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<h5 class="subsubsection">A.2.2.4 Calling Variadic Functions</h5>

<p><a name="index-variadic-functions_002c-calling-3720"></a><a name="index-calling-variadic-functions-3721"></a><a name="index-declaring-variadic-functions-3722"></a>
You don't have to do anything special to call a variadic function.
Just put the arguments (required arguments, followed by optional ones)
inside parentheses, separated by commas, as usual.  But you must declare
the function with a prototype and know how the argument values are converted.

   <p>In principle, functions that are <em>defined</em> to be variadic must also
be <em>declared</em> to be variadic using a function prototype whenever
you call them.  (See <a href="Variadic-Prototypes.html#Variadic-Prototypes">Variadic Prototypes</a>, for how.)  This is because
some C compilers use a different calling convention to pass the same set
of argument values to a function depending on whether that function
takes variable arguments or fixed arguments.

   <p>In practice, the GNU C compiler always passes a given set of argument
types in the same way regardless of whether they are optional or
required.  So, as long as the argument types are self-promoting, you can
safely omit declaring them.  Usually it is a good idea to declare the
argument types for variadic functions, and indeed for all functions.
But there are a few functions which it is extremely convenient not to
have to declare as variadic&mdash;for example, <code>open</code> and
<code>printf</code>.

   <p><a name="index-default-argument-promotions-3723"></a><a name="index-argument-promotion-3724"></a>Since the prototype doesn't specify types for optional arguments, in a
call to a variadic function the <dfn>default argument promotions</dfn> are
performed on the optional argument values.  This means the objects of
type <code>char</code> or <code>short&nbsp;int</code><!-- /@w --> (whether signed or not) are
promoted to either <code>int</code> or <code>unsigned&nbsp;int</code><!-- /@w -->, as
appropriate; and that objects of type <code>float</code> are promoted to type
<code>double</code>.  So, if the caller passes a <code>char</code> as an optional
argument, it is promoted to an <code>int</code>, and the function can access
it with <code>va_arg (</code><var>ap</var><code>, int)</code>.

   <p>Conversion of the required arguments is controlled by the function
prototype in the usual way: the argument expression is converted to the
declared argument type as if it were being assigned to a variable of
that type.

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