Date: Fri, 3 May 1996 13:59:26 +1000 From: Bruce Evans <bde@zeta.org.au> To: freebsd-hackers@freebsd.org, jmacd@deceit.xcf.berkeley.edu Subject: Re: stdio problem Message-ID: <199605030359.NAA14153@godzilla.zeta.org.au>
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> close(0);
> if(!freopen(argv[1], "r", stdin)) {
This fails because freopen() gets its plumbing tangled in an attempt
to support freopen()ing /dev/stdin. It's not clear if this is required
to work. ANSI doesn't apply since close() isn't in ANSI, and POSIX has
about 4 pages of rules that I don't completely understand (the rules
are mostly about unavoidable interactions, e.g., for i/o done on the
fd's by a child process, and not about abusing close() when fclose()
would work).
>is "wrong". The pracical application here is that if you close 0
>and then exec GNU diff3 you'll encounter this problem.
This is a completely different situation. The stdio streams before exec
are only related to the stdio streams after exec by the implicit
fdopen()s of the first 3 file descriptors. FreeBSD's stdio apparently
gets these fdopen()s wrong by statically initializing the FILE structs.
This fails if any of the descriptors is closed or has abnormal access
flags. If STDIN_FILENO is closed, then the state is similar to the one
for closing STDIN_FILENO without telling stdio, and freopen() fails for
the same reason.
>Is it legal
>to close your standard input and then fork/exec other programs?
Yes. Of course, many programs won't be able to handle this.
Bruce
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