VA_ARG(9F) Kernel Functions for Drivers VA_ARG(9F)

NAME

va_arg, va_start, va_copy, va_end - handle variable argument list

SYNOPSIS

#include <sys/varargs.h>


void va_start(va_list pvar, name);


type va_arg(va_list pvar, type);


void va_copy(va_list dest, va_list src);


void va_end(va_list pvar);

INTERFACE LEVEL

illumos DDI specific (illumos DDI).

PARAMETERS

va_start()
pvar
Pointer to variable argument list.


name
Identifier of rightmost parameter in the function definition.


va_arg()
pvar
Pointer to variable argument list.


type
Type name of the next argument to be returned.


va_copy()
dest
Destination variable argument list.


src
Source variable argument list.


va_end()
pvar
Pointer to variable argument list.

DESCRIPTION

This set of macros allows portable procedures that accept variable
argument lists to be written. Routines that have variable argument
lists but do not use the varargs macros are inherently non-portable,
as different machines use different argument-passing conventions.
Routines that accept a variable argument list can use these macros to
traverse the list.


va_list is the type defined for the variable used to traverse the
list of arguments.


va_start() is called to initialize pvar to the beginning of the
variable argument list. va_start() must be invoked before any access
to the unnamed arguments. The parameter name is the identifier of the
rightmost parameter in the variable parameter list in the function
definition (the one just before the ", ..."). If this parameter is
declared with the register storage class or with a function or array
type, or with a type that is not compatible with the type that
results after application of the default argument promotions, the
behavior is undefined.


va_arg() expands to an expression that has the type and value of the
next argument in the call. The parameter pvar must be initialized by
va_start(). Each invocation of va_arg() modifies pvar so that the
values of successive arguments are returned in turn. The parameter
type is the type name of the next argument to be returned. The type
name must be specified in such a way that the type of pointer to an
object that has the specified type can be obtained by postfixing a *
to type. If there is no actual next argument, or if type is not
compatible with the type of the actual next argument (as promoted
according to the default argument promotions), the behavior is
undefined.


The va_copy() macro saves the state represented by the va_list src in
the va_list dest. The va_list passed as dest should not be
initialized by a previous call to va_start(). It then must be passed
to va_end() before being reused as a parameter to va_start() or as
the dest parameter of a subsequent call to va_copy(). The behavior is
undefined if any of these restrictions are not met.


The va_end() macro is used to clean up. It invalidates pvar for use
(unless va_start() is invoked again).


Multiple traversals, each bracketed by a call to va_start() and
va_end(), are possible.

EXAMPLES

Example 1: Creating a Variable Length Command

The following example uses these routines to create a variable length
command. This might be useful for a device that provides for a
variable-length command set. ncmdbytes is the number of bytes in the
command. The new command is written to cmdp.


static void
xx_write_cmd(uchar_t *cmdp, int ncmdbytes, ...)
{
va_list ap;
int i;

/*
* Write variable-length command to destination
*/
va_start(ap, ncmdbytes);
for (i = 0; i < ncmdbytes; i++) {
*cmdp++ = va_arg(ap, uchar_t);
}
va_end(ap);
}

SEE ALSO

vcmn_err(9F), vsprintf(9F)

NOTES

It is up to the calling routine to specify in some manner how many
arguments there are, since it is not always possible to determine the
number of arguments from the stack frame.


Specifying a second argument of char or short to va_arg makes your
code non-portable, because arguments seen by the called function are
not char or short. C converts char and short arguments to int before
passing them to a function.

May 12, 2021 VA_ARG(9F)