varargs

- handle variable argument list

Synopsis

#include <varargs.h>
va_alist
va_dcl
va_list pvar;

void va_start(va_listpvar);

type va_arg(va_list pvar, type);

void va_end(va_list pvar);

Description

This set of macros allows portable procedures that accept variable argument lists to be written. Routines that have variable argument lists (such as printf(3C)) but do not use varargs are inherently non-portable, as different machines use different argument-passing conventions.

va_alist is used as the parameter list in a function header.

va_dcl is a declaration for va_alist. No semicolon should follow va_dcl.

va_list is a type defined for the variable used to traverse the list.

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

va_arg will return the next argument in the list pointed to by pvar. type is the type the argument is expected to be. Different types can be mixed, but it is up to the routine to know what type of argument is expected, as it cannot be determined at runtime.

va_end is used to clean up.

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

Examples

Example 1 A sample program.

This example is a possible implementation of execl (see exec(2) ).

#include <unistd.h>
#include <varargs.h>
#define MAXARGS    100
/*    execl is called by
        execl(file, arg1, arg2, ..., NULL);
*/
execl(va_alist)
va_dcl
{
    va_list ap;
    char *file;
    char *args[MAXARGS];        /* assumed big enough*/
    int argno = 0;

    va_start(ap);
    file = va_arg(ap, char *);
    while ((args[argno++] = va_arg(ap, char *)) != 0)
        ;
    va_end(ap);
    return execv(file, args);
}

See Also

exec(2), printf(3C), vprintf(3C), stdarg(3EXT)

Notes

It is up to the calling routine to specify in some manner how many arguments there are, since it is not always possible to determine the number of arguments from the stack frame. For example, execl is passed a zero pointer to signal the end of the list. printf can tell how many arguments are there by the format.

It is non-portable to specify a second argument of char, short, or float to va_arg, since arguments seen by the called function are not char, short, or float. C converts char and short arguments to int and converts float arguments to double before passing them to a function.

stdarg is the preferred interface.