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>Chapter 52. PostgreSQL Coding Conventions</TD
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><H1
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><A
NAME="SOURCE-CONVENTIONS"
>52.4. Miscellaneous Coding Conventions</A
></H1
><DIV
CLASS="SIMPLESECT"
><H2
CLASS="SIMPLESECT"
><A
NAME="AEN116119"
>52.4.1. C Standard</A
></H2
><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 <TT
CLASS="LITERAL"
>static inline</TT
> and
     <TT
CLASS="LITERAL"
>_Static_assert()</TT
> 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"
><H2
CLASS="SIMPLESECT"
><A
NAME="AEN116126"
>52.4.2. Function-Like Macros and Inline Functions</A
></H2
><P
>     Both, macros with arguments and <TT
CLASS="LITERAL"
>static inline</TT
>
     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) &#62; (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 <TT
CLASS="LITERAL"
>CurrentMemoryContext</TT
>, which is only
     available in the backend, is referenced and the function thus
     hidden with a <TT
CLASS="LITERAL"
>#ifndef FRONTEND</TT
>. This rule
     exists because some compilers emit references to symbols
     contained in inline functions even if the function is not used.
    </P
></DIV
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><H2
CLASS="SIMPLESECT"
><A
NAME="AEN116135"
>52.4.3. Writing Signal Handlers</A
></H2
><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 <TT
CLASS="LITERAL"
>volatile sig_atomic_t</TT
>. A few
     functions in <TT
CLASS="COMMAND"
>postgres</TT
> 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>
     <TT
CLASS="VARNAME"
>errno</TT
> is saved and restored because
     <CODE
CLASS="FUNCTION"
>SetLatch()</CODE
> might change it. If that were not done
     interrupted code that's currently inspecting <TT
CLASS="VARNAME"
>errno</TT
> might see the wrong
     value.
    </P
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