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$Header: /cvsroot/pgsql/doc/src/sgml/ecpg.sgml,v 1.29 2001/11/08 23:37:50 petere Exp $
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 <chapter id="ecpg">
  <docinfo>
   <authorgroup>
    <author>
     <firstname>Linus</firstname>
     <surname>Tolke</surname>
    </author>
    <author>
     <firstname>Michael</firstname>
     <surname>Meskes</surname>
    </author>
   </authorgroup>
   <copyright>
    <year>1996-1997</year>
    <holder>Linus Tolke</holder>
   </copyright>
   <copyright>
    <year>1998</year>
    <holder>Michael Meskes</holder>
   </copyright>
   <date>Transcribed 1998-02-12</date>
  </docinfo>

  <title><application>ecpg</application> - Embedded <acronym>SQL</acronym> 
   in <acronym>C</acronym></title>

  <para>
   This describes the embedded <acronym>SQL</acronym> package for
   <productname>Postgres</productname>. It works with
   <acronym>C</acronym> and <acronym>C++</acronym>. It was written by
   Linus Tolke (<email>linus@epact.se</email>) and Michael Meskes
   (<email>meskes@debian.org</email>). The package is installed with the
   <productname>Postgres</> distribution, and carries a similar license.
  </para>

  <sect1 id="ecpg-why">
   <title>Why Embedded <acronym>SQL</acronym>?</title>

   <para>
    Embedded <acronym>SQL</acronym> has advantages over other methods
    for handling <acronym>SQL</acronym> queries. It takes care of
    the tedious passing of information to and from variables in your
    <acronym>C</acronym> or <acronym>C++</acronym> program. Many
    <acronym>RDBMS</acronym> packages support this embedded language.
   </para>

   <para> 
    There is an ANSI standard describing how the embedded language
    should work. <application>ecpg</application> was designed to match
    this standard as much as possible. It is possible to port embedded
    <acronym>SQL</acronym> programs written for other
    <acronym>RDBMS</acronym> to <productname>Postgres</productname>.
   </para>
  </sect1>

  <sect1 id="ecpg-concept">
   <title>The Concept</title>

   <para>
    You write your program in <acronym>C/C++</acronym> with special
    <acronym>SQL</acronym> constructs. When declaring variables to be
    used in <acronym>SQL</acronym> statements, you need to put them in a
    special <command>declare</> section. You use a special syntax for the
    <acronym>SQL</acronym> queries.
   </para>

   <para>
    Before compiling you run the file through the embedded
    <acronym>SQL</acronym> <acronym>C</acronym> preprocessor and it
    converts the <acronym>SQL</acronym> statements you used to function
    calls with the variables used as arguments. Both query input and
    result output variables are passed.
   </para>

   <para>
    After compiling, you must link with a special library that contains
    needed functions. These functions fetch information from the
    arguments, perform the <acronym>SQL</acronym> query using the
    <filename>libpq</filename> interface, and put the result in the
    arguments specified for output.
   </para>
  </sect1>

  <sect1 id="ecpg-use">
   <title>How To Use <application>ecpg</application></title>

   <para>
    This section describes how to use <application>ecpg</application>.
   </para>

   <sect2>
    <title>Preprocessor</title>

    <para>
     The preprocessor is called <application>ecpg</application>. After
     installation it resides in the <productname>Postgres</productname>
     <filename>bin/</filename> directory.
    </para>
   </sect2>
   <sect2>
    <title>Library</title>

    <para>
     The <application>ecpg</application> library is called
     <filename>libecpg.a</filename> or <filename>libecpg.so</filename>.
     Additionally, the library uses the <filename>libpq</filename>
     library for communication to the
     <productname>Postgres</productname> server. You will have to link
     your program using <parameter>-lecpg -lpq</parameter>.
    </para>

    <para>
     The library has some methods that are <quote>hidden</quote> but may prove
     useful.

     <itemizedlist>
      <listitem>
       <para>
        <function>ECPGdebug(int <replaceable>on</replaceable>, FILE
        *<replaceable>stream</replaceable>)</function> turns on debug
        logging if called with the first argument non-zero. Debug
        logging is done on <replaceable>stream</replaceable>. Most
        <acronym>SQL</acronym> statement log their arguments and results.
       </para>

       <para>
        The most important function , <function>ECPGdo</function>, logs
        all <acronym>SQL</acronym> statements with both the expanded
        string, i.e. the string with all the input variables inserted,
        and the result from the <productname>Postgres</productname>
        server. This can be very useful when searching for errors in
        your <acronym>SQL</acronym> statements.
       </para>
      </listitem>

      <listitem>
       <para>
        <function>ECPGstatus()</function>
        This method returns TRUE if we are connected to a database and FALSE if not.
       </para>
      </listitem>
     </itemizedlist>
    </para>
   </sect2>

   <sect2>
    <title>Error handling</title>

    <para>
     To detect errors from the <productname>Postgres</productname>
     server, include a line like: 
<programlisting>
exec sql include sqlca; 
</programlisting>
     in the include section of your file. This will define a struct and
     a variable with the name <parameter>sqlca</parameter> as follows:
<programlisting>
struct sqlca
{
 char sqlcaid[8];
 long sqlabc;
 long sqlcode;
 struct
 {
  int sqlerrml;
  char sqlerrmc[70];
 } sqlerrm;
 char sqlerrp[8];
 long sqlerrd[6];
 /* 0: empty                                         */
 /* 1: OID of processed tuple if applicable          */
 /* 2: number of rows processed in an INSERT, UPDATE */
 /*    or DELETE statement                           */
 /* 3: empty                                         */
 /* 4: empty                                         */
 /* 5: empty                                         */
 char sqlwarn[8];
 /* 0: set to 'W' if at least one other is 'W'       */
 /* 1: if 'W' at least one character string          */
 /*    value was truncated when it was               */
 /*    stored into a host variable.                  */
 /* 2: empty                                         */
 /* 3: empty                                         */
 /* 4: empty                                         */
 /* 5: empty                                         */
 /* 6: empty                                         */
 /* 7: empty                                         */
 char sqlext[8];
} sqlca;
</programlisting>
    </para>

    <para>
     If an no error occurred in the last <acronym>SQL</acronym> statement.
     <parameter>sqlca.sqlcode</parameter> will be 0 (ECPG_NO_ERROR). If
     <parameter>sqlca.sqlcode</parameter> is less that zero, this is a
     serious error, like the database definition does not match the
     query. If it is greater than zero, it is a normal error like the
     table did not contain the requested row.
    </para>

    <para>
     <parameter>sqlca.sqlerrm.sqlerrmc</parameter> will contain a string
     that describes the error. The string ends with the line number in
     the source file.
    </para>

    <para>
     These are the errors that can occur:

     <variablelist>
      <varlistentry>
       <term><computeroutput>-12, Out of memory in line %d.</computeroutput></term>
       <listitem>
        <para>
         Should not normally occur. This indicates your virtual memory is
         exhausted.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term><computeroutput>-200 (ECPG_UNSUPPORTED): Unsupported type %s on line %d.</computeroutput></term>
       <listitem>
        <para>
         Should not normally occur. This indicates the preprocessor has
         generated something that the library does not know about.
         Perhaps you are running incompatible versions of the
         preprocessor and the library.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term><computeroutput>-201 (ECPG_TOO_MANY_ARGUMENTS): Too many arguments line %d.</computeroutput></term>
       <listitem>
        <para>
         This means that <productname>Postgres</productname> has
         returned more arguments than we have matching variables.
         Perhaps you have forgotten a couple of the host variables in
         the <command>INTO :var1,:var2</command>-list.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term><computeroutput>-202 (ECPG_TOO_FEW_ARGUMENTS): Too few arguments line %d.</computeroutput></term>
       <listitem>
        <para>
         This means that <productname>Postgres</productname> has
         returned fewer arguments than we have host variables. Perhaps
         you have too many host variables in the <command>INTO
         :var1,:var2</command>-list.
        </para> 
       </listitem>
      </varlistentry>

      <varlistentry>
       <term><computeroutput>-203 (ECPG_TOO_MANY_MATCHES): Too many matches line %d.</computeroutput></term>
       <listitem>
        <para>
         This means the query has returned several rows but the
         variables specified are not arrays. The
         <command>SELECT</command> command was not unique.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term><computeroutput>-204 (ECPG_INT_FORMAT): Not correctly formatted int type: %s line %d.</computeroutput></term>
       <listitem>
        <para>
         This means the host variable is of type <type>int</type> and
         the field in the <productname>Postgres</productname> database
         is of another type and contains a value that cannot be
         interpreted as an <type>int</type>. The library uses
         <function>strtol()</function> for this conversion.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term><computeroutput>-205 (ECPG_UINT_FORMAT): Not correctly formatted unsigned type: %s line %d.</computeroutput></term>
       <listitem>
        <para>
         This means the host variable is of type <type>unsigned
         int</type> and the field in the
         <productname>Postgres</productname> database is of another type
         and contains a value that cannot be interpreted as an
         <type>unsigned int</type>. The library uses
         <function>strtoul()</function> for this conversion.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term><computeroutput>-206 (ECPG_FLOAT_FORMAT): Not correctly formatted floating point type: %s line %d.</computeroutput></term>
       <listitem>
        <para>
         This means the host variable is of type <type>float</type> and
         the field in the <productname>Postgres</productname> database
         is of another type and contains a value that cannot be
         interpreted as a <type>float</type>. The library uses
         <function>strtod()</function> for this conversion.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term><computeroutput>-207 (ECPG_CONVERT_BOOL): Unable to convert %s to bool on line %d.</computeroutput></term>
       <listitem>
        <para>
         This means the host variable is of type <type>bool</type> and
         the field in the <productname>Postgres</productname> database
         is neither <literal>'t'</> nor <literal>'f'</>.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term><computeroutput>-208 (ECPG_EMPTY): Empty query line %d.</computeroutput></term>
       <listitem>
        <para>
         <productname>Postgres</productname> returned <symbol>PGRES_EMPTY_QUERY</symbol>, probably
         because the query indeed was empty.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term><computeroutput>-209 (ECPG_MISSING_INDICATOR): NULL value without indicator in line %d.</computeroutput></term>
       <listitem>
        <para>
         <productname>Postgres</productname> returned <symbol>ECPG_MISSING_INDICATOR</symbol>
         because a NULL was returned and no NULL indicator variable was supplied.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term><computeroutput>-210 (ECPG_NO_ARRAY): Variable is not an array in line %d.</computeroutput></term>
       <listitem>
        <para>
         <productname>Postgres</productname> returned <symbol>ECPG_NO_ARRAY</symbol>
         because an ordinary variable was used in a place that requires
         an array.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term><computeroutput>-211 (ECPG_DATA_NOT_ARRAY): Data read from backend is not an array in line %d.</computeroutput></term>
       <listitem>
        <para>
         <productname>Postgres</productname> returned <symbol>ECPG_DATA_NOT_ARRAY</symbol>
         because the database returned an ordinary variable in a place
         that requires array value.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term><computeroutput>-220 (ECPG_NO_CONN): No such connection %s in line %d.</computeroutput></term>
       <listitem>
        <para>
         The program tried to access a connection that does not exist.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term><computeroutput>-221 (ECPG_NOT_CONN): Not connected in line %d.</computeroutput></term>
       <listitem>
        <para>
         The program tried to access a connection that does exist but is
         not open.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term><computeroutput>-230 (ECPG_INVALID_STMT): Invalid statement name %s in line %d.</computeroutput></term>
       <listitem>
        <para>
         The statement you are trying to use has not been prepared.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term><computeroutput>-240 (ECPG_UNKNOWN_DESCRIPTOR): Descriptor %s not found in line %d.</computeroutput></term>
       <listitem>
        <para>
         The descriptor specified was not foundstatement you are trying to use has not been prepared.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term><computeroutput>-241 (ECPG_INVALID_DESCRIPTOR_INDEX): Descriptor index out of range in line %d.</computeroutput></term>
       <listitem>
        <para>
         The descriptor index specified was out of range.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term><computeroutput>-242 (ECPG_UNKNOWN_DESCRIPTOR_ITEM): Descriptor %s not found in line %d.</computeroutput></term>
       <listitem>
        <para>
         The descriptor specified was not foundstatement you are trying to use has not been prepared.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term><computeroutput>-243 (ECPG_VAR_NOT_NUMERIC): Variable is not a numeric type in line %d.</computeroutput></term>
       <listitem>
        <para>
         The database returned a numeric value and the variable was not
         numeric.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term><computeroutput>-244 (ECPG_VAR_NOT_CHAR): Variable is not a character type in line %d.</computeroutput></term>
       <listitem>
        <para>
         The database returned a non-numeric value and the variable was
         numeric.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term><computeroutput>-400 (ECPG_PGSQL): Postgres error: %s line %d.</computeroutput></term>
       <listitem>
        <para>
         Some <productname>Postgres</productname> error. 
         The message contains the error message from the
         <productname>Postgres</productname> backend.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term><computeroutput>-401 (ECPG_TRANS): Error in transaction processing line %d.</computeroutput></term>
       <listitem>
        <para>
         <productname>Postgres</productname> signaled that we cannot start,
         commit or rollback the transaction.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term><computeroutput>-402 (ECPG_CONNECT): Could not connect to database %s in line %d.</computeroutput></term>
       <listitem>
        <para>
         The connect to the database did not work.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term><computeroutput>100 (ECPG_NOT_FOUND): Data not found line %d.</computeroutput></term>
       <listitem>
        <para>
         This is a <quote>normal</quote> error that tells you that what you are querying cannot
         be found or you are at the end of the cursor.
        </para>
       </listitem>
      </varlistentry>

     </variablelist>
    </para>
   </sect2>
  </sect1>

  <sect1 id="ecpg-limitations">
   <title>Limitations</title>

   <para>
    What will never be included and why it cannot be done:

    <variablelist>
     <varlistentry>
      <term>Oracle's single tasking</term>
      <listitem>
       <para>
        Oracle version 7.0 on <systemitem class="osname">AIX</> 3 uses OS-supported locks in shared
        memory that allow an application designer to link an application
        in a <quote>single tasking</quote> way. Instead of starting one client
        process per application process, both the database part and the
        application part run in the same process. In later versions of
        Oracle this is no longer supported.
       </para>

       <para>
        This would require a total redesign of the
        <productname>Postgres</productname> access model and the
        performance gain does not justify the effort.
       </para>
      </listitem>
     </varlistentry>
    </variablelist>
   </para>
  </sect1>

  <sect1 id="ecpg-porting">
   <title>Porting From Other <acronym>RDBMS</acronym> Packages</title>

   <para>
    The design of <application>ecpg</application> follows the SQL
    standard. Porting from a standard RDBMS should not be a problem.
    Unfortunately there is no such thing as a standard RDBMS. Therefore
    <application>ecpg</application> tries to understand syntax
    extensions as long as they do not create conflicts with the
    standard.
   </para>

   <para>
    The following list shows all the known incompatibilities. If you
    find one not listed please notify the developers. Note, however, that
    we list only incompatibilities from a precompiler of another RDBMS
    to <application>ecpg</application> and not
    <application>ecpg</application> features that these RDBMS do not
    support.
   </para>

   <para>
    <variablelist>
     <varlistentry>
      <term>Syntax of FETCH</term>
      <listitem>
       <para>
        The standard syntax for FETCH is:
       </para>
       <para>
        FETCH [direction] [amount] IN|FROM <replaceable>cursor</replaceable>.
       </para>
       <para>
        <application>ORACLE</application>, however, does not use the keywords IN
        or FROM. This feature cannot be added since it would create parsing
        conflicts.
       </para>
      </listitem>
     </varlistentry>
    </variablelist>
   </para>
  </sect1>

  <sect1 id="ecpg-develop">
   <title>For the Developer</title>

   <para>
    This section explain how <application>ecpg</application>
    works internally. It contains valuable information to help users
    understand how to use <application>ecpg</application>.
   </para>

   <sect2>
    <title>ToDo List</title>

    <para>
     This version of the preprocessor has some flaws:

     <variablelist>
      <varlistentry>
       <term>Library functions</term>
       <listitem>
        <para>
         to_date et al. does not exist. However,
         <productname>Postgres</productname> has some good conversion
         routines so you probably won't miss them.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Structures and unions</term>
       <listitem>
        <para>
         Structures and unions have to be defined in the
         <command>declare</> section.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Missing statements</term>
       <listitem>
        <para>
         The following statements are not implemented thus far:

         <itemizedlist>
          <listitem>
           <para>
            <literal>exec sql allocate</>
           </para>
          </listitem>

          <listitem>
           <para>
            <literal>exec sql deallocate</>
           </para>
          </listitem>
 
          <listitem>
           <para>
            <literal>SQLSTATE</>
           </para>
          </listitem>
         </itemizedlist>
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>message 'no data found'</term>
       <listitem>
        <para>
         The error message for <quote>no data</quote> in:
         <programlisting>
exec sql insert select from statement
        </programlisting>
         has to be <literal>100</>.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term><literal>sqlwarn[6]</literal></term>
       <listitem>
        <para>
         <literal>sqlwarn[6]</literal> should be <literal>W</> if the <command>PRECISION</>
         or <command>SCALE</> value specified in a <command>SET
         DESCRIPTOR</> statement was ignored.
        </para>
       </listitem>
      </varlistentry>
     </variablelist>
    </para>
   </sect2>

   <sect2>
    <title>The Preprocessor</title>

    <para>
     The first four lines written by <command>ecpg</command> to the output are fixed lines.
     Two are comments and two are include lines necessary to interface
     to the library.
    </para>

    <para>
     Then the preprocessor reads through the file and writes output.
     Normally it just echoes everything to the output.
    </para>

    <para>
     When it sees an <command>EXEC SQL</command> statement, it
     intervenes and changes it. The <command>EXEC SQL</command>
     statement can be one of these:

     <variablelist>
      <varlistentry>
       <term>Declare sections</term>
       <listitem>
        <para>
         <command>Declare</> sections begin with:
<programlisting>
exec sql begin declare section;
</programlisting>
         and end with:
<programlisting>
exec sql end declare section;
</programlisting>
         In this section only variable declarations are allowed. Every
         variable declared within this section is stored in a list
         of variables indexed by name together with its corresponding
         type.
        </para>

        <para>
         In particular the definition of a structure or union also must
         be listed inside a <command>declare</> section. Otherwise
         <application>ecpg</application> cannot handle these types since
         it does not know the definition.
        </para>

        <para>
         The declaration is also echoed to the file to make it a normal
         C variable.
        </para>

        <para>
         The special types <type>VARCHAR</type> and <type>VARCHAR2</type> are converted into a named struct
         for every variable. A declaration like:
<programlisting>
VARCHAR var[180];
</programlisting>
         is converted into:
<programlisting>
struct varchar_var { int len; char arr[180]; } var;
</programlisting>
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Include statements</term>
       <listitem>
        <para>
         An include statement looks like:
<programlisting>
exec sql include filename;
</programlisting>
         Note that this is NOT the same as:
<programlisting>
#include &lt;filename.h&gt;
</programlisting>
        </para>

        <para>
         Instead the file specified is parsed by
         <application>ecpg</application> so the contents of the file are
         included in the resulting C code. This way you are able to
         specify EXEC SQL commands in an include file.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Connect statement</term>
       <listitem>
        <para>
         A connect statement looks like:
<programlisting>
exec sql connect to <replaceable>connection target</replaceable>;
</programlisting>
         It creates a connection to the specified database.
        </para>

        <para>
         The <replaceable>connection target</replaceable> can be specified in the
         following ways:
         <itemizedlist>
          <listitem>
           <simpara>
            <literal>dbname[@server][:port][as <replaceable>connection
            name</replaceable>][user <replaceable>user name</replaceable>]</literal>
           </simpara>
          </listitem>

          <listitem>
           <simpara>
            <literal>tcp:postgresql://server[:port][/dbname][as
            <replaceable>connection name</replaceable>][user <replaceable>user name</replaceable>]</literal>
           </simpara>
          </listitem>

          <listitem>
           <simpara>
            <literal>unix:postgresql://server[:port][/dbname][as
            <replaceable>connection name</replaceable>][user <replaceable>user name</replaceable>]</literal>
           </simpara>
          </listitem>
 
          <listitem>
           <simpara>
            <literal><replaceable>character variable</replaceable>[as
            <replaceable>connection name</replaceable>][user <replaceable>user name</replaceable>]</literal>
           </simpara>
          </listitem>
 
          <listitem>
           <simpara>
            <literal><replaceable>character string</replaceable>[as
            <replaceable>connection name</replaceable>][<replaceable>user</replaceable>]</literal>
           </simpara>
          </listitem>
 
          <listitem>
           <simpara>
            <literal>default</literal>
           </simpara>
          </listitem>
 
          <listitem>
           <simpara>
            <literal>user</literal>
           </simpara>
          </listitem>
         </itemizedlist>
        </para>

        <para>
         There are also different ways to specify the user name:

         <itemizedlist>
          <listitem>
           <simpara>
            <literal><replaceable>userid</replaceable></literal>
           </simpara>
          </listitem>

          <listitem>
           <simpara>
            <literal><replaceable>userid</replaceable>/<replaceable>password</replaceable></literal>
           </simpara>
          </listitem>

          <listitem>
           <simpara>
            <literal><replaceable>userid</replaceable> identified by <replaceable>password</replaceable></literal>
           </simpara>
          </listitem>

          <listitem>
           <simpara>
            <literal><replaceable>userid</replaceable> using <replaceable>password</replaceable></literal>
           </simpara>
          </listitem>
         </itemizedlist>
        </para>

        <para> 
         Finally, the <replaceable>userid</replaceable> and <replaceable>password</replaceable> may be a constant text, a
         character variable, or a character string.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Disconnect statements</term>
       <listitem>
        <para>
         A disconnect statement looks like:
         <programlisting>
exec sql disconnect [<replaceable>connection target</replaceable>];
         </programlisting>
         It closes the connection to the specified database.
        </para>

        <para>
         The <replaceable>connection target</replaceable> can be specified in the
         following ways:

         <itemizedlist>
          <listitem>
           <simpara>
            <literal><replaceable>connection name</replaceable></literal>
           </simpara>
          </listitem>

          <listitem>
           <simpara>
           <literal>default</literal>
           </simpara>
          </listitem>

          <listitem>
           <simpara>
           <literal>current</literal>
           </simpara>
          </listitem>

          <listitem>
           <simpara>
           <literal>all</literal>
           </simpara>
          </listitem>
         </itemizedlist>
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Open cursor statement</term>
       <listitem>
        <para>
         An open cursor statement looks like:
<programlisting>
exec sql open <replaceable>cursor</replaceable>;
</programlisting>
         and is not copied to the output. Instead, the cursor's
         <command>DECLARE</> command is used because it opens the cursor
         as well.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Commit statement</term>
       <listitem>
        <para>
         A commit statement looks like:
<programlisting>
exec sql commit;
</programlisting>
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Rollback statement</term>
       <listitem>
        <para>
         A rollback statement looks like:
<programlisting>
exec sql rollback;
</programlisting>
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Other statements</term>
       <listitem>
        <para>
         Other <acronym>SQL</acronym> statements are used by
         starting with <command>exec sql</command> and ending with
         <command>;</command>. Everything in between is treated as an
         <acronym>SQL</acronym> statement and parsed for variable
         substitution.
        </para>

        <para>
         Variable substitution occurs when a symbol starts with a colon
         (<command>:</command>). The variable with that name is looked
         up among the variables that were previously declared within a
         <command>declare</> section. Depending on whether the variable is
         being use for input or output, a pointer to the variable is
         output to allow access by the function.
        </para>

        <para>
         For every variable that is part of the <acronym>SQL</acronym>
         query, the function gets other arguments:

         <itemizedlist>
          <listitem>
           <para>
            The type as a special symbol.
           </para>
          </listitem>

          <listitem>
           <para> 
            A pointer to the value or a pointer to the pointer.
           </para>
          </listitem>

          <listitem>
           <para>
            The size of the variable if it is a <type>char</type> or <type>varchar</type>.
           </para>
          </listitem>

          <listitem>
           <para>
            The number of elements in the array (for array fetches).
           </para>
          </listitem>

          <listitem>
           <para>
            The offset to the next element in the array (for array fetches).
           </para>
          </listitem>

          <listitem>
           <para>
            The type of the indicator variable as a special symbol.
           </para>
          </listitem>

          <listitem>
           <para>
            A pointer to the value of the indicator variable or a pointer to the pointer of the indicator variable.
           </para>
          </listitem>

          <listitem>
           <para>
            0.
           </para>
          </listitem>

          <listitem>
           <para>
            Number of elements in the indicator array (for array fetches).
           </para>
          </listitem>

          <listitem>
           <para>
            The offset to the next element in the indicator array (for
            array fetches).
           </para>
          </listitem>
         </itemizedlist>
        </para>

       </listitem>
      </varlistentry>
     </variablelist>
    </para>
   </sect2>

   <sect2>
    <title>A Complete Example</title>

    <para>
     Here is a complete example describing the output of the preprocessor of a
     file <filename>foo.pgc</filename>:
<programlisting>
exec sql begin declare section;
int index;
int result;
exec sql end declare section;
...
exec sql select res into :result from mytable where index = :index;
</programlisting>
     is translated into:
<programlisting>
/* Processed by ecpg (2.6.0) */
/* These two include files are added by the preprocessor */
#include &lt;ecpgtype.h&gt;;
#include &lt;ecpglib.h&gt;;

/* exec sql begin declare section */

#line 1 "foo.pgc"

 int index;
 int result;
/* exec sql end declare section */
...
ECPGdo(__LINE__, NULL, "select  res  from mytable where index = ?     ",
        ECPGt_int,&amp;(index),1L,1L,sizeof(int),
        ECPGt_NO_INDICATOR, NULL , 0L, 0L, 0L, ECPGt_EOIT,
        ECPGt_int,&amp;(result),1L,1L,sizeof(int),
        ECPGt_NO_INDICATOR, NULL , 0L, 0L, 0L, ECPGt_EORT);
#line 147 "foo.pgc"
</programlisting>
     (The indentation in this manual is added for readability and not
     something the preprocessor does.)
    </para>
   </sect2>

   <sect2>
    <title>The Library</title>

    <para>
     The most important function in the library is
     <function>ECPGdo</function>. It takes a variable number of
     arguments. Hopefully there are no computers that limit the
     number of variables that can be accepted by a <function>varargs()</function> function. This
     can easily add up to 50 or so arguments.
    </para>

    <para>
     The arguments are:

     <variablelist>
      <varlistentry>
       <term>A line number</term>
       <listitem>
        <para>
         This is a line number of the original line; used in error messages only.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>A string</term>
       <listitem>
        <para>
         This is the <acronym>SQL</acronym> query that is to be issued.
         It is modified by the input variables, i.e. the variables that
         where not known at compile time but are to be entered in the
         query. Where the variables should go the string contains
         <literal>?</literal>.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Input variables</term>
       <listitem>
        <para>
         As described in the section about the preprocessor, every input variable
         gets ten arguments.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term><literal>ECPGt_EOIT</literal></term>
       <listitem>
        <para>
         An enum telling that there are no more input variables.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>Output variables</term>
       <listitem>
        <para>
         As described in the section about the preprocessor, every input variable
         gets ten arguments. These variables are filled by the function.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term>ECPGt_EORT</term>
       <listitem>
        <para>
         An enum telling that there are no more variables.
        </para>
       </listitem>
      </varlistentry>
     </variablelist>
    </para>

    <para>
     All <acronym>SQL</acronym> statements are performed in one
     transaction unless you issue a commit transaction. To accomplish
     this auto-transaction behavior, the first statement and the first
     statement after a commit or rollback always begins a new transaction. To
     disable this feature, use the <option>-t</option> command-line
     option.
    </para>

   </sect2>
  </sect1>
 </chapter>

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