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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>35.16. Internals</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-informix-compat.html" title="35.15. Informix Compatibility Mode" /><link rel="next" href="information-schema.html" title="Chapter 36. The Information Schema" /></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">35.16. Internals</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="ecpg-informix-compat.html" title="35.15. Informix Compatibility Mode">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="information-schema.html" title="Chapter 36. The Information Schema">Next</a></td></tr></table><hr></hr></div><div class="sect1" id="ECPG-DEVELOP"><div class="titlepage"><div><div><h2 class="title" style="clear: both">35.16. Internals</h2></div></div></div><p>
- This section explains how <span class="application">ECPG</span> works
- internally. This information can occasionally be useful to help
- users understand how to use <span class="application">ECPG</span>.
- </p><p>
- The first four lines written by <code class="command">ecpg</code> to the
- output are fixed lines. Two are comments and two are include
- lines necessary to interface to the library. Then the
- preprocessor reads through the file and writes output. Normally
- it just echoes everything to the output.
- </p><p>
- When it sees an <code class="command">EXEC SQL</code> statement, it
- intervenes and changes it. The command starts with <code class="command">EXEC
- SQL</code> and ends with <code class="command">;</code>. Everything in
- between is treated as an <acronym class="acronym">SQL</acronym> statement and
- parsed for variable substitution.
- </p><p>
- Variable substitution occurs when a symbol starts with a colon
- (<code class="literal">:</code>). The variable with that name is looked up
- among the variables that were previously declared within a
- <code class="literal">EXEC SQL DECLARE</code> section.
- </p><p>
- The most important function in the library is
- <code class="function">ECPGdo</code>, which takes care of executing most
- commands. It takes a variable number of arguments. This can easily
- add up to 50 or so arguments, and we hope this will not be a
- problem on any platform.
- </p><p>
- The arguments are:
-
- </p><div class="variablelist"><dl class="variablelist"><dt><span class="term">A line number</span></dt><dd><p>
- This is the line number of the original line; used in error
- messages only.
- </p></dd><dt><span class="term">A string</span></dt><dd><p>
- This is the <acronym class="acronym">SQL</acronym> command 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
- command. Where the variables should go the string contains
- <code class="literal">?</code>.
- </p></dd><dt><span class="term">Input variables</span></dt><dd><p>
- Every input variable causes ten arguments to be created. (See below.)
- </p></dd><dt><span class="term"><em class="parameter"><code>ECPGt_EOIT</code></em></span></dt><dd><p>
- An <code class="type">enum</code> telling that there are no more input
- variables.
- </p></dd><dt><span class="term">Output variables</span></dt><dd><p>
- Every output variable causes ten arguments to be created.
- (See below.) These variables are filled by the function.
- </p></dd><dt><span class="term"><em class="parameter"><code>ECPGt_EORT</code></em></span></dt><dd><p>
- An <code class="type">enum</code> telling that there are no more variables.
- </p></dd></dl></div><p>
- </p><p>
- For every variable that is part of the <acronym class="acronym">SQL</acronym>
- command, the function gets ten arguments:
-
- </p><div class="orderedlist"><ol class="orderedlist" type="1"><li class="listitem"><p>
- The type as a special symbol.
- </p></li><li class="listitem"><p>
- A pointer to the value or a pointer to the pointer.
- </p></li><li class="listitem"><p>
- The size of the variable if it is a <code class="type">char</code> or <code class="type">varchar</code>.
- </p></li><li class="listitem"><p>
- The number of elements in the array (for array fetches).
- </p></li><li class="listitem"><p>
- The offset to the next element in the array (for array fetches).
- </p></li><li class="listitem"><p>
- The type of the indicator variable as a special symbol.
- </p></li><li class="listitem"><p>
- A pointer to the indicator variable.
- </p></li><li class="listitem"><p>
- 0
- </p></li><li class="listitem"><p>
- The number of elements in the indicator array (for array fetches).
- </p></li><li class="listitem"><p>
- The offset to the next element in the indicator array (for
- array fetches).
- </p></li></ol></div><p>
- </p><p>
- Note that not all SQL commands are treated in this way. For
- instance, an open cursor statement like:
- </p><pre class="programlisting">
- EXEC SQL OPEN <em class="replaceable"><code>cursor</code></em>;
- </pre><p>
- is not copied to the output. Instead, the cursor's
- <code class="command">DECLARE</code> command is used at the position of the <code class="command">OPEN</code> command
- because it indeed opens the cursor.
- </p><p>
- Here is a complete example describing the output of the
- preprocessor of a file <code class="filename">foo.pgc</code> (details might
- change with each particular version of the preprocessor):
- </p><pre class="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;
- </pre><p>
- is translated into:
- </p><pre class="programlisting">
- /* Processed by ecpg (2.6.0) */
- /* These two include files are added by the preprocessor */
- #include <ecpgtype.h>;
- #include <ecpglib.h>;
-
- /* 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,&(index),1L,1L,sizeof(int),
- ECPGt_NO_INDICATOR, NULL , 0L, 0L, 0L, ECPGt_EOIT,
- ECPGt_int,&(result),1L,1L,sizeof(int),
- ECPGt_NO_INDICATOR, NULL , 0L, 0L, 0L, ECPGt_EORT);
- #line 147 "foo.pgc"
-
- </pre><p>
- (The indentation here is added for readability and not
- something the preprocessor does.)
- </p></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="ecpg-informix-compat.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="information-schema.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">35.15. <span class="productname">Informix</span> Compatibility Mode </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> Chapter 36. The Information Schema</td></tr></table></div></body></html>
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