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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>56.4. Miscellaneous Coding Conventions</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="error-style-guide.html" title="56.3. Error Message Style Guide" /><link rel="next" href="nls.html" title="Chapter 57. Native Language Support" /></head><body id="docContent" class="container-fluid col-10"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">56.4. Miscellaneous Coding Conventions</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="error-style-guide.html" title="56.3. Error Message Style Guide">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="source.html" title="Chapter 56. PostgreSQL Coding Conventions">Up</a></td><th width="60%" align="center">Chapter 56. PostgreSQL Coding Conventions</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 15.6 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="nls.html" title="Chapter 57. Native Language Support">Next</a></td></tr></table><hr /></div><div class="sect1" id="SOURCE-CONVENTIONS"><div class="titlepage"><div><div><h2 class="title" style="clear: both">56.4. Miscellaneous Coding Conventions</h2></div></div></div><div class="simplesect" id="id-1.10.7.5.2"><div class="titlepage"><div><div><h3 class="title">C Standard</h3></div></div></div><p>
Code in <span class="productname">PostgreSQL</span> should only rely on language
features available in the C99 standard. That means a conforming
C99 compiler has to be able to compile postgres, at least aside
from a few platform dependent pieces.
</p><p>
A few features included in the C99 standard are, at this time, not
permitted to be used in core <span class="productname">PostgreSQL</span>
code. This currently includes variable length arrays, intermingled
declarations and code, <code class="literal">//</code> comments, universal
character names. Reasons for that include portability and historical
practices.
</p><p>
Features from later revisions of the C standard or compiler specific
features can be used, if a fallback is provided.
</p><p>
For example <code class="literal">_Static_assert()</code> and
<code class="literal">__builtin_constant_p</code> are currently used, even though
they are from newer revisions of the C standard and a
<span class="productname">GCC</span> extension respectively. If not available
we respectively fall back to using a C99 compatible replacement that
performs the same checks, but emits rather cryptic messages and do not
use <code class="literal">__builtin_constant_p</code>.
</p></div><div class="simplesect" id="id-1.10.7.5.3"><div class="titlepage"><div><div><h3 class="title">Function-Like Macros and Inline Functions</h3></div></div></div><p>
Both macros with arguments and <code class="literal">static inline</code>
functions may be used. The latter are preferable if there are
multiple-evaluation hazards when written as a macro, as e.g., the
case with
</p><pre class="programlisting">
#define Max(x, y) ((x) > (y) ? (x) : (y))
</pre><p>
or when the macro would be very long. In other cases it's only
possible to use macros, or at least easier. For example because
expressions of various types need to be passed to the macro.
</p><p>
When the definition of an inline function references symbols
(i.e., variables, functions) that are only available as part of the
backend, the function may not be visible when included from frontend
code.
</p><pre class="programlisting">
#ifndef FRONTEND
static inline MemoryContext
MemoryContextSwitchTo(MemoryContext context)
{
MemoryContext old = CurrentMemoryContext;
CurrentMemoryContext = context;
return old;
}
#endif /* FRONTEND */
</pre><p>
In this example <code class="literal">CurrentMemoryContext</code>, which is only
available in the backend, is referenced and the function thus
hidden with a <code class="literal">#ifndef FRONTEND</code>. This rule
exists because some compilers emit references to symbols
contained in inline functions even if the function is not used.
</p></div><div class="simplesect" id="id-1.10.7.5.4"><div class="titlepage"><div><div><h3 class="title">Writing Signal Handlers</h3></div></div></div><p>
To be suitable to run inside a signal handler code has to be
written very carefully. The fundamental problem is that, unless
blocked, a signal handler can interrupt code at any time. If code
inside the signal handler uses the same state as code outside
chaos may ensue. As an example consider what happens if a signal
handler tries to acquire a lock that's already held in the
interrupted code.
</p><p>
Barring special arrangements code in signal handlers may only
call async-signal safe functions (as defined in POSIX) and access
variables of type <code class="literal">volatile sig_atomic_t</code>. A few
functions in <code class="command">postgres</code> are also deemed signal safe, importantly
<code class="function">SetLatch()</code>.
</p><p>
In most cases signal handlers should do nothing more than note
that a signal has arrived, and wake up code running outside of
the handler using a latch. An example of such a handler is the
following:
</p><pre class="programlisting">
static void
handle_sighup(SIGNAL_ARGS)
{
int save_errno = errno;
got_SIGHUP = true;
SetLatch(MyLatch);
errno = save_errno;
}
</pre><p>
<code class="varname">errno</code> is saved and restored because
<code class="function">SetLatch()</code> might change it. If that were not done
interrupted code that's currently inspecting <code class="varname">errno</code> might see the wrong
value.
</p></div><div class="simplesect" id="id-1.10.7.5.5"><div class="titlepage"><div><div><h3 class="title">Calling Function Pointers</h3></div></div></div><p>
For clarity, it is preferred to explicitly dereference a function pointer
when calling the pointed-to function if the pointer is a simple variable,
for example:
</p><pre class="programlisting">
(*emit_log_hook) (edata);
</pre><p>
(even though <code class="literal">emit_log_hook(edata)</code> would also work).
When the function pointer is part of a structure, then the extra
punctuation can and usually should be omitted, for example:
</p><pre class="programlisting">
paramInfo->paramFetch(paramInfo, paramId);
</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="error-style-guide.html" title="56.3. Error Message Style Guide">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="source.html" title="Chapter 56. PostgreSQL Coding Conventions">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="nls.html" title="Chapter 57. Native Language Support">Next</a></td></tr><tr><td width="40%" align="left" valign="top">56.3. Error Message Style Guide </td><td width="20%" align="center"><a accesskey="h" href="index.html" title="PostgreSQL 15.6 Documentation">Home</a></td><td width="40%" align="right" valign="top"> Chapter 57. Native Language Support</td></tr></table></div></body></html>
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