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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@postgresql.org" /><meta name="generator" content="DocBook XSL Stylesheets V1.79.1" /><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 11beta1 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>

</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">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="source.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="nls.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">54.3. Error Message Style Guide </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> Chapter 55. Native Language Support</td></tr></table></div></body></html>

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