Argument Macros - The GNU C Library

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##### A.2.2.5 Argument Access Macros

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

— Data Type: va_list

> The type `va_list` is used for argument pointer variables.

— Macro: void va_start (va_list ap, last-required)

> 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`.

— Macro: type va_arg (va_list ap, type)

> 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.

— Macro: void va_end (va_list ap)

> 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.

— Macro: void __va_copy (va_list dest, va_list src)

> 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
           ...
         }