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STDARG
!!!STDARG
NAME
SYNOPSIS
DESCRIPTION
EXAMPLES
CONFORMING TO
COMPATIBILITY
BUGS
----
!!NAME


stdarg - variable argument lists
!!SYNOPSIS


__#include __


__void va_start( va_list__ ''ap''__,__
''last''__);__''
type'' __va_arg( va_list__ ''ap''__,__
''type''__);
void va_end( va_list__ ''ap''__);__
!!DESCRIPTION


A function may be called with a varying number of arguments
of varying types. The include file ''stdarg.h'' declares
a type __va_list__ and defines three macros for stepping
through a list of arguments whose number and types are not
known to the called function.


The called function must declare an object of type
__va_list__ which is used by the macros __va_start__,
__va_arg__, and __va_end__.


The __va_start__ macro initializes ''ap'' for
subsequent use by __va_arg__ and __va_end__, and must
be called first.


The parameter ''last'' is the name of the last parameter
before the variable argument list, i.e., the last parameter
of which the calling function knows the type.


Because the address of this parameter is used in the
__va_start__ macro, it should not be declared as a
register variable, or as a function or an array
type.


The __va_start__ macro returns no value.


The __va_arg__ macro expands to an expression that has
the type and value of the next argument in the call. The
parameter ''ap'' is the __va_list ap__ initialized by
__va_start__. Each call to __va_arg__ modifies
''ap'' so that the next call returns the next argument.
The parameter ''type'' is a type name specified so that
the type of a pointer to an object that has the specified
type can be obtained simply by adding a * to
''type''.


If there is no next argument, or if ''type'' is not
compatible with the type of the actual next argument (as
promoted according to the default argument promotions),
random errors will occur.


The first use of the __va_arg__ macro after that of the
__va_start__ macro returns the argument after
''last''. Successive invocations return the values of the
remaining arguments.


The __va_end__ macro handles a normal return from the
function whose variable argument list was initialized by
__va_start__.


The __va_end__ macro returns no value.
!!EXAMPLES


The function ''foo'' takes a string of format characters
and prints out the argument associated with each format
character based on the type.


void foo(char *fmt, ...)
{
va_list ap;
int d;
char c, *p, *s;
va_start(ap, fmt);
while (*fmt)
switch(*fmt++) {
case 's':           /* string */
s = va_arg(ap, char *);
printf(
!!CONFORMING TO


The __va_start__, __va_arg__, and __va_end__ macros
conform to ANSI X3.159-1989 (``ANSI C'').
!!COMPATIBILITY


These macros are ''not'' compatible with the historic
macros they replace. A backward compatible version can be
found in the include file ''varargs.h''.
!!BUGS


Unlike the __varargs__ macros, the __stdarg__ macros
do not permit programmers to code a function with no fixed
arguments. This problem generates work mainly when
converting __varargs__ code to __stdarg__ code, but it
also creates difficulties for variadic functions that wish
to pass all of their arguments on to a function that takes a
__va_list__ argument, such as
vfprintf(3).
----
19 pages link to stdarg(3):
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Last edited on Tuesday, June 4, 2002 12:24:35 am by "perry"
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