Path: utzoo!utgpu!utstat!jarvis.csri.toronto.edu!rutgers!apple!ames!pasteur!ucbvax!ulysses!hector!ekrell
From: ekrell@hector.UUCP (Eduardo Krell)
Newsgroups: comp.unix.wizards
Subject: Re: shell file descriptor programming (was: Unlinked temp files)
Message-ID: <11540@ulysses.homer.nj.att.com>
Date: 18 May 89 02:22:27 GMT
References: <871@marvin.Solbourne.COM> <1015@philmds.UUCP> <296@tree.UUCP> <4542@psuvax1.cs.psu.edu> <134@minya.UUCP> <11529@ulysses.homer.nj.att.com> <8473@chinet.chi.il.us>
Sender: netnews@ulysses.homer.nj.att.com
Reply-To: ekrell@hector.UUCP (Eduardo Krell)
Organization: AT&T Bell Laboratories
Lines: 37

In article <8473@chinet.chi.il.us> les@chinet.chi.il.us (Leslie Mikesell) writes:

>How about the man page for exec(2) where it says that file descriptors
>open in the calling process remain open in the new process?

That man page says nothing about individual programs.

>>The only standard file descriptors that all programs expect to be
>>opened are 0, 1, and 2.
>
>Is this documented somewhere?

It's tradition. Why do you think stdio only defines stdin, stdout and
stderr?

>If I:
>exec 3<foo 4>bar
>under what conditions should I not expect to be able to access these
>files?

Those files are opened to the shell. The shell has every right to
exec a process with the close-on-exec flags on all file descriptors
other than 0, 1, 2 and any file descriptors indicated in the command line
(as in "program 3<foo 4>bar").

The real problem is that the close-on-exec flag was a late arrival
into Unix and so the default exec() behavior remained not to use
the close-on-exec flag (because there was no such a flag when it
all started). I think this is a mistake. File descriptors are part
of the environment of a process, and like environment variables,
shouldn't be exported by the shell unless it's done explicitly
(by something like the overloading of the "export" builtin I
described).
    
Eduardo Krell                   AT&T Bell Laboratories, Murray Hill, NJ

UUCP: {att,decvax,ucbvax}!ulysses!ekrell  Internet: ekrell@ulysses.att.com