fflush on closed std stream

[Jeff Johnston]

Eric Blake wrote:
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> POSIX is clear that calling fclose(f), fflush(f) gives unspecified
> behavior.  And in general, this is true, since FILE* are allocated by
> f[d]open and fclose frees that memory.  But there is a special case of
> std{in,out,err}, where the FILE object persists for the lifetime of the
> program, even when the program is started with the underlying fd's
> inherited as closed.
>
> Also, POSIX states that fflush must set errno to EBADF if called on a
> stream with an invalid fd, but after fclose (stdout), newlib currently
> treats fflush(stdout) as success (although the fact that the program has
> already done undefined behavior means that we can't claim it is wrong,
> just poor QoI).
>
> I recently ran into a problem where the m4 program, when run under cygwin
> 1.5, is able to successfully report fclose() failures inside an atexit()
> handler.  But it is reporting that error via a generic error() method that
> blindly calls fflush(stdout) prior to writing to stderr.  Fortunately,
> most platforms tolerate this (particularly if you don't care about errno
> or the return status after the failed fflush).  But when running the same
> m4 test case under cygwin 1.7, and when coupled with the libsigsegv
> library, the attempt to _flockfile the closed stdout results in a
> segfault.  http://cygwin.com/ml/cygwin/2009-07/msg00655.html
>
> This patch appears to fix the issue.  Is it safe to probe fp->_flags prior
> to the _flockfile, in order to skip the lock if the stream is not
> currently visiting a file?  Or do I have to approach this in some other
> manner?
>
>   
Probing fp->_flags should be ok.  I do not think we should be setting 
errno and returning failure.  There is nothing to flush.  Running a test 
of fclose(stdout); fflush(stdout);  on glibc returns 0 from fflush.  
Newlib should do the same.

-- Jeff J.
> 2009-07-18  Eric Blake  <ebb9@byu.net>
>
> 	Avoid a fault from locking a closed standard file.
> 	* libc/stdio/fflush.c (_fflush_r): Give up early if stream has
> 	been previously closed.
>
> - --
> Don't work too hard, make some time for fun as well!
>
> Eric Blake             ebb9@byu.net
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