Date: Mon, 20 Nov 2000 12:19:59 +0200 (SAST) From: Robert Nordier <rnordier@nordier.com> To: drosih@rpi.edu (Garance A Drosihn) Cc: hackers@FreeBSD.ORG Subject: Re: fclose vs ferror (from libc/getcap) Message-ID: <200011201020.MAA23349@siri.nordier.com> In-Reply-To: <p04330101b63e2c0b9192@[128.113.24.47]> from "Garance A Drosihn" at Nov 19, 2000 08:27:56 PM
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Garance A Drosihn wrote:
> As mentioned in pr bin/22965, I stumbled across a bug
> in libc/gen/getcap.c when compiling it under other
> platforms. The basic problem is some code which does:
>
> (void)fclose(pfp);
> if (ferror(pfp)) {
> ...do stuff...
> }
>
> I find it surprising that the above works under FreeBSD.
> (not only that, it seems to work OK under several other
> OS's too). The fclose is supposed to throw away the
> stream, but the ferror (apparently) still works with
> that pointer.
>
> The PR includes a patch for getcap.c (which is just to
> use the result from fclose to determine if an error
> occurred), but I also wonder if we should do something
> so that this combination would ALWAYS fail. It seems
> to me that fclose should clear out whatever fields that
> ferror is using for sanity-checking, and ferror should
> always return an errno of 'bad parameter', or something
> like that.
>
> Or is there some reason that we DO want ferror to work
> on streams which have already been closed?
Bear in mind that ferror is one of a number of stdio functions that
are often implemented as macros. For instance: ferror and (say)
getc might look like this:
#define ferror(f) ((f)->fl & _FERR)
#define getc(f) ((f)->p < (f)->xr ? *(f)->p++ : fgetc(f))
Given the intentionally minimalist way those functions are written,
to do any consistent and obligatory sanity-checking on (FILE *) would
cause a big change in the actual code generated, and the amount of
code generated.
I think the best way to do what you want is to create a separate
debugging library.
The point about
(void)fclose(pfp);
if (ferror(pfp)) {
...do stuff...
}
is that it's a silly thing to do deliberately, but if I was porting
some hairy old C code I'd tend to expect it to work. C is not a
language in which you go out of your way to prevent people making
mistakes.
--
Robert Nordier
rnordier@nordier.com
rnordier@FreeBSD.org
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