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stdarg - Handles a variable-length parameter list
Standard C Library (libc.so, libc.a)
#include <stdarg.h>
va_list
void va_start (
va_list argp, parmN );
type va_arg (
va_list argp,
type );
void va_end (
va_list argp );
The stdarg set of macros allows you to write portable functions that accept a variable number of parameters. Subroutines that have variable-length parameter lists (such as the printf() function), but that do not use the stdarg macros, are inherently nonportable because different systems use different parameter-passing conventions.
The stdarg macros are as follows:
Your function can traverse, or scan, the parameter list more than once. Start each traversal with a call to va_start() and end it with va_end().
The following example is a possible implementation of the
execl() function:
#include <stdarg.h>
#define MAXargS 100
/*
** execl is called by
** execl(file, arg1, arg2, . . . , (char *) 0);
*/
execl(char * file, . . .)
{ va_list ap;
char *file;
char *args[MAXargS];
int argno = 0;
va_start(ap, file);
while ((args[argno++] = va_arg(ap, char *)) != (char *) 0)
; /* Empty loop body */
va_end(ap);
return (execv(file, args));
}
The calling routine is responsible for specifying the number of parameters because it is not always possible to determine this from the stack frame. For example, the execl() function is passed a null pointer to signal the end of the list. The printf() function determines the number of parameters from its fmt parameter.
AES Support Level: Temporary use
Functions: exec(2), printf(3), varargs(3), vprintf(3) delim off