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  1. <?xml version="1.0" encoding="UTF-8" standalone="no"?>

  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>35.9. 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 V1.79.1" /><link rel="prev" href="ecpg-errors.html" title="35.8. Error Handling" /><link rel="next" href="ecpg-process.html" title="35.10. Processing Embedded SQL Programs" /></head><body><div xmlns="http://www.w3.org/TR/xhtml1/transitional" class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">35.9. Preprocessor Directives</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="ecpg-errors.html" title="35.8. Error Handling">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="ecpg.html" title="Chapter 35. ECPG - Embedded SQL in C">Up</a></td><th width="60%" align="center">Chapter 35. <span xmlns="http://www.w3.org/1999/xhtml" class="application">ECPG</span> - Embedded <acronym xmlns="http://www.w3.org/1999/xhtml" class="acronym">SQL</acronym> in C</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 12.4 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="ecpg-process.html" title="35.10. Processing Embedded SQL Programs">Next</a></td></tr></table><hr></hr></div><div class="sect1" id="ECPG-PREPROC"><div class="titlepage"><div><div><h2 class="title" style="clear: both">35.9. Preprocessor Directives</h2></div></div></div><div class="toc"><dl class="toc"><dt><span class="sect2"><a href="ecpg-preproc.html#ECPG-INCLUDE">35.9.1. Including Files</a></span></dt><dt><span class="sect2"><a href="ecpg-preproc.html#ECPG-DEFINE">35.9.2. The define and undef Directives</a></span></dt><dt><span class="sect2"><a href="ecpg-preproc.html#ECPG-IFDEF">35.9.3. ifdef, ifndef, else, elif, 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">35.9.1. Including Files</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">35.9.2. The define and undef Directives</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></div><div class="sect2" id="ECPG-IFDEF"><div class="titlepage"><div><div><h3 class="title">35.9.3. ifdef, ifndef, else, elif, and endif Directives</h3></div></div></div><p>

  81.    You can use the following directives to compile code sections conditionally:

  82. 

  83.    </p><div class="variablelist"><dl class="variablelist"><dt><span class="term"><code class="literal">EXEC SQL ifdef <em class="replaceable"><code>name</code></em>;</code></span></dt><dd><p>

  84.       Checks a <em class="replaceable"><code>name</code></em> and processes subsequent lines if

  85.       <em class="replaceable"><code>name</code></em> has been created with <code class="literal">EXEC SQL define

  86.       <em class="replaceable"><code>name</code></em></code>.

  87.      </p></dd><dt><span class="term"><code class="literal">EXEC SQL ifndef <em class="replaceable"><code>name</code></em>;</code></span></dt><dd><p>

  88.       Checks a <em class="replaceable"><code>name</code></em> and processes subsequent lines if

  89.       <em class="replaceable"><code>name</code></em> has <span class="emphasis"><em>not</em></span> been created with

  90.       <code class="literal">EXEC SQL define <em class="replaceable"><code>name</code></em></code>.

  91.      </p></dd><dt><span class="term"><code class="literal">EXEC SQL else;</code></span></dt><dd><p>

  92.       Starts processing an alternative section to a section introduced by

  93.       either <code class="literal">EXEC SQL ifdef <em class="replaceable"><code>name</code></em></code> or

  94.       <code class="literal">EXEC SQL ifndef <em class="replaceable"><code>name</code></em></code>.

  95.      </p></dd><dt><span class="term"><code class="literal">EXEC SQL elif <em class="replaceable"><code>name</code></em>;</code></span></dt><dd><p>

  96.       Checks <em class="replaceable"><code>name</code></em> and starts an alternative section if

  97.       <em class="replaceable"><code>name</code></em> has been created with <code class="literal">EXEC SQL define

  98.       <em class="replaceable"><code>name</code></em></code>.

  99.      </p></dd><dt><span class="term"><code class="literal">EXEC SQL endif;</code></span></dt><dd><p>

  100.       Ends an alternative section.

  101.      </p></dd></dl></div><p>

  102.    </p><p>

  103.     Example:

  104. </p><pre class="programlisting">

  105. EXEC SQL ifndef TZVAR;

  106. EXEC SQL SET TIMEZONE TO 'GMT';

  107. EXEC SQL elif TZNAME;

  108. EXEC SQL SET TIMEZONE TO TZNAME;

  109. EXEC SQL else;

  110. EXEC SQL SET TIMEZONE TO TZVAR;

  111. EXEC SQL endif;

  112. </pre><p>

  113.    </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">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="ecpg.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="ecpg-process.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">35.8. Error Handling </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> 35.10. Processing Embedded SQL Programs</td></tr></table></div></body></html>
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