Content-type: text/html Man page of varargs

varargs

Section: C Library Functions (3)
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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. Unsigned char or short arguments 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 because the type cannot be determined at run time.

 

DESCRIPTION

NOTE: When writing new code, it is recommended that you use stdarg instead of varargs. They both perform the same function, but stdarg is standards-compliant and varargs complies only with BSD conventions.

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_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), stdarg(3), vprintf(3) delim off


 

Index

NAME
LIBRARY
SYNOPSIS
PARAMETERS
DESCRIPTION
EXAMPLE
NOTES
RELATED INFORMATION

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Time: 02:41:29 GMT, October 02, 2010