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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 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 rou-
tine 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 vari-
able-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 argu-
ments.
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
varargs(3)
