varargs(5) File Formats Manual varargs(5)
Name
varargs - handle variable argument list
Syntax
#include <varargs.h>
va_alist
va_dcl
void va_start(pvar)
va_list pvar;
type va_arg(pvar, type)
va_list pvar;
void va_end(pvar)
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 but that do not use are inherently nonportable, 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. A semicolon should not 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 Returns 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. This information cannot be determined at run
time.
va_end is used to clean up.
Multiple traversals, each bracketed by va_start ... va_end, are possible.
The calling routine must specify how many arguments there are, because it is not always possible to determine this from the stack frame.
For example, is passed a zero pointer to signal the end of the list. The routine can tell how many arguments there are by the format.
It is nonportable to specify a second argument of char, short, or float to va_arg, because 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 func-
tion.
Examples
The following example presents an implementation of
#include <varargs.h>
#define MAXARGS 100
/* execl is called by
execl(file, arg1, arg2, ..., (char *)0);
*/
execl(va_alist)
va_dcl
{
va_list ap;
char *file;
char *args[MAXARGS];
int argno = 0;
va_start(ap);
file = va_arg(ap, char *);
while ((args[argno++] = va_arg(ap, char *)) != (char *)0)
;
va_end(ap);
return execv(file, args);
}
See Also
exec(2), printf(3s), vprintf(3s)
RISC varargs(5)
