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Argument Access Macros

Here are descriptions of the macros used to retrieve variable arguments. These macros are defined in the header file stdarg.h.

va_list Data Type
The type va_list is used for argument pointer variables.

void va_start (va_list ap, last-required) Macro
This macro initializes the argument pointer variable ap to point to the first of the optional arguments of the current function; last-required must be the last required argument to the function.

See Old Varargs, for an alternate definition of va_start found in the header file varargs.h.

type va_arg (va_list ap, type) Macro
The va_arg macro returns the value of the next optional argument, and modifies the value of ap to point to the subsequent argument. Thus, successive uses of va_arg return successive optional arguments.

The type of the value returned by va_arg is type as specified in the call. type must be a self-promoting type (not char or short int or float) that matches the type of the actual argument.

void va_end (va_list ap) Macro
This ends the use of ap. After a va_end call, further va_arg calls with the same ap may not work. You should invoke va_end before returning from the function in which va_start was invoked with the same ap argument.

In the GNU C library, va_end does nothing, and you need not ever use it except for reasons of portability.

Sometimes it is necessary to parse the list of parameters more than once or one wants to remember a certain position in the parameter list. To do this, one will have to make a copy of the current value of the argument. But va_list is an opaque type and one cannot necessarily assign the value of one variable of type va_list to another variable of the same type.

void __va_copy (va_list dest, va_list src) Macro
The __va_copy macro allows copying of objects of type va_list even if this is not an integral type. The argument pointer in dest is initialized to point to the same argument as the pointer in src.

This macro is a GNU extension but it will hopefully also be available in the next update of the ISO C standard.

If you want to use __va_copy you should always be prepared for the possibility that this macro will not be available. On architectures where a simple assignment is invalid, hopefully __va_copy will be available, so one should always write something like this: 

{
  va_list ap, save;
  ...
#ifdef __va_copy
  __va_copy (save, ap);
#else
  save = ap;
#endif
  ...
}