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authorDaniel Baumann <daniel.baumann@progress-linux.org>2024-04-16 19:46:48 +0000
committerDaniel Baumann <daniel.baumann@progress-linux.org>2024-04-16 19:46:48 +0000
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Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>
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+<!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>36.9. 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="36.8. Error Handling" /><link rel="next" href="ecpg-process.html" title="36.10. 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">36.9. Preprocessor Directives</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="ecpg-errors.html" title="36.8. Error Handling">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="ecpg.html" title="Chapter 36. ECPG — Embedded SQL in C">Up</a></td><th width="60%" align="center">Chapter 36. <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 15.4 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="ecpg-process.html" title="36.10. 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">36.9. Preprocessor Directives</h2></div></div></div><div class="toc"><dl class="toc"><dt><span class="sect2"><a href="ecpg-preproc.html#ECPG-INCLUDE">36.9.1. Including Files</a></span></dt><dt><span class="sect2"><a href="ecpg-preproc.html#ECPG-DEFINE">36.9.2. The define and undef Directives</a></span></dt><dt><span class="sect2"><a href="ecpg-preproc.html#ECPG-IFDEF">36.9.3. ifdef, ifndef, elif, else, and endif Directives</a></span></dt></dl></div><p>
+ Several preprocessor directives are available that modify how
+ the <code class="command">ecpg</code> preprocessor parses and processes a
+ file.
+ </p><div class="sect2" id="ECPG-INCLUDE"><div class="titlepage"><div><div><h3 class="title">36.9.1. Including Files</h3></div></div></div><p>
+ To include an external file into your embedded SQL program, use:
+</p><pre class="programlisting">
+EXEC SQL INCLUDE <em class="replaceable"><code>filename</code></em>;
+EXEC SQL INCLUDE &lt;<em class="replaceable"><code>filename</code></em>&gt;;
+EXEC SQL INCLUDE "<em class="replaceable"><code>filename</code></em>";
+</pre><p>
+ The embedded SQL preprocessor will look for a file named
+ <code class="literal"><em class="replaceable"><code>filename</code></em>.h</code>,
+ preprocess it, and include it in the resulting C output. Thus,
+ embedded SQL statements in the included file are handled correctly.
+ </p><p>
+ The <code class="command">ecpg</code> preprocessor will search a file at
+ several directories in following order:
+
+ </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>
+
+ But when <code class="literal">EXEC SQL INCLUDE
+ "<em class="replaceable"><code>filename</code></em>"</code> is used, only the
+ current directory is searched.
+ </p><p>
+ In each directory, the preprocessor will first look for the file
+ name as given, and if not found will append <code class="literal">.h</code>
+ to the file name and try again (unless the specified file name
+ already has that suffix).
+ </p><p>
+ Note that <code class="command">EXEC SQL INCLUDE</code> is <span class="emphasis"><em>not</em></span> the same as:
+</p><pre class="programlisting">
+#include &lt;<em class="replaceable"><code>filename</code></em>.h&gt;
+</pre><p>
+ because this file would not be subject to SQL command preprocessing.
+ Naturally, you can continue to use the C
+ <code class="literal">#include</code> directive to include other header
+ files.
+ </p><div class="note"><h3 class="title">Note</h3><p>
+ The include file name is case-sensitive, even though the rest of
+ the <code class="literal">EXEC SQL INCLUDE</code> command follows the normal
+ SQL case-sensitivity rules.
+ </p></div></div><div class="sect2" id="ECPG-DEFINE"><div class="titlepage"><div><div><h3 class="title">36.9.2. The define and undef Directives</h3></div></div></div><p>
+ Similar to the directive <code class="literal">#define</code> that is known from C,
+ embedded SQL has a similar concept:
+</p><pre class="programlisting">
+EXEC SQL DEFINE <em class="replaceable"><code>name</code></em>;
+EXEC SQL DEFINE <em class="replaceable"><code>name</code></em> <em class="replaceable"><code>value</code></em>;
+</pre><p>
+ So you can define a name:
+</p><pre class="programlisting">
+EXEC SQL DEFINE HAVE_FEATURE;
+</pre><p>
+ And you can also define constants:
+</p><pre class="programlisting">
+EXEC SQL DEFINE MYNUMBER 12;
+EXEC SQL DEFINE MYSTRING 'abc';
+</pre><p>
+ Use <code class="literal">undef</code> to remove a previous definition:
+</p><pre class="programlisting">
+EXEC SQL UNDEF MYNUMBER;
+</pre><p>
+ </p><p>
+ Of course you can continue to use the C versions <code class="literal">#define</code>
+ and <code class="literal">#undef</code> in your embedded SQL program. The difference
+ is where your defined values get evaluated. If you use <code class="literal">EXEC SQL
+ DEFINE</code> then the <code class="command">ecpg</code> preprocessor evaluates the defines and substitutes
+ the values. For example if you write:
+</p><pre class="programlisting">
+EXEC SQL DEFINE MYNUMBER 12;
+...
+EXEC SQL UPDATE Tbl SET col = MYNUMBER;
+</pre><p>
+ then <code class="command">ecpg</code> will already do the substitution and your C compiler will never
+ see any name or identifier <code class="literal">MYNUMBER</code>. Note that you cannot use
+ <code class="literal">#define</code> for a constant that you are going to use in an
+ embedded SQL query because in this case the embedded SQL precompiler is not
+ able to see this declaration.
+ </p></div><div class="sect2" id="ECPG-IFDEF"><div class="titlepage"><div><div><h3 class="title">36.9.3. ifdef, ifndef, elif, else, and endif Directives</h3></div></div></div><p>
+ You can use the following directives to compile code sections conditionally:
+
+ </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>
+ Checks a <em class="replaceable"><code>name</code></em> and processes subsequent lines if
+ <em class="replaceable"><code>name</code></em> has been defined via <code class="literal">EXEC SQL define
+ <em class="replaceable"><code>name</code></em></code>.
+ </p></dd><dt><span class="term"><code class="literal">EXEC SQL ifndef <em class="replaceable"><code>name</code></em>;</code></span></dt><dd><p>
+ Checks a <em class="replaceable"><code>name</code></em> and processes subsequent lines if
+ <em class="replaceable"><code>name</code></em> has <span class="emphasis"><em>not</em></span> been defined via
+ <code class="literal">EXEC SQL define <em class="replaceable"><code>name</code></em></code>.
+ </p></dd><dt><span class="term"><code class="literal">EXEC SQL elif <em class="replaceable"><code>name</code></em>;</code></span></dt><dd><p>
+ Begins an optional alternative section after an
+ <code class="literal">EXEC SQL ifdef <em class="replaceable"><code>name</code></em></code> or
+ <code class="literal">EXEC SQL ifndef <em class="replaceable"><code>name</code></em></code>
+ directive. Any number of <code class="literal">elif</code> sections can appear.
+ Lines following an <code class="literal">elif</code> will be processed
+ if <em class="replaceable"><code>name</code></em> has been
+ defined <span class="emphasis"><em>and</em></span> no previous section of the same
+ <code class="literal">ifdef</code>/<code class="literal">ifndef</code>...<code class="literal">endif</code>
+ construct has been processed.
+ </p></dd><dt><span class="term"><code class="literal">EXEC SQL else;</code></span></dt><dd><p>
+ Begins an optional, final alternative section after an
+ <code class="literal">EXEC SQL ifdef <em class="replaceable"><code>name</code></em></code> or
+ <code class="literal">EXEC SQL ifndef <em class="replaceable"><code>name</code></em></code>
+ directive. Subsequent lines will be processed if no previous section
+ of the same
+ <code class="literal">ifdef</code>/<code class="literal">ifndef</code>...<code class="literal">endif</code>
+ construct has been processed.
+ </p></dd><dt><span class="term"><code class="literal">EXEC SQL endif;</code></span></dt><dd><p>
+ Ends an
+ <code class="literal">ifdef</code>/<code class="literal">ifndef</code>...<code class="literal">endif</code>
+ construct. Subsequent lines are processed normally.
+ </p></dd></dl></div><p>
+ </p><p>
+ <code class="literal">ifdef</code>/<code class="literal">ifndef</code>...<code class="literal">endif</code>
+ constructs can be nested, up to 127 levels deep.
+ </p><p>
+ This example will compile exactly one of the three <code class="literal">SET
+ TIMEZONE</code> commands:
+</p><pre class="programlisting">
+EXEC SQL ifdef TZVAR;
+EXEC SQL SET TIMEZONE TO TZVAR;
+EXEC SQL elif TZNAME;
+EXEC SQL SET TIMEZONE TO TZNAME;
+EXEC SQL else;
+EXEC SQL SET TIMEZONE TO 'GMT';
+EXEC SQL endif;
+</pre><p>
+ </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="36.8. Error Handling">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="ecpg.html" title="Chapter 36. ECPG — Embedded SQL in C">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="ecpg-process.html" title="36.10. Processing Embedded SQL Programs">Next</a></td></tr><tr><td width="40%" align="left" valign="top">36.8. Error Handling </td><td width="20%" align="center"><a accesskey="h" href="index.html" title="PostgreSQL 15.4 Documentation">Home</a></td><td width="40%" align="right" valign="top"> 36.10. Processing Embedded SQL Programs</td></tr></table></div></body></html> \ No newline at end of file
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