEventProc);
            mydata *res_data = dup_mydata(conn_data);

            /* associate app specific data with result (copy it from conn) */
            PQsetResultInstanceData(e->result, myEventProc, res_data);
            break;
        }

        case PGEVT_RESULTCOPY:
        {
            PGEventResultCopy *e = (PGEventResultCopy *)evtInfo;
            mydata *src_data = PQresultInstanceData(e->src, myEventProc);
            mydata *dest_data = dup_mydata(src_data);

            /* associate app specific data with result (copy it from a result) */
            PQsetResultInstanceData(e->dest, myEventProc, dest_data);
            break;
        }

        case PGEVT_RESULTDESTROY:
        {
            PGEventResultDestroy *e = (PGEventResultDestroy *)evtInfo;
            mydata *data = PQresultInstanceData(e->result, myEventProc);

            /* free instance data because the result is being destroyed */
            if (data)
              free_mydata(data);
            break;
        }

        /* unknown event id, just return TRUE. */
        default:
            break;
    }

    return TRUE; /* event processing succeeded */
}
]]>
</programlisting>
  </sect2>
 </sect1>

 <sect1 id="libpq-envars">
  <title>Environment Variables</title>

  <indexterm zone="libpq-envars">
   <primary>environment variable</primary>
  </indexterm>

  <para>
   The following environment variables can be used to select default
   connection parameter values, which will be used by
   <function>PQconnectdb</>, <function>PQsetdbLogin</> and
   <function>PQsetdb</> if no value is directly specified by the calling
   code.  These are useful to avoid hard-coding database connection
   information into simple client applications, for example.

   <itemizedlist>
    <listitem>
     <para>
      <indexterm>
       <primary><envar>PGHOST</envar></primary>
      </indexterm>
      <envar>PGHOST</envar> sets the database server name.
      If this begins with a slash, it specifies Unix-domain communication
      rather than TCP/IP communication; the value is then the name of the
      directory in which the socket file is stored (in a default installation
      setup this would be <filename>/tmp</filename>).
     </para>
    </listitem>

    <listitem>
     <para>
      <indexterm>
       <primary><envar>PGHOSTADDR</envar></primary>
      </indexterm>
      <envar>PGHOSTADDR</envar> specifies the numeric IP address of the database
      server.  This can be set instead of or in addition to <envar>PGHOST</envar>
      to avoid DNS lookup overhead. See the documentation of
      these parameters, under <function>PQconnectdb</function> above, for details
      on their interaction.
     </para>
     <para>
      When neither <envar>PGHOST</envar> nor <envar>PGHOSTADDR</envar> is set,
      the default behavior is to connect using a local Unix-domain socket; or on
      machines without Unix-domain sockets, <application>libpq</application> will
      attempt to connect to <literal>localhost</>.
     </para>
    </listitem>

    <listitem>
     <para>
      <indexterm>
       <primary><envar>PGPORT</envar></primary>
      </indexterm>
      <envar>PGPORT</envar> sets the TCP port number or Unix-domain socket
      file extension for communicating with the
      <productname>PostgreSQL</productname> server.
     </para>
    </listitem>

    <listitem>
     <para>
      <indexterm>
       <primary><envar>PGDATABASE</envar></primary>
      </indexterm>
      <envar>PGDATABASE</envar>  sets the
      <productname>PostgreSQL</productname> database name.
     </para>
    </listitem>

    <listitem>
     <para>
      <indexterm>
       <primary><envar>PGUSER</envar></primary>
      </indexterm>
      <envar>PGUSER</envar> sets the user name used to connect to the
      database.
     </para>
    </listitem>

    <listitem>
     <para>
      <indexterm>
       <primary><envar>PGPASSWORD</envar></primary>
      </indexterm>
      <envar>PGPASSWORD</envar> sets the password used if the server
      demands password authentication.  Use of this environment variable
      is not recommended for security reasons (some operating systems
      allow non-root users to see process environment variables via
      <application>ps</>); instead consider using  the
      <filename>~/.pgpass</> file (see <xref linkend="libpq-pgpass">).
     </para>
    </listitem>

    <listitem>
     <para>
      <indexterm>
       <primary><envar>PGPASSFILE</envar></primary>
      </indexterm>
      <envar>PGPASSFILE</envar> specifies the name of the password file to
      use for lookups.  If not set, it defaults to <filename>~/.pgpass</>
      (see <xref linkend="libpq-pgpass">).
     </para>
    </listitem>

    <listitem>
     <para>
      <indexterm>
       <primary><envar>PGSERVICE</envar></primary>
      </indexterm>
      <envar>PGSERVICE</envar>
      sets the service name to be looked up in
      <filename>pg_service.conf</filename>.  This offers a shorthand way
      of setting all the parameters.
     </para>
    </listitem>

    <listitem>
     <para>
      <indexterm>
       <primary><envar>PGREALM</envar></primary>
      </indexterm>
      <envar>PGREALM</envar> sets the Kerberos realm to  use  with
      <productname>PostgreSQL</productname>, if  it is different from the
      local realm.  If <envar>PGREALM</envar> is set,
      <application>libpq</application> applications  will  attempt
      authentication  with  servers for this realm and use separate ticket
      files to avoid conflicts with  local ticket  files.   This
      environment  variable is only used if Kerberos authentication is
      selected by the server.
     </para>
    </listitem>

    <listitem>
     <para>
      <indexterm>
       <primary><envar>PGOPTIONS</envar></primary>
      </indexterm>
      <envar>PGOPTIONS</envar> sets additional run-time options for the
      <productname>PostgreSQL</productname> server.  For example, setting
      <envar>PGOPTIONS</envar> to <literal>-c geqo=off</> sets the session's
      value of the <varname>geqo</> parameter to <literal>off</>.
      For a detailed discussion of the available options consult <xref
      linkend="runtime-config">.
     </para>
    </listitem>

    <listitem>
     <para>
      <indexterm>
       <primary><envar>PGSSLMODE</envar></primary>
      </indexterm>
      <envar>PGSSLMODE</envar> determines whether and with what priority
      an <acronym>SSL</> connection will be negotiated with the server.
      There are four modes: <literal>disable</> will attempt only an
      unencrypted <acronym>SSL</> connection; <literal>allow</> will
      negotiate, trying first a non-<acronym>SSL</> connection, then if
      that fails, trying an <acronym>SSL</> connection; <literal>prefer</>
      (the default) will negotiate, trying first an <acronym>SSL</>
      connection, then if that fails, trying a regular non-<acronym>SSL</>
      connection; <literal>require</> will try only an <acronym>SSL</>
      connection. If <productname>PostgreSQL</> is compiled without SSL
      support, using option <literal>require</> will cause an error, while
      options <literal>allow</> and <literal>prefer</> will be accepted
      but <application>libpq</> will not in fact attempt an <acronym>SSL</>
      connection.
     </para>
    </listitem>

    <listitem>
     <para>
      <indexterm>
       <primary><envar>PGSSLVERIFY</envar></primary>
      </indexterm>
      <envar>PGSSLVERIFY</envar> controls how libpq verifies the certificate on the
       server when performing an <acronym>SSL</> connection. There are
       three options: <literal>none</> disables verification completely
       (not recommended!); <literal>cert</> enables verification that
       the certificate chains to a known CA only; <literal>cn</> will
       both verify that the certificate chains to a known CA and that
       the <literal>cn</> attribute of the certificate matches the
       hostname the connection is being made to (default).
     </para>
    </listitem>

    <listitem>
     <para>
      <indexterm>
       <primary><envar>PGREQUIRESSL</envar></primary>
      </indexterm>
      <envar>PGREQUIRESSL</envar> sets whether or not the connection must
      be made over <acronym>SSL</acronym>. If set to <quote>1</quote>,
      <application>libpq</> will refuse to connect if the server does not
      accept an <acronym>SSL</acronym> connection (equivalent to
      <literal>sslmode</> <literal>prefer</>).  This option is deprecated
      in favor of the <literal>sslmode</> setting, and is only available
      if <productname>PostgreSQL</> is compiled with SSL support.
     </para>
    </listitem>

    <listitem>
     <para>
      <indexterm>
       <primary><envar>PGSSLCERT</envar></primary>
      </indexterm>
      <envar>PGSSLCERT</envar> specifies the location for the client
      certificate to use if the server requests one.
     </para>
    </listitem>

    <listitem>
     <para>
      <indexterm>
       <primary><envar>PGSSLKEY</envar></primary>
      </indexterm>
      <envar>PGSSLKEY</envar> specifies the location for the secret key
      used for the client certificate. It can either specify a filename
      that will be used instead of the default
      <filename>~/.postgresql/postgresql.key</>, or can specify an external
      engine (engines are <productname>OpenSSL</> loadable modules). The
      external engine specification should consist of a colon-separated
      engine name and an engine-specific key identifier.
     </para>
    </listitem>

    <listitem>
     <para>
      <indexterm>
       <primary><envar>PGSSLROOTCERT</envar></primary>
      </indexterm>
      <envar>PGSSLROOTCERT</envar> specifies the file name where the SSL
      root certificate is stored.
     </para>
    </listitem>

    <listitem>
     <para>
      <indexterm>
       <primary><envar>PGSSLCRL</envar></primary>
      </indexterm>
      <envar>PGSSLCRL</envar> specifies the file name where the SSL certificate
      revocation list is stored.
     </para>
    </listitem>

    <listitem>
     <para>
      <indexterm>
       <primary><envar>PGKRBSRVNAME</envar></primary>
      </indexterm>
      <envar>PGKRBSRVNAME</envar> sets the Kerberos service name to use
      when authenticating with Kerberos 5 or GSSAPI.
     </para>
    </listitem>

    <listitem>
     <para>
      <indexterm>
       <primary><envar>PGGSSLIB</envar></primary>
      </indexterm>
      <envar>PGGSSLIB</envar> sets the GSS library to use for GSSAPI
      authentication.
     </para>
    </listitem>

    <listitem>
     <para>
      <indexterm>
       <primary><envar>PGCONNECT_TIMEOUT</envar></primary>
      </indexterm>
      <envar>PGCONNECT_TIMEOUT</envar> sets the maximum number of seconds
      that <application>libpq</application> will wait when attempting to
      connect to the <productname>PostgreSQL</productname> server.  If
      unset or set to zero, <application>libpq</application> will wait
      indefinitely.  It is not recommended to set the timeout to less than
      2 seconds.
     </para>
    </listitem>
   </itemizedlist>
  </para>

  <para>
   The following environment variables can be used to specify default
   behavior for each <productname>PostgreSQL</productname> session.  (See
   also the <xref linkend="sql-alteruser" endterm="sql-alteruser-title">
   and <xref linkend="sql-alterdatabase" endterm="sql-alterdatabase-title">
   commands for ways to set default behavior on a per-user or per-database
   basis.)

   <itemizedlist>
    <listitem>
     <para>
      <indexterm>
       <primary><envar>PGDATESTYLE</envar></primary>
      </indexterm>
      <envar>PGDATESTYLE</envar> sets the default style of date/time
      representation.  (Equivalent to <literal>SET datestyle TO
      ...</literal>.)
     </para>
    </listitem>

    <listitem>
     <para>
      <indexterm>
       <primary><envar>PGTZ</envar></primary>
      </indexterm>
      <envar>PGTZ</envar> sets the default time zone.  (Equivalent to
      <literal>SET timezone TO ...</literal>.)
     </para>
    </listitem>

    <listitem>
     <para>
      <indexterm>
       <primary><envar>PGCLIENTENCODING</envar></primary>
      </indexterm>
      <envar>PGCLIENTENCODING</envar> sets the default client character
      set encoding.  (Equivalent to <literal>SET client_encoding TO
      ...</literal>.)
     </para>
    </listitem>

    <listitem>
     <para>
      <indexterm>
       <primary><envar>PGGEQO</envar></primary>
      </indexterm>
      <envar>PGGEQO</envar> sets the default mode for the genetic query
      optimizer.  (Equivalent to <literal>SET geqo TO ...</literal>.)
     </para>
    </listitem>
   </itemizedlist>

   Refer to the <acronym>SQL</acronym> command <xref linkend="sql-set"
   endterm="sql-set-title"> for information on correct values for these
   environment variables.
  </para>

  <para>
   The following environment variables determine internal behavior of
   <application>libpq</application>; they override compiled-in defaults.

   <itemizedlist>
    <listitem>
     <para>
      <indexterm>
       <primary><envar>PGSYSCONFDIR</envar></primary>
      </indexterm>
      <envar>PGSYSCONFDIR</envar> sets the directory containing the
      <filename>pg_service.conf</> file.
     </para>
    </listitem>

    <listitem>
     <para>
      <indexterm>
       <primary><envar>PGLOCALEDIR</envar></primary>
      </indexterm>
      <envar>PGLOCALEDIR</envar> sets the directory containing the
      <literal>locale</> files for message internationalization.
     </para>
    </listitem>
   </itemizedlist>
  </para>

 </sect1>


 <sect1 id="libpq-pgpass">
  <title>The Password File</title>

  <indexterm zone="libpq-pgpass">
   <primary>password file</primary>
  </indexterm>
  <indexterm zone="libpq-pgpass">
   <primary>.pgpass</primary>
  </indexterm>

  <para>
   The file <filename>.pgpass</filename> in a user's home directory or the
   file referenced by <envar>PGPASSFILE</envar> can contain passwords to
   be used if the connection requires a password (and no password has been
   specified  otherwise). On Microsoft Windows the file is named
   <filename>%APPDATA%\postgresql\pgpass.conf</> (where
   <filename>%APPDATA%</> refers to the Application Data subdirectory in
   the user's profile).
  </para>

  <para>
   This file should contain lines of the following format:
   <synopsis>
    <replaceable>hostname</replaceable>:<replaceable>port</replaceable>:<replaceable>database</replaceable>:<replaceable>username</replaceable>:<replaceable>password</replaceable>
   </synopsis>
   Each of the first four fields can be a literal value, or
   <literal>*</literal>, which matches anything.  The password field from
   the first line that matches the current connection parameters will be
   used.  (Therefore, put more-specific entries first when you are using
   wildcards.) If an entry needs to contain <literal>:</literal> or
   <literal>\</literal>, escape this character with <literal>\</literal>.
   A host name of <literal>localhost</> matches both TCP (host name
   <literal>localhost</>) and Unix domain socket (<literal>pghost</> empty
   or the default socket directory) connections coming from the local
   machine.
  </para>

  <para>
   On Unix systems, the permissions on <filename>.pgpass</filename> must
   disallow any access to world or group; achieve this by the command
   <command>chmod 0600 ~/.pgpass</command>.  If the permissions are less
   strict than this, the file will be ignored.  On Microsoft Windows, it
   is assumed that the file is stored in a directory that is secure, so
   no special permissions check is made.
  </para>
 </sect1>


 <sect1 id="libpq-pgservice">
  <title>The Connection Service File</title>

  <indexterm zone="libpq-pgservice">
   <primary>connection service file</primary>
  </indexterm>
  <indexterm zone="libpq-pgservice">
   <primary>pg_service.conf</primary>
  </indexterm>

  <para>
   The connection service file allows libpq connection parameters to be
   associated with a single service name. That service name can then be
   specified by a libpq connection, and the associated settings will be
   used. This allows connection parameters to be modified without requiring
   a recompile of the libpq application. The service name can also be
   specified using the <envar>PGSERVICE</envar> environment variable.
  </para>

  <para>
   To use this feature, copy
   <filename>share/pg_service.conf.sample</filename> to
   <filename>etc/pg_service.conf</filename> and edit the file to add
   service names and parameters. This file can be used for client-only
   installs too. The file's location can also be specified by the
   <envar>PGSYSCONFDIR</envar> environment variable.
  </para>
 </sect1>


 <sect1 id="libpq-ldap">
  <title>LDAP Lookup of Connection Parameters</title>

  <indexterm zone="libpq-ldap">
   <primary>LDAP connection parameter lookup</primary>
  </indexterm>

  <para>
   If <application>libpq</application> has been compiled with LDAP support (option
   <literal><option>--with-ldap</option></literal> for <command>configure</command>)
   it is possible to retrieve connection options like <literal>host</literal>
   or <literal>dbname</literal> via LDAP from a central server.
   The advantage is that if the connection parameters for a database change,
   the connection information doesn't have to be updated on all client machines.
  </para>

  <para>
   LDAP connection parameter lookup uses the connection service file
   <filename>pg_service.conf</filename> (see <xref
   linkend="libpq-pgservice">).  A line in a
   <filename>pg_service.conf</filename> stanza that starts with
   <literal>ldap://</literal> will be recognized as an LDAP URL and an
   LDAP query will be performed. The result must be a list of
   <literal>keyword = value</literal> pairs which will be used to set
   connection options.  The URL must conform to RFC 1959 and be of the
   form
   <synopsis>
    ldap://[<replaceable>hostname</replaceable>[:<replaceable>port</replaceable>]]/<replaceable>search_base</replaceable>?<replaceable>attribute</replaceable>?<replaceable>search_scope</replaceable>?<replaceable>filter</replaceable>
   </synopsis>
   where <replaceable>hostname</replaceable> defaults to
   <literal>localhost</literal> and <replaceable>port</replaceable>
   defaults to 389.
  </para>

  <para>
   Processing of <filename>pg_service.conf</filename> is terminated after
   a successful LDAP lookup, but is continued if the LDAP server cannot
   be contacted.  This is to provide a fallback with further LDAP URL
   lines that point to different LDAP servers, classical <literal>keyword
   = value</literal> pairs, or default connection options.  If you would
   rather get an error message in this case, add a syntactically incorrect
   line after the LDAP URL.
  </para>

  <para>
   A sample LDAP entry that has been created with the LDIF file
   <synopsis>
    version:1
    dn:cn=mydatabase,dc=mycompany,dc=com
    changetype:add
    objectclass:top
    objectclass:groupOfUniqueNames
    cn:mydatabase
    uniqueMember:host=dbserver.mycompany.com
    uniqueMember:port=5439
    uniqueMember:dbname=mydb
    uniqueMember:user=mydb_user
    uniqueMember:sslmode=require
   </synopsis>
   might be queried with the following LDAP URL:
   <synopsis>
    ldap://ldap.mycompany.com/dc=mycompany,dc=com?uniqueMember?one?(cn=mydatabase)
   </synopsis>
  </para>

  <para>
   You can also mix regular service file entries with LDAP lookups.
   A complete example for a stanza in <filename>pg_service.conf</filename>
   would be:
   <synopsis>
    # only host and port are stored in LDAP, specify dbname and user explicitly
    [customerdb]
    dbname=customer
    user=appuser
    ldap://ldap.acme.com/cn=dbserver,cn=hosts?pgconnectinfo?base?(objectclass=*)
   </synopsis>
  </para>

 </sect1>


 <sect1 id="libpq-ssl">
  <title>SSL Support</title>

  <indexterm zone="libpq-ssl">
   <primary>SSL</primary>
  </indexterm>

  <para>
   <productname>PostgreSQL</> has native support for using <acronym>SSL</>
   connections to encrypt client/server communications for increased
   security. See <xref linkend="ssl-tcp"> for details about the server-side
   <acronym>SSL</> functionality.
  </para>

  <para>
   <application>libpq</application> reads the system-wide
   <productname>OpenSSL</productname> configuration file. By default, this
   file is named <filename>openssl.cnf</filename> and is located in the
   directory reported by <literal>openssl version -d</>.  This default
   can be overridden by setting environment variable
   <envar>OPENSSL_CONF</envar> to the name of the desired configuration
   file.
  </para>

  <para>
   When the <literal>sslverify</> parameter is set to <literal>cn</> or
   <literal>cert</>, libpq will verify that the server certificate is
   trustworthy by checking the certificate chain up to a <acronym>CA</>.
   For this to work, place the certificate of a trusted <acronym>CA</>
   in the file <filename>~/.postgresql/root.crt</> in the user's home directory.
   (On Microsoft Windows the file is named
   <filename>%APPDATA%\postgresql\root.crt</filename>.)
   <application>libpq</application> will then verify that the server's
   certificate is signed by one of the trusted certificate authorities.
   The SSL connection will fail if the server does not present a trusted
   certificate.  Certificate Revocation List (CRL) entries are also checked
   if the file <filename>~/.postgresql/root.crl</filename> exists
   (<filename>%APPDATA%\postgresql\root.crl</filename> on Microsoft
   Windows).
   The location of the root certificate store and the CRL can be overridden
   by the connection parameters <literal>sslrootcert</> and <literal>sslcrl</>
   or the environment variables <envar>PGSSLROOTCERT</> and <envar>PGSSLCRL</>.
  </para>

  <para>
   If the server requests a trusted client certificate,
   <application>libpq</application> will send the certificate stored in
   file <filename>~/.postgresql/postgresql.crt</> in the user's home
   directory.  The certificate must be signed by one of the certificate
   authorities (<acronym>CA</acronym>) trusted by the server.  A matching
   private key file <filename>~/.postgresql/postgresql.key</> must also
   be present. The private
   key file must not allow any access to world or group; achieve this by the
   command <command>chmod 0600 ~/.postgresql/postgresql.key</command>.
   On Microsoft Windows these files are named
   <filename>%APPDATA%\postgresql\postgresql.crt</filename> and
   <filename>%APPDATA%\postgresql\postgresql.key</filename>, and there
   is no special permissions check since the directory is presumed secure.
   The location of the certificate and key files can be overridden by the
   connection parameters <literal>sslcert</> and <literal>sslkey</> or the
   environment variables <envar>PGSSLCERT</> and <envar>PGSSLKEY</>.
  </para>

  <para>
   If you are using <acronym>SSL</> inside your application (in addition
   to inside <application>libpq</application>), you can use
   <function>PQinitSSL(int)</> to tell <application>libpq</application>
   that the <acronym>SSL</> library has already been initialized by your
   application.
   <!-- If this URL changes replace it with a URL to www.archive.org. -->
   See <ulink
   url="http://h71000.www7.hp.com/doc/83final/BA554_90007/ch04.html"></ulink>
   for details on the SSL API.
  </para>

  <table id="libpq-ssl-file-usage">
   <title>Libpq/Client SSL File Usage</title>
   <tgroup cols="3">
    <thead>
     <row>
      <entry>File</entry>
      <entry>Contents</entry>
      <entry>Effect</entry>
     </row>
    </thead>

    <tbody>

     <row>
      <entry><filename>~/.postgresql/postgresql.crt</></entry>
      <entry>client certificate</entry>
      <entry>requested by server</entry>
     </row>

     <row>
      <entry><filename>~/.postgresql/postgresql.key</></entry>
      <entry>client private key</entry>
      <entry>proves client certificate sent by owner; does not indicate
      certificate owner is trustworthy</entry>
     </row>

     <row>
      <entry><filename>~/.postgresql/root.crt</></entry>
      <entry>trusted certificate authorities</entry>
      <entry>checks server certificate is signed by a trusted certificate
      authority</entry>
     </row>

     <row>
      <entry><filename>~/.postgresql/root.crl</></entry>
      <entry>certificates revoked by certificate authorities</entry>
      <entry>server certificate must not be on this list</entry>
     </row>

    </tbody>
   </tgroup>
  </table>

 </sect1>


 <sect1 id="libpq-threading">
  <title>Behavior in Threaded Programs</title>

  <indexterm zone="libpq-threading">
   <primary>threads</primary>
   <secondary>with libpq</secondary>
  </indexterm>

  <para>
   <application>libpq</application> is reentrant and thread-safe if the
   <filename>configure</filename> command-line option
   <literal>--enable-thread-safety</> was used when the
   <productname>PostgreSQL</productname> distribution was built.  In
   addition, you might need to use additional compiler command-line
   options when you compile your application code.  Refer to your
   system's documentation for information about how to build
   thread-enabled applications, or look in
   <filename>src/Makefile.global</filename> for <literal>PTHREAD_CFLAGS</>
   and <literal>PTHREAD_LIBS</>.  This function allows the querying of
   <application>libpq</application>'s thread-safe status:
  </para>

  <variablelist>
   <varlistentry>
    <term>
     <function>PQisthreadsafe</function>
     <indexterm>
      <primary>PQisthreadsafe</primary>
     </indexterm>
    </term>

    <listitem>
     <para>
      Returns the thread safety status of the
      <application>libpq</application> library.
      <synopsis>
       int PQisthreadsafe();
      </synopsis>
     </para>

     <para>
      Returns 1 if the <application>libpq</application> is thread-safe
      and 0 if it is not.
     </para>
    </listitem>
   </varlistentry>
  </variablelist>

  <para>
   One thread restriction is that no two threads attempt to manipulate
   the same <structname>PGconn</> object at the same time. In particular,
   you cannot issue concurrent commands from different threads through
   the same connection object. (If you need to run concurrent commands,
   use multiple connections.)
  </para>

  <para>
   <structname>PGresult</> objects are read-only after creation, and so
   can be passed around freely between threads.
  </para>

  <para>
   The deprecated functions <function>PQrequestCancel</function> and
   <function>PQoidStatus</function> are not thread-safe and should not be
   used in multithread programs.  <function>PQrequestCancel</function>
   can be replaced by <function>PQcancel</function>.
   <function>PQoidStatus</function> can be replaced by
   <function>PQoidValue</function>.
  </para>

  <para>
   If you are using Kerberos inside your application (in addition to inside
   <application>libpq</application>), you will need to do locking around
   Kerberos calls because Kerberos functions are not thread-safe.  See
   function <function>PQregisterThreadLock</> in the
   <application>libpq</application> source code for a way to do cooperative
   locking between <application>libpq</application> and your application.
  </para>

  <para>
   If you experience problems with threaded applications, run the program
   in <filename>src/tools/thread</> to see if your platform has
   thread-unsafe functions.  This program is run by
   <filename>configure</filename>, but for binary distributions your
   library might not match the library used to build the binaries.
  </para>
 </sect1>


 <sect1 id="libpq-build">
  <title>Building <application>libpq</application> Programs</title>

  <indexterm zone="libpq-build">
   <primary>compiling</primary>
   <secondary>libpq applications</secondary>
  </indexterm>

  <para>
   To build (i.e., compile and link) a program using
   <application>libpq</application> you need to do all of the following
   things:

   <itemizedlist>
    <listitem>
     <para>
      Include the <filename>libpq-fe.h</filename> header file:
<programlisting>
#include &lt;libpq-fe.h&gt;
</programlisting>
      If you failed to do that then you will normally get error messages
      from your compiler similar to
<screen>
foo.c: In function `main':
foo.c:34: `PGconn' undeclared (first use in this function)
foo.c:35: `PGresult' undeclared (first use in this function)
foo.c:54: `CONNECTION_BAD' undeclared (first use in this function)
foo.c:68: `PGRES_COMMAND_OK' undeclared (first use in this function)
foo.c:95: `PGRES_TUPLES_OK' undeclared (first use in this function)
</screen>
     </para>
    </listitem>

    <listitem>
     <para>
      Point your compiler to the directory where the <productname>PostgreSQL</> header
      files were installed, by supplying the
      <literal>-I<replaceable>directory</replaceable></literal> option
      to your compiler.  (In some cases the compiler will look into
      the directory in question by default, so you can omit this
      option.)  For instance, your compile command line could look
      like:
<programlisting>
cc -c -I/usr/local/pgsql/include testprog.c
</programlisting>
      If you are using makefiles then add the option to the
      <varname>CPPFLAGS</varname> variable:
<programlisting>
CPPFLAGS += -I/usr/local/pgsql/include
</programlisting>
     </para>

     <para>
      If there is any chance that your program might be compiled by
      other users then you should not hardcode the directory location
      like that.  Instead, you can run the utility
      <command>pg_config</command><indexterm><primary>pg_config</><secondary
      sortas="libpq">with libpq</></> to find out where the header
      files are on the local system:
<screen>
<prompt>$</prompt> pg_config --includedir
<computeroutput>/usr/local/include</computeroutput>
</screen>
     </para>

     <para>
      Failure to specify the correct option to the compiler will
      result in an error message such as
<screen>
testlibpq.c:8:22: libpq-fe.h: No such file or directory
</screen>
     </para>
    </listitem>

    <listitem>
     <para>
      When linking the final program, specify the option
      <literal>-lpq</literal> so that the <application>libpq</application>
      library gets pulled in, as well as the option
      <literal>-L<replaceable>directory</replaceable></literal> to point
      the compiler to the directory where the
      <application>libpq</application> library resides.  (Again, the
      compiler will search some directories by default.)  For maximum
      portability, put the <option>-L</option> option before the
      <option>-lpq</option> option.  For example:
<programlisting>
cc -o testprog testprog1.o testprog2.o -L/usr/local/pgsql/lib -lpq
</programlisting>
     </para>

     <para>
      You can find out the library directory using
      <command>pg_config</command> as well:
<screen>
<prompt>$</prompt> pg_config --libdir
<computeroutput>/usr/local/pgsql/lib</computeroutput>
</screen>
     </para>

     <para>
      Error messages that point to problems in this area could look like
      the following.
<screen>
testlibpq.o: In function `main':
testlibpq.o(.text+0x60): undefined reference to `PQsetdbLogin'
testlibpq.o(.text+0x71): undefined reference to `PQstatus'
testlibpq.o(.text+0xa4): undefined reference to `PQerrorMessage'
</screen>
      This means you forgot <option>-lpq</option>.
<screen>
/usr/bin/ld: cannot find -lpq
</screen>
      This means you forgot the <option>-L</option> option or did not
      specify the right directory.
     </para>
    </listitem>
   </itemizedlist>
  </para>

 </sect1>


 <sect1 id="libpq-example">
  <title>Example Programs</title>

  <para>
   These examples and others can be found in the
   directory <filename>src/test/examples</filename> in the source code
   distribution.
  </para>

  <example id="libpq-example-1">
   <title><application>libpq</application> Example Program 1</title>

<programlisting>
<![CDATA[
/*
 * testlibpq.c
 *
 *      Test the C version of libpq, the PostgreSQL frontend library.
 */
#include <stdio.h>
#include <stdlib.h>
#include "libpq-fe.h"

static void
exit_nicely(PGconn *conn)
{
    PQfinish(conn);
    exit(1);
}

int
main(int argc, char **argv)
{
    const char *conninfo;
    PGconn     *conn;
    PGresult   *res;
    int         nFields;
    int         i,
                j;

    /*
     * If the user supplies a parameter on the command line, use it as the
     * conninfo string; otherwise default to setting dbname=postgres and using
     * environment variables or defaults for all other connection parameters.
     */
    if (argc > 1)
        conninfo = argv[1];
    else
        conninfo = "dbname = postgres";

    /* Make a connection to the database */
    conn = PQconnectdb(conninfo);

    /* Check to see that the backend connection was successfully made */
    if (PQstatus(conn) != CONNECTION_OK)
    {
        fprintf(stderr, "Connection to database failed: %s",
                PQerrorMessage(conn));
        exit_nicely(conn);
    }

    /*
     * Our test case here involves using a cursor, for which we must be inside
     * a transaction block.  We could do the whole thing with a single
     * PQexec() of "select * from pg_database", but that's too trivial to make
     * a good example.
     */

    /* Start a transaction block */
    res = PQexec(conn, "BEGIN");
    if (PQresultStatus(res) != PGRES_COMMAND_OK)
    {
        fprintf(stderr, "BEGIN command failed: %s", PQerrorMessage(conn));
        PQclear(res);
        exit_nicely(conn);
    }

    /*
     * Should PQclear PGresult whenever it is no longer needed to avoid memory
     * leaks
     */
    PQclear(res);

    /*
     * Fetch rows from pg_database, the system catalog of databases
     */
    res = PQexec(conn, "DECLARE myportal CURSOR FOR select * from pg_database");
    if (PQresultStatus(res) != PGRES_COMMAND_OK)
    {
        fprintf(stderr, "DECLARE CURSOR failed: %s", PQerrorMessage(conn));
        PQclear(res);
        exit_nicely(conn);
    }
    PQclear(res);

    res = PQexec(conn, "FETCH ALL in myportal");
    if (PQresultStatus(res) != PGRES_TUPLES_OK)
    {
        fprintf(stderr, "FETCH ALL failed: %s", PQerrorMessage(conn));
        PQclear(res);
        exit_nicely(conn);
    }

    /* first, print out the attribute names */
    nFields = PQnfields(res);
    for (i = 0; i < nFields; i++)
        printf("%-15s", PQfname(res, i));
    printf("\n\n");

    /* next, print out the rows */
    for (i = 0; i < PQntuples(res); i++)
    {
        for (j = 0; j < nFields; j++)
            printf("%-15s", PQgetvalue(res, i, j));
        printf("\n");
    }

    PQclear(res);

    /* close the portal ... we don't bother to check for errors ... */
    res = PQexec(conn, "CLOSE myportal");
    PQclear(res);

    /* end the transaction */
    res = PQexec(conn, "END");
    PQclear(res);

    /* close the connection to the database and cleanup */
    PQfinish(conn);

    return 0;
}
]]>
</programlisting>
  </example>

  <example id="libpq-example-2">
   <title><application>libpq</application> Example Program 2</title>

<programlisting>
<![CDATA[
/*
 * testlibpq2.c
 *      Test of the asynchronous notification interface
 *
 * Start this program, then from psql in another window do
 *   NOTIFY TBL2;
 * Repeat four times to get this program to exit.
 *
 * Or, if you want to get fancy, try this:
 * populate a database with the following commands
 * (provided in src/test/examples/testlibpq2.sql):
 *
 *   CREATE TABLE TBL1 (i int4);
 *
 *   CREATE TABLE TBL2 (i int4);
 *
 *   CREATE RULE r1 AS ON INSERT TO TBL1 DO
 *     (INSERT INTO TBL2 VALUES (new.i); NOTIFY TBL2);
 *
 * and do this four times:
 *
 *   INSERT INTO TBL1 VALUES (10);
 */
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#include <sys/time.h>
#include "libpq-fe.h"

static void
exit_nicely(PGconn *conn)
{
    PQfinish(conn);
    exit(1);
}

int
main(int argc, char **argv)
{
    const char *conninfo;
    PGconn     *conn;
    PGresult   *res;
    PGnotify   *notify;
    int         nnotifies;

    /*
     * If the user supplies a parameter on the command line, use it as the
     * conninfo string; otherwise default to setting dbname=postgres and using
     * environment variables or defaults for all other connection parameters.
     */
    if (argc > 1)
        conninfo = argv[1];
    else
        conninfo = "dbname = postgres";

    /* Make a connection to the database */
    conn = PQconnectdb(conninfo);

    /* Check to see that the backend connection was successfully made */
    if (PQstatus(conn) != CONNECTION_OK)
    {
        fprintf(stderr, "Connection to database failed: %s",
                PQerrorMessage(conn));
        exit_nicely(conn);
    }

    /*
     * Issue LISTEN command to enable notifications from the rule's NOTIFY.
     */
    res = PQexec(conn, "LISTEN TBL2");
    if (PQresultStatus(res) != PGRES_COMMAND_OK)
    {
        fprintf(stderr, "LISTEN command failed: %s", PQerrorMessage(conn));
        PQclear(res);
        exit_nicely(conn);
    }

    /*
     * should PQclear PGresult whenever it is no longer needed to avoid memory
     * leaks
     */
    PQclear(res);

    /* Quit after four notifies are received. */
    nnotifies = 0;
    while (nnotifies < 4)
    {
        /*
         * Sleep until something happens on the connection.  We use select(2)
         * to wait for input, but you could also use poll() or similar
         * facilities.
         */
        int         sock;
        fd_set      input_mask;

        sock = PQsocket(conn);

        if (sock < 0)
            break;              /* shouldn't happen */

        FD_ZERO(&input_mask);
        FD_SET(sock, &input_mask);

        if (select(sock + 1, &input_mask, NULL, NULL, NULL) < 0)
        {
            fprintf(stderr, "select() failed: %s\n", strerror(errno));
            exit_nicely(conn);
        }

        /* Now check for input */
        PQconsumeInput(conn);
        while ((notify = PQnotifies(conn)) != NULL)
        {
            fprintf(stderr,
                    "ASYNC NOTIFY of '%s' received from backend pid %d\n",
                    notify->relname, notify->be_pid);
            PQfreemem(notify);
            nnotifies++;
        }
    }

    fprintf(stderr, "Done.\n");

    /* close the connection to the database and cleanup */
    PQfinish(conn);

    return 0;
}
]]>
</programlisting>
  </example>

  <example id="libpq-example-3">
   <title><application>libpq</application> Example Program 3</>

<programlisting>
<![CDATA[
/*
 * testlibpq3.c
 *      Test out-of-line parameters and binary I/O.
 *
 * Before running this, populate a database with the following commands
 * (provided in src/test/examples/testlibpq3.sql):
 *
 * CREATE TABLE test1 (i int4, t text, b bytea);
 *
 * INSERT INTO test1 values (1, 'joe''s place', '\\000\\001\\002\\003\\004');
 * INSERT INTO test1 values (2, 'ho there', '\\004\\003\\002\\001\\000');
 *
 * The expected output is:
 *
 * tuple 0: got
 *  i = (4 bytes) 1
 *  t = (11 bytes) 'joe's place'
 *  b = (5 bytes) \000\001\002\003\004
 *
 * tuple 0: got
 *  i = (4 bytes) 2
 *  t = (8 bytes) 'ho there'
 *  b = (5 bytes) \004\003\002\001\000
 */
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include "libpq-fe.h"

/* for ntohl/htonl */
#include <netinet/in.h>
#include <arpa/inet.h>


static void
exit_nicely(PGconn *conn)
{
    PQfinish(conn);
    exit(1);
}

/*
 * This function prints a query result that is a binary-format fetch from
 * a table defined as in the comment above.  We split it out because the
 * main() function uses it twice.
 */
static void
show_binary_results(PGresult *res)
{
    int         i,
                j;
    int         i_fnum,
                t_fnum,
                b_fnum;

    /* Use PQfnumber to avoid assumptions about field order in result */
    i_fnum = PQfnumber(res, "i");
    t_fnum = PQfnumber(res, "t");
    b_fnum = PQfnumber(res, "b");

    for (i = 0; i < PQntuples(res); i++)
    {
        char       *iptr;
        char       *tptr;
        char       *bptr;
        int         blen;
        int         ival;

        /* Get the field values (we ignore possibility they are null!) */
        iptr = PQgetvalue(res, i, i_fnum);
        tptr = PQgetvalue(res, i, t_fnum);
        bptr = PQgetvalue(res, i, b_fnum);

        /*
         * The binary representation of INT4 is in network byte order, which
         * we'd better coerce to the local byte order.
         */
        ival = ntohl(*((uint32_t *) iptr));

        /*
         * The binary representation of TEXT is, well, text, and since libpq
         * was nice enough to append a zero byte to it, it'll work just fine
         * as a C string.
         *
         * The binary representation of BYTEA is a bunch of bytes, which could
         * include embedded nulls so we have to pay attention to field length.
         */
        blen = PQgetlength(res, i, b_fnum);

        printf("tuple %d: got\n", i);
        printf(" i = (%d bytes) %d\n",
               PQgetlength(res, i, i_fnum), ival);
        printf(" t = (%d bytes) '%s'\n",
               PQgetlength(res, i, t_fnum), tptr);
        printf(" b = (%d bytes) ", blen);
        for (j = 0; j < blen; j++)
            printf("\\%03o", bptr[j]);
        printf("\n\n");
    }
}

int
main(int argc, char **argv)
{
    const char *conninfo;
    PGconn     *conn;
    PGresult   *res;
    const char *paramValues[1];
    int         paramLengths[1];
    int         paramFormats[1];
    uint32_t    binaryIntVal;

    /*
     * If the user supplies a parameter on the command line, use it as the
     * conninfo string; otherwise default to setting dbname=postgres and using
     * environment variables or defaults for all other connection parameters.
     */
    if (argc > 1)
        conninfo = argv[1];
    else
        conninfo = "dbname = postgres";

    /* Make a connection to the database */
    conn = PQconnectdb(conninfo);

    /* Check to see that the backend connection was successfully made */
    if (PQstatus(conn) != CONNECTION_OK)
    {
        fprintf(stderr, "Connection to database failed: %s",
                PQerrorMessage(conn));
        exit_nicely(conn);
    }

    /*
     * The point of this program is to illustrate use of PQexecParams() with
     * out-of-line parameters, as well as binary transmission of data.
     *
     * This first example transmits the parameters as text, but receives the
     * results in binary format.  By using out-of-line parameters we can
     * avoid a lot of tedious mucking about with quoting and escaping, even
     * though the data is text.  Notice how we don't have to do anything
     * special with the quote mark in the parameter value.
     */

    /* Here is our out-of-line parameter value */
    paramValues[0] = "joe's place";

    res = PQexecParams(conn,
                       "SELECT * FROM test1 WHERE t = $1",
                       1,       /* one param */
                       NULL,    /* let the backend deduce param type */
                       paramValues,
                       NULL,    /* don't need param lengths since text */
                       NULL,    /* default to all text params */
                       1);      /* ask for binary results */

    if (PQresultStatus(res) != PGRES_TUPLES_OK)
    {
        fprintf(stderr, "SELECT failed: %s", PQerrorMessage(conn));
        PQclear(res);
        exit_nicely(conn);
    }

    show_binary_results(res);

    PQclear(res);

    /*
     * In this second example we transmit an integer parameter in binary
     * form, and again retrieve the results in binary form.
     *
     * Although we tell PQexecParams we are letting the backend deduce
     * parameter type, we really force the decision by casting the parameter
     * symbol in the query text.  This is a good safety measure when sending
     * binary parameters.
     */

    /* Convert integer value "2" to network byte order */
    binaryIntVal = htonl((uint32_t) 2);

    /* Set up parameter arrays for PQexecParams */
    paramValues[0] = (char *) &binaryIntVal;
    paramLengths[0] = sizeof(binaryIntVal);
    paramFormats[0] = 1;        /* binary */

    res = PQexecParams(conn,
                       "SELECT * FROM test1 WHERE i = $1::int4",
                       1,       /* one param */
                       NULL,    /* let the backend deduce param type */
                       paramValues,
                       paramLengths,
                       paramFormats,
                       1);      /* ask for binary results */

    if (PQresultStatus(res) != PGRES_TUPLES_OK)
    {
        fprintf(stderr, "SELECT failed: %s", PQerrorMessage(conn));
        PQclear(res);
        exit_nicely(conn);
    }

    show_binary_results(res);

    PQclear(res);

    /* close the connection to the database and cleanup */
    PQfinish(conn);

    return 0;
}
]]>
</programlisting>
  </example>

 </sect1>
</chapter>
