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<h3 class="section">Prototypes and Old-Style Function Definitions</h3>
<p>GNU C extends ISO C to allow a function prototype to override a later
old-style non-prototype definition. Consider the following example:
<pre class="example"> /* Use prototypes unless the compiler is old-fashioned. */
#ifdef __STDC__
#define P(x) x
#else
#define P(x) ()
#endif
/* Prototype function declaration. */
int isroot P((uid_t));
/* Old-style function definition. */
int
isroot (x) /* ??? lossage here ??? */
uid_t x;
{
return x == 0;
}
</pre>
<p>Suppose the type <code>uid_t</code> happens to be <code>short</code>. ISO C does
not allow this example, because subword arguments in old-style
non-prototype definitions are promoted. Therefore in this example the
function definition's argument is really an <code>int</code>, which does not
match the prototype argument type of <code>short</code>.
<p>This restriction of ISO C makes it hard to write code that is portable
to traditional C compilers, because the programmer does not know
whether the <code>uid_t</code> type is <code>short</code>, <code>int</code>, or
<code>long</code>. Therefore, in cases like these GNU C allows a prototype
to override a later old-style definition. More precisely, in GNU C, a
function prototype argument type overrides the argument type specified
by a later old-style definition if the former type is the same as the
latter type before promotion. Thus in GNU C the above example is
equivalent to the following:
<pre class="example"> int isroot (uid_t);
int
isroot (uid_t x)
{
return x == 0;
}
</pre>
<p>GNU C++ does not support old-style function definitions, so this
extension is irrelevant.
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