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: typeva_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
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.
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_copywill be available,
so one should always write something like this: