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Subject: Re: fwrite inconsistency
To: JChing AT adtech-inc DOT com (Jimen.Ching)
Date: Fri, 11 Aug 2000 19:42:58 -0700 (PDT)
Cc: cygwin AT sources DOT redhat DOT com
In-Reply-To: <B9906C3F4BB3D311A60F009027463EE9B64EE7@mgmgrand.adtech-inc.com> from "Jimen.Ching" at Aug 08, 2000 12:18:46 PM
From: "J. J. Farrell" <jjf AT bcs DOT org DOT uk>
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> From: "Jimen.Ching" <JChing AT adtech-inc DOT com>
> 
> The problem below produces "0 0" as output on HPUX and Solaris 2.5 with gcc
> 2.8.x.  But using gcc 2.95.x on cygwin, I get "0 1".  I also tried using
> Borland BCC, and I also got "0 1".  Is GCC trying to behave like MS
> compilers under cygwin?
> 
> Is it correct for size2 to contain 1?  What does ANSI/ISO say about this?
> 
> -----------------------------------------------------
> #include <stdio.h>
> 
> int
> main(int argc, char *argv[])
> 	{
> 	int size1, size2;
> 	FILE *fp;
> 
> 	fp = fopen("tst.log", "w");
> 	size1 = fwrite(0, 1, 0, fp);
> 	size2 = fwrite(0, 0, 1, fp);
> 	printf("%d %d\n", size1, size2);
> 
> 	return 0;
> 	}

Standard C says that your program produces undefined behaviour
because of the first parameter you give to the fwrite() calls.
The rest of this discussion assumes that your program gives the
same results with a valid first parameter.

Standard C doesn't comment on the effect of giving 0 for "size"
or "nmemb". It's clear from the definition that the first case
should always return 0. The value returned in the second case
depends on the implementor's view of whether or not a zero-length
write succeeds.

XPG3 and the Standard Unix Specification explicitly say

    If size or nitems is 0, fwrite() returns 0

I haven't checked, but it's very likely that POSIX says the same.

This looks like a bug in cygwin's fwrite().


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