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   2 <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>34.9. Preprocessor Directives</title><link rel="stylesheet" type="text/css" href="stylesheet.css" /><link rev="made" href="pgsql-docs@lists.postgresql.org" /><meta name="generator" content="DocBook XSL Stylesheets Vsnapshot" /><link rel="prev" href="ecpg-errors.html" title="34.8. Error Handling" /><link rel="next" href="ecpg-process.html" title="34.10. Processing Embedded SQL Programs" /></head><body id="docContent" class="container-fluid col-10"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">34.9. Preprocessor Directives</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="ecpg-errors.html" title="34.8. Error Handling">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="ecpg.html" title="Chapter 34. ECPG — Embedded SQL in C">Up</a></td><th width="60%" align="center">Chapter 34. <span class="application">ECPG</span> — Embedded <acronym class="acronym">SQL</acronym> in C</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 18.0 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="ecpg-process.html" title="34.10. Processing Embedded SQL Programs">Next</a></td></tr></table><hr /></div><div class="sect1" id="ECPG-PREPROC"><div class="titlepage"><div><div><h2 class="title" style="clear: both">34.9. Preprocessor Directives <a href="#ECPG-PREPROC" class="id_link">#</a></h2></div></div></div><div class="toc"><dl class="toc"><dt><span class="sect2"><a href="ecpg-preproc.html#ECPG-INCLUDE">34.9.1. Including Files</a></span></dt><dt><span class="sect2"><a href="ecpg-preproc.html#ECPG-DEFINE">34.9.2. The define and undef Directives</a></span></dt><dt><span class="sect2"><a href="ecpg-preproc.html#ECPG-IFDEF">34.9.3. ifdef, ifndef, elif, else, and endif Directives</a></span></dt></dl></div><p>
   3    Several preprocessor directives are available that modify how
   4    the <code class="command">ecpg</code> preprocessor parses and processes a
   5    file.
   6   </p><div class="sect2" id="ECPG-INCLUDE"><div class="titlepage"><div><div><h3 class="title">34.9.1. Including Files <a href="#ECPG-INCLUDE" class="id_link">#</a></h3></div></div></div><p>
   7     To include an external file into your embedded SQL program, use:
   8 </p><pre class="programlisting">
   9 EXEC SQL INCLUDE <em class="replaceable"><code>filename</code></em>;
  10 EXEC SQL INCLUDE &lt;<em class="replaceable"><code>filename</code></em>&gt;;
  11 EXEC SQL INCLUDE "<em class="replaceable"><code>filename</code></em>";
  12 </pre><p>
  13     The embedded SQL preprocessor will look for a file named
  14     <code class="literal"><em class="replaceable"><code>filename</code></em>.h</code>,
  15     preprocess it, and include it in the resulting C output.  Thus,
  16     embedded SQL statements in the included file are handled correctly.
  17    </p><p>
  18     The <code class="command">ecpg</code> preprocessor will search a file at
  19     several directories in following order:
  20
  21     </p><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">current directory</li><li class="listitem"><code class="filename">/usr/local/include</code></li><li class="listitem">PostgreSQL include directory, defined at build time (e.g., <code class="filename">/usr/local/pgsql/include</code>)</li><li class="listitem"><code class="filename">/usr/include</code></li></ul></div><p>
  22
  23     But when <code class="literal">EXEC SQL INCLUDE
  24     "<em class="replaceable"><code>filename</code></em>"</code> is used, only the
  25     current directory is searched.
  26    </p><p>
  27     In each directory, the preprocessor will first look for the file
  28     name as given, and if not found will append <code class="literal">.h</code>
  29     to the file name and try again (unless the specified file name
  30     already has that suffix).
  31    </p><p>
  32     Note that <code class="command">EXEC SQL INCLUDE</code> is <span class="emphasis"><em>not</em></span> the same as:
  33 </p><pre class="programlisting">
  34 #include &lt;<em class="replaceable"><code>filename</code></em>.h&gt;
  35 </pre><p>
  36     because this file would not be subject to SQL command preprocessing.
  37     Naturally, you can continue to use the C
  38     <code class="literal">#include</code> directive to include other header
  39     files.
  40    </p><div class="note"><h3 class="title">Note</h3><p>
  41      The include file name is case-sensitive, even though the rest of
  42      the <code class="literal">EXEC SQL INCLUDE</code> command follows the normal
  43      SQL case-sensitivity rules.
  44     </p></div></div><div class="sect2" id="ECPG-DEFINE"><div class="titlepage"><div><div><h3 class="title">34.9.2. The define and undef Directives <a href="#ECPG-DEFINE" class="id_link">#</a></h3></div></div></div><p>
  45     Similar to the directive <code class="literal">#define</code> that is known from C,
  46     embedded SQL has a similar concept:
  47 </p><pre class="programlisting">
  48 EXEC SQL DEFINE <em class="replaceable"><code>name</code></em>;
  49 EXEC SQL DEFINE <em class="replaceable"><code>name</code></em> <em class="replaceable"><code>value</code></em>;
  50 </pre><p>
  51     So you can define a name:
  52 </p><pre class="programlisting">
  53 EXEC SQL DEFINE HAVE_FEATURE;
  54 </pre><p>
  55     And you can also define constants:
  56 </p><pre class="programlisting">
  57 EXEC SQL DEFINE MYNUMBER 12;
  58 EXEC SQL DEFINE MYSTRING 'abc';
  59 </pre><p>
  60     Use <code class="literal">undef</code> to remove a previous definition:
  61 </p><pre class="programlisting">
  62 EXEC SQL UNDEF MYNUMBER;
  63 </pre><p>
  64    </p><p>
  65     Of course you can continue to use the C versions <code class="literal">#define</code>
  66     and <code class="literal">#undef</code> in your embedded SQL program. The difference
  67     is where your defined values get evaluated. If you use <code class="literal">EXEC SQL
  68     DEFINE</code> then the <code class="command">ecpg</code> preprocessor evaluates the defines and substitutes
  69     the values. For example if you write:
  70 </p><pre class="programlisting">
  71 EXEC SQL DEFINE MYNUMBER 12;
  72 ...
  73 EXEC SQL UPDATE Tbl SET col = MYNUMBER;
  74 </pre><p>
  75     then <code class="command">ecpg</code> will already do the substitution and your C compiler will never
  76     see any name or identifier <code class="literal">MYNUMBER</code>. Note that you cannot use
  77     <code class="literal">#define</code> for a constant that you are going to use in an
  78     embedded SQL query because in this case the embedded SQL precompiler is not
  79     able to see this declaration.
  80    </p><p>
  81     If multiple input files are named on the <code class="command">ecpg</code>
  82     preprocessor's command line, the effects of <code class="literal">EXEC SQL
  83     DEFINE</code> and <code class="literal">EXEC SQL UNDEF</code> do not carry
  84     across files: each file starts with only the symbols defined
  85     by <code class="option">-D</code> switches on the command line.
  86    </p></div><div class="sect2" id="ECPG-IFDEF"><div class="titlepage"><div><div><h3 class="title">34.9.3. ifdef, ifndef, elif, else, and endif Directives <a href="#ECPG-IFDEF" class="id_link">#</a></h3></div></div></div><p>
  87    You can use the following directives to compile code sections conditionally:
  88
  89    </p><div class="variablelist"><dl class="variablelist"><dt id="ECPG-IFDEF-IFDEF"><span class="term"><code class="literal">EXEC SQL ifdef <em class="replaceable"><code>name</code></em>;</code></span> <a href="#ECPG-IFDEF-IFDEF" class="id_link">#</a></dt><dd><p>
  90       Checks a <em class="replaceable"><code>name</code></em> and processes subsequent lines if
  91       <em class="replaceable"><code>name</code></em> has been defined via <code class="literal">EXEC SQL define
  92       <em class="replaceable"><code>name</code></em></code>.
  93      </p></dd><dt id="ECPG-IFDEF-IFNDEF"><span class="term"><code class="literal">EXEC SQL ifndef <em class="replaceable"><code>name</code></em>;</code></span> <a href="#ECPG-IFDEF-IFNDEF" class="id_link">#</a></dt><dd><p>
  94       Checks a <em class="replaceable"><code>name</code></em> and processes subsequent lines if
  95       <em class="replaceable"><code>name</code></em> has <span class="emphasis"><em>not</em></span> been defined via
  96       <code class="literal">EXEC SQL define <em class="replaceable"><code>name</code></em></code>.
  97      </p></dd><dt id="ECPG-IFDEF-ELIF"><span class="term"><code class="literal">EXEC SQL elif <em class="replaceable"><code>name</code></em>;</code></span> <a href="#ECPG-IFDEF-ELIF" class="id_link">#</a></dt><dd><p>
  98       Begins an optional alternative section after an
  99       <code class="literal">EXEC SQL ifdef <em class="replaceable"><code>name</code></em></code> or
 100       <code class="literal">EXEC SQL ifndef <em class="replaceable"><code>name</code></em></code>
 101       directive.  Any number of <code class="literal">elif</code> sections can appear.
 102       Lines following an <code class="literal">elif</code> will be processed
 103       if <em class="replaceable"><code>name</code></em> has been
 104       defined <span class="emphasis"><em>and</em></span> no previous section of the same
 105       <code class="literal">ifdef</code>/<code class="literal">ifndef</code>...<code class="literal">endif</code>
 106       construct has been processed.
 107      </p></dd><dt id="ECPG-IFDEF-ELSE"><span class="term"><code class="literal">EXEC SQL else;</code></span> <a href="#ECPG-IFDEF-ELSE" class="id_link">#</a></dt><dd><p>
 108       Begins an optional, final alternative section after an
 109       <code class="literal">EXEC SQL ifdef <em class="replaceable"><code>name</code></em></code> or
 110       <code class="literal">EXEC SQL ifndef <em class="replaceable"><code>name</code></em></code>
 111       directive.  Subsequent lines will be processed if no previous section
 112       of the same
 113       <code class="literal">ifdef</code>/<code class="literal">ifndef</code>...<code class="literal">endif</code>
 114       construct has been processed.
 115      </p></dd><dt id="ECPG-IFDEF-ENDIF"><span class="term"><code class="literal">EXEC SQL endif;</code></span> <a href="#ECPG-IFDEF-ENDIF" class="id_link">#</a></dt><dd><p>
 116       Ends an
 117       <code class="literal">ifdef</code>/<code class="literal">ifndef</code>...<code class="literal">endif</code>
 118       construct.  Subsequent lines are processed normally.
 119      </p></dd></dl></div><p>
 120    </p><p>
 121     <code class="literal">ifdef</code>/<code class="literal">ifndef</code>...<code class="literal">endif</code>
 122     constructs can be nested, up to 127 levels deep.
 123    </p><p>
 124     This example will compile exactly one of the three <code class="literal">SET
 125     TIMEZONE</code> commands:
 126 </p><pre class="programlisting">
 127 EXEC SQL ifdef TZVAR;
 128 EXEC SQL SET TIMEZONE TO TZVAR;
 129 EXEC SQL elif TZNAME;
 130 EXEC SQL SET TIMEZONE TO TZNAME;
 131 EXEC SQL else;
 132 EXEC SQL SET TIMEZONE TO 'GMT';
 133 EXEC SQL endif;
 134 </pre><p>
 135    </p></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="ecpg-errors.html" title="34.8. Error Handling">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="ecpg.html" title="Chapter 34. ECPG — Embedded SQL in C">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="ecpg-process.html" title="34.10. Processing Embedded SQL Programs">Next</a></td></tr><tr><td width="40%" align="left" valign="top">34.8. Error Handling </td><td width="20%" align="center"><a accesskey="h" href="index.html" title="PostgreSQL 18.0 Documentation">Home</a></td><td width="40%" align="right" valign="top"> 34.10. Processing Embedded SQL Programs</td></tr></table></div></body></html>