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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>54.4. 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="54.3. Error Message Style Guide" /><link rel="next" href="nls.html" title="Chapter 55. Native Language Support" /></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">54.4. Miscellaneous Coding Conventions</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="error-style-guide.html" title="54.3. Error Message Style Guide">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="source.html" title="Chapter 54. PostgreSQL Coding Conventions">Up</a></td><th width="60%" align="center">Chapter 54. PostgreSQL Coding Conventions</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 11.5 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="nls.html" title="Chapter 55. Native Language Support">Next</a></td></tr></table><hr></hr></div><div class="sect1" id="SOURCE-CONVENTIONS"><div class="titlepage"><div><div><h2 class="title" style="clear: both">54.4. Miscellaneous Coding Conventions</h2></div></div></div><div class="simplesect" id="id-1.10.6.5.2"><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 C89 standard. That means a conforming
C89 compiler has to be able to compile postgres, at least aside
from a few platform dependent pieces. Features from later
revision of the C standard or compiler specific features can be
used, if a fallback is provided.
</p><p>
For example <code class="literal">static inline</code> and
<code class="literal">_StaticAssert()</code> are currently used, even
though they are from newer revisions of the C standard. If not
available we respectively fall back to defining the functions
without inline, and to using a C89 compatible replacement that
performs the same checks, but emits rather cryptic messages.
</p></div><div class="simplesect" id="id-1.10.6.5.3"><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-1.10.6.5.4"><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-1.10.6.5.5"><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>
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