varargs(3)varargs(3)NAME
varargs, va_arg, va_start, va_end - Handle a variable-length parameter
list
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 );
LIBRARY
Standard C Library (libc)
PARAMETERS
Specifies a variable that the varargs macros use to keep track of the
current location in the parameter list. Do not modify this variable.
Specifies the type to which the expected argument will be converted
when passed as an argument. Unsigned char or short arguments are con‐
verted 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: Initializes argp to point to the
beginning of the list. The va_start() macro will be invoked before any
access to the unnamed arguments. Returns the next parameter in the
list pointed to by argp. 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(). Is used in the function header to declare a variable argu‐
ment list. va_dcl is a declaration for va_dcl. Note that there is no
semicolon after va_dcl.
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
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)); }
SEE ALSO
Functions: exec(2), printf(3), stdarg(3), vprintf(3)varargs(3)
