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Command: va_arg | Section: 3 | Source: Digital UNIX | File: va_arg.3.gz
varargs(3) Library Functions Manual varargs(3)
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 con-
verted when passed as an argument. Unsigned char or short argu-
ments 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 in-
stead 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 differ-
ent 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 un-
named 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() func-
tion:
#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 parame-
ters 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
varargs(3)