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Name

va_arg, va_start, va_end - handle variable argument list

Synopsis

#include <sys/varargs.h>

void va_start(va_list pvar, void name);

type va_arg(va_list pvar, void type);

void va_end(va_list pvar);

Interface Level

Solaris DDI specific (Solaris DDI).

Arguments

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_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 so that the type of a pointer to an object that has the specified type can be obtained simply 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_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

The following example uses these routines to create a variable length command. This may be useful for a device which 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(u_char *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, u_char);
    }
    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.

It is non-portable to specify a second argument of char or short to va_arg, 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.

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