Matthew Seaman m.seaman at
Tue Apr 13 09:40:48 PDT 2004

On Tue, Apr 13, 2004 at 09:45:39AM -0500, Brian Henning wrote:
> I have a question about the stdio.h file.
> In the following function signature what does the __P do? 
> Why is the __P needed?
> int (*_close) __P((void *));

History.  Backwards compatability.  This construct dates from the
times of K&R C, before the ANSI standard that said function prototypes
should contain argument type declarations.  If you examine the
<sys/cdefs.h> header file, you'll see that macro is designed to expand
to either an empty pair of brackets -- () -- which is the old-style
prototype, or the string enclosed within it -- (void *) -- which is
the "new" style prototype.  I say "new" because prototypes like that
have been standard for more than 10 years.

On any compiler you encounter nowadays that declaration will be
resolved to:

  int (*_close) (void *)

meaning _close is a pointer to a function taking an arbitrary pointer
as argument and returning an int.

Yes -- the __P() construct is pretty much obsolete nowadays, but it's
probably more trouble than it's worth to try and strip it out of



Dr Matthew J Seaman MA, D.Phil.                       26 The Paddocks
                                                      Savill Way
PGP:         Marlow
Tel: +44 1628 476614                                  Bucks., SL7 1TH UK
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