Calling Variadics - The GNU C Library

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##### A.2.2.4 Calling Variadic Functions

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.

In principle, functions that are defined to be variadic must also be declared to be variadic using a function prototype whenever you call them. (See Variadic Prototypes, 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.

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—for example, `open` and `printf`.

Since the prototype doesn't specify types for optional arguments, in a call to a variadic function the default argument promotions are performed on the optional argument values. This means the objects of type `char` or `short int` (whether signed or not) are promoted to either `int` or `unsigned int`, as appropriate; and that objects of type `float` are promoted to type `double`. So, if the caller passes a `char` as an optional argument, it is promoted to an `int`, and the function can access it with `va_arg (`ap`, int)`.

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.