stdarg(3)stdarg(3)NAMEstdarg - Handle a variable-length parameter list
SYNOPSIS
#include <stdarg.h>
va_list
void va_start(
va_list argp,
parmN ); type va_arg(
va_list argp,
type ); void va_end(
va_list argp );
LIBRARY
Standard C Library (libc)
PARAMETERS
Specifies a variable that the stdarg macros use to keep track of the
current location in the parameter list. Do not modify this variable.
Specifies the last named parameter (the one just before the "..." in
the execl() definition in the Example section). There must be at least
one named parameter. 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 con‐
verted 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
The stdarg 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 stdarg macros, are inherently nonportable because dif‐
ferent systems use different parameter-passing conventions.
The stdarg macros are as follows: Defines the type of the variable used
to traverse the list. Initializes argp to point to the first unnamed
argument. 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().
NOTES
The calling routine is responsible for specifying the number of param‐
eters 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 <stdarg.h> #define MAXargS 100 /* ** execl is called by **
execl(file, arg1, arg2, . . . , (char *) 0); */ execl(char * file, . .
.) { va_list ap;
char *file;
char *args[MAXargS];
int argno = 0;
va_start(ap, file);
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), varargs(3), vprintf(3)stdarg(3)
