<!-- $PostgreSQL: pgsql/doc/src/sgml/libpq.sgml,v 1.237 2007/07/08 18:28:55 tgl 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 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 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.
      </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>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>krbsrvname</literal></term>
     <listitem>
      <para>
       Kerberos service name to use when authenticating with Kerberos 5.
       This must match the service name specified in the server
       configuration for Kerberos authentication to succeed. (See also
       <xref linkend="kerberos-auth">.)
      </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;  /* Character to display for 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>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</></></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</></></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</></></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</></></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</></></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</></></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</></></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</></></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</></></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</></></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>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.)
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</></></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</></></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</></></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 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>PQconnectionUsedPassword</function><indexterm><primary>PQconnectionUsedPassword</></></term>
     <listitem>
      <para>
       Returns true (1) if the connection authentication method
       required a password to be supplied. Returns false (0) if not.
<synopsis>
int PQconnectionUsedPassword(const PGconn *conn);
</synopsis>
      </para>

      <para>
       This function can be applied after either successful or failed
       connection attempts.  In the case of failure, it can for example
       be used to decide whether to prompt the user for 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</></></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</></></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>
    </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</></></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</></></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</></></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</></></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</></></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</></></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</></></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>

<varlistentry>
<term><function>PQmakeEmptyPGresult</function><indexterm><primary>PQmakeEmptyPGresult</></></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>.  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>
</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</></></term>
<listitem>
<para>
          Returns the number of rows (tuples)
          in the query result.
<synopsis>
int PQntuples(const PGresult *res);
</synopsis>
</para>
</listitem>
</varlistentry>

<varlistentry>
<term><function>PQnfields</function><indexterm><primary>PQnfields</></></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</></></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</></></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</></></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</></></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</></></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</></></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</></></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</></></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</></></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</></></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</></></term>
<indexterm><primary>null value</><secondary sortas="libpq">in libpq</></indexterm><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</></></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</></></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</></></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</></></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</></></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</></></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</></></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</></></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</></>
   <indexterm zone="libpq-exec-escape-string"><primary>PQescapeString</></>
   <indexterm zone="libpq-exec-escape-string">
     <primary>escaping strings</>
     <secondary>in libpq</secondary>
   </>

<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</>
   <secondary sortas="libpq">in libpq</>
  </indexterm>

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

  <varlistentry>
  <term><function>PQfreemem</function><indexterm><primary>PQfreemem</></></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>
  </variablelist>
   
 </sect2>
</sect1>

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

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

<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</></></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</></></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</></></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</></></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</></></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</></></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</></></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</></></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</></></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</></></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</></></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</></></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</><secondary>SQL command</></>

<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</></></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</></></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</></></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</></></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</></>

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

<varlistentry>
<term><function>PQbinaryTuples</function><indexterm><primary>PQbinaryTuples</><secondary>with COPY</></></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</><secondary>with COPY</></></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</></></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</></></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</></></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</></></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</></></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</></></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</></></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</></></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>PQsetErrorVerbosity</function><indexterm><primary>PQsetErrorVerbosity</></></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</></></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</></></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>PQencryptPassword</function><indexterm><primary>PQencryptPassword</></></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>
</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-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.
</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>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>PGSSLKEY</envar></primary>
</indexterm>
<envar>PGSSLKEY</envar>
specifies the hardware token that stores the secret key for the client
certificate. The value of this variable should consist
of a colon-separated engine name (engines are <productname>OpenSSL</>
loadable modules) and an engine-specific key identifier.  If this is not
set, the secret key must be kept in a file.
</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.
</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 (hostname
<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>
</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>
   If the server demands a client certificate, 
   <application>libpq</application>
   will send the certificate stored in file
   <filename>~/.postgresql/postgresql.crt</> within the user's home directory.
   A matching private key file <filename>~/.postgresql/postgresql.key</>
   must also be present, unless the secret key for the certificate is stored
   in a hardware token, as specified by <envar>PGSSLKEY</envar>.
   (On Microsoft Windows these files are named
   <filename>%APPDATA%\postgresql\postgresql.crt</filename> and
   <filename>%APPDATA%\postgresql\postgresql.key</filename>.)
   The private key file must not be world-readable.
  </para>

  <para>
   If the environment variable <envar>PGSSLKEY</envar> is set, its value
   should consist of a colon-separated engine name and key identifier. In
   this case, <application>libpq</application> will load the specified
   engine, i.e. the <productname>OpenSSL</> module which supports special
   hardware, and reference the key with the specified identifier.
   Identifiers are engine-specific. Typically, cryptography hardware tokens
   do not reveal secret keys to the application. Instead, applications
   delegate all cryptography operations which require the secret key to
   the hardware token.
  </para>

  <para>
   If the file <filename>~/.postgresql/root.crt</> is present in the user's
   home directory,
   <application>libpq</application> will use the certificate list stored
   therein to verify the server's certificate.
   (On Microsoft Windows the file is named
   <filename>%APPDATA%\postgresql\root.crt</filename>.)
   The SSL connection will
   fail if the server does not present a certificate; therefore, to
   use this feature the server must have a <filename>server.crt</> file.
   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).
  </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.
  </para>


</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</></></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>
/*
 * testlibpq.c
 *
 *      Test the C version of libpq, the PostgreSQL frontend library.
 */
#include &lt;stdio.h&gt;
#include &lt;stdlib.h&gt;
#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 &gt; 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 &lt; nFields; i++)
        printf("%-15s", PQfname(res, i));
    printf("\n\n");

    /* next, print out the rows */
    for (i = 0; i &lt; PQntuples(res); i++)
    {
        for (j = 0; j &lt; 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>
/*
 * 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 &lt;stdio.h&gt;
#include &lt;stdlib.h&gt;
#include &lt;string.h&gt;
#include &lt;errno.h&gt;
#include &lt;sys/time.h&gt;
#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 &gt; 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 &lt; 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 &lt; 0)
            break;              /* shouldn't happen */

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

        if (select(sock + 1, &amp;input_mask, NULL, NULL, NULL) &lt; 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-&gt;relname, notify-&gt;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>
/*
 * 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 &lt;stdio.h&gt;
#include &lt;stdlib.h&gt;
#include &lt;string.h&gt;
#include &lt;sys/types.h&gt;
#include "libpq-fe.h"

/* for ntohl/htonl */
#include &lt;netinet/in.h&gt;
#include &lt;arpa/inet.h&gt;


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 &lt; 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 &lt; 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 &gt; 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 *) &amp;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>
