(libc.info.gz) Calling Variadics

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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.  ( 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.
 

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(libc.info.gz) How Many Arguments

(libc.info.gz) How Variadic

(libc.info.gz) Argument Macros