STDARG(3)

NAME
stdarg - variable argument list

SYNOPSIS
#include <stdarg.h>
void va_start(va_list ap, argtypeN parmN)
type va_arg(va_list ap, type)
void va_end(va_list ap)

DESCRIPTION
This set of macros provides a means of writing portable procedures that accept variable argument lists. Routines having variable argument lists (such as printf(3)) that do not use stdarg are inherently nonportable, since different machines use different argument passing conventions.

A function that accepts a variable argument list is declared with "..." at the end of its parameter list. It must have at least one normal argument before the "...". For example:

    int printf(const char *format, ...) { /* code */ }
    int fprintf(FILE *stream, const char *format, ...) { /* code */ }

va_list is a type which is used for the variable ap within the body of a variable argument function which is used to traverse the list.

va_start (ap, parmN) is called to initialize ap to the beginning of the list. The last true parameter of the function, parmN, must be supplied to allow va_start to compute the address of the first variable parameter.

va_arg (ap, type) will return the next argument in the list pointed to by ap. Type is the type to which the expected argument will be converted when passed as an argument.

Different types can be mixed, but it is up to the routine to know what type of argument is expected, since it cannot be determined at runtime.

va_end (ap) must be used to finish up.

Multiple traversals, each bracketed by va_start ... va_end, are possible.

EXAMPLE
	#include <stdarg.h>

	execl(const char *path, ...)
	{
		va_list ap;
		char *args[100];
		int argno = 0;
		va_start(ap, path);
		while ((args[argno++] = va_arg(ap, char *)) != NULL) {}
		va_end(ap);
		return execv(path, args);
	}

NOTES
It is up to the calling routine to determine how many arguments there are, since it is not possible to determine this from the stack frame. For example, execl passes a null pointer to signal the end of the list. Printf can tell how many arguments are supposed to be there by the format.

The macros va_start and va_end may be arbitrarily complex; for example, va_start might contain an opening brace, which is closed by a matching brace in va_end. Thus, they should only be used where they could be placed within a single complex statement.

BUGS
It is impossible to properly show the macros as C declarations as is done in the synopsis. They can never be coded as C functions, because all three macros use their arguments by address, and the type field is certainly impossible. Just look at them as being part of the C language, like sizeof.