@(#)stdarg.3 4.1 (Berkeley) 05/15/85

.AT 3 varargs - variable argument list

#include <varargs.h>
function(va_alist)
va_dcl
va_list pvar;
va_start(pvar);
f = va_arg(pvar, type);
va_end(pvar);

This set of macros allows portable procedures that accept variable argument lists to be written. Routines which have 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 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, since it cannot be determined at runtime.

va_end ( pvar ) is used to finish up.

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

 #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);
 }

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 there by the format.