NAME

varargs, va_arg, va_start, va_end − Handles a variable-length parameter list

LIBRARY

Standard C Library (libc.a)

SYNOPSIS

#include <varargs.h>

va_alist
va_dcl
void va_start (
va_alist argp );
type va_arg (
va_alist argp,
type );
void va_end (
va_alist argp );

PARAMETERS

argp Specifies a variable that the varargs macros use to keep track of the current location in the parameter list. Do not modify this variable. 

type Specifies the type to which the expected argument will be converted when passed as an argument. In C, arguments that are char or short should be accessed as int; unsigned char or short are converted to unsigned int, and float arguments are converted to double. 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. 

DESCRIPTION

The varargs 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 varargs macros, are inherently nonportable because different systems use different parameter-passing conventions. 

The varargs macros are as follows:

va_alist()
Defines the type of the variable used to traverse the list.

va_start()
Initializes argp to point to the beginning of the list.  The va_start() macro will be invoked before any access to the unnamed arguments. 

va_argp()
A variable that the varargs macros use to keep track of the current location in the parameter list.  Do not modify this variable. 

va_arg()
Returns the next parameter in the list pointed to by argp. 

va_end()
Cleans up at the end.

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(). 

EXAMPLE

The following example is a possible implementation of the execl() function:

#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)
      ;   /∗ Empty loop body ∗/
   va_end(ap);
   return (execv(file, args));
}

NOTES

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

RELATED INFORMATION

Functions: exec(2), printf(3), vprintf(3)