VARARGS(3)

NAME
varargs - variable argument list

SYNOPSIS
#include <varargs.h>

function (va_alist)
va_dcl
va_list pvar;
va_start (pvar);
f = va_arg (pvar, type);
va_end (pvar);

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 varargs are inherently nonportable, since different machines use different argument passing conventions.

va_alist is used in a function header to declare a variable argument list.

va_dcl is a declaration for va_alist. Note that there is no semicolon after va_dcl.

va_list is a type which can be used for the variable pvar, which is used to traverse the list. One such variable must always be declared.

va_start (pvar) is called to initialize pvar to the beginning of the list.

va_arg (pvar, type) will return the next argument in the list pointed to by pvar. Type is the type to which the expected argument will be converted when passed as an argument. In standard C, arguments that are char or short should be accessed as int, unsigned char or unsigned short are converted to unsigned int, and float arguments are converted to double. 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 (pvar) is used to finish up.

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

EXAMPLE
	#include <varargs.h>
	execl(va_alist)
	va_dcl
	{
		va_list ap;
		char *file;
		char *args[100];
		int argno = 0;
		va_start(ap);
		file = va_arg(ap, char *);
		while (args[argno++] = va_arg(ap, char *))
			;
		va_end(ap);
		return execv(file, args);
	}

BUGS
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 0 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.