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)