Newsgroups: comp.lang.c
Path: utzoo!henry
From: henry@utzoo.uucp (Henry Spencer)
Subject: Re: The free() thing and function names
Message-ID: <1989Sep15.183248.2955@utzoo.uucp>
Organization: U of Toronto Zoology
References: <1989Sep14.022055.5961@twwells.com> <841@uniol.UUCP>
Date: Fri, 15 Sep 89 18:32:48 GMT

In article <841@uniol.UUCP> lehners@uniol.UUCP (Joerg Lehners) writes:
>What does the standard say about standardized function overriding ?

The answer is a little complicated.  Assuming you just want to use a
standard function's name for your own purposes, you can always do this
provided (a) it doesn't begin with __ or _[A-Z], and (b) you do not
#include the standard header declaring that function.  If either of these
restrictions is violated, behavior is "undefined", i.e. it's up to your 
compiler and it's not portable.

If what you want is to supply, say, your own atof() function, *and* have
[say] scanf() use it, there is no portable way to do this.  ANSI C
essentially demands that scanf() not call atof() at all, but some internal
version with a name in the implementation-reserved name space (see clause
(a) above).  This is so you can innocently re-use the name, as above,
without breaking scanf().  Your compiler *might* permit you to override
the function scanf() is using, but the name will be implementation-specific
and the overriding will violate clause (a) above.

>   a.) C-Compiler: just the C keywords are reserved,
>       function overriding possible and possible more.
>   b.) C-System (C-Compiler plus standardized function package [often
>       called 'library']: C keywords reserved; return type, argument types
>       of standardized functions reserved; and more.

It is not necessarily possible for an implementation to make a clean split
between the two, since some of the standard functions may be wired into
the compiler.  ANSI C does permit that, subject to the requirement that
the relevant headers be #included, so that innocent name re-use is not
an issue.  X3J11 was not at all keen on having multiple versions of C,
so it generally didn't provide the sort of choice you suggest.  (It did
end up being talked into providing one choice, between "hosted" and
"free-standing" implementations, which has some similarity to your request
but isn't the same thing.)
-- 
V7 /bin/mail source: 554 lines.|     Henry Spencer at U of Toronto Zoology
1989 X.400 specs: 2200+ pages. | uunet!attcan!utzoo!henry henry@zoo.toronto.edu