NAME

varargs − variable argument list

USAGE

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

DESCRIPTION

This set of macros provides a way to write portable procedures that accept variable argument lists.  Routines with variable argument lists (such as printf(3)) that do not use varargs are inherently difficult to port, 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 that 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 expected type of the argument.  Different types can be mixed, but the routine should 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. 

NOTES

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 zero to signal the end of the list.  Printf can tell from the format how many arguments are supposed to be there. 

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); }