ok@quintus.UUCP (Richard A. O'Keefe) (12/23/87)
I was under the impression that if you had a plain Bourne shell
which had been invoked as part of logging in, and you ran a program
without specifying any I/O redirection, you would have precisely
three file descriptors already connected: ^^^^^^^^^
0 - stdin
1 - stdout
2 - stderr
A particular program we have which is part of a larger suite expects
to be called with fds 3 and 4 connected (to pipes, as it happens).
Part of the testing procedure when we put this thing on a new system
is to run this program on its own, e.g.
$ component 3<test-script 4>test-output
$ diff test-output expected-output
At the moment we are putting this thing up on an Apollo running SR9.5,
and the tester didn't realise this thing had arguments, and ran
$ component
with no I/O redirection. We were surprised to see output coming out on
the terminal that was intended for fd 4.
This can be reproduced by running the following script:
#!/bin/sh
cat >four.c <<'EOF'
main()
{
static char message[] = "You should NOT see this!\n";
int rc;
extern int errno;
rc = write(4, message, -1+sizeof message);
printf("sizeof message = %d, rc = %d, errno = %d\n",
sizeof message, rc, errno);
exit(0);
}
EOF
cc -o four four.c
echo This one IS supposed to write the message
./four 3</dev/null 4>/dev/tty
echo This one is NOT supposed to write the message
./four
The C shell acts as expected. On the other hand, it doesn't permit
the redirection we really want.
The question: are file descriptors other than 0,1,2 MEANT to be
preconnected in the Apollo system, and if so, which,
and to what?
if we close 3 and 4, will something break?
are there other "UNIX" systems which do this?gwyn@brl-smoke.ARPA (Doug Gwyn ) (01/06/88)
In article <491@cresswell.quintus.UUCP> ok@quintus.UUCP (Richard A. O'Keefe) writes: > if we close 3 and 4, will something break? > are there other "UNIX" systems which do this? I seem to recall that on Ninth Edition UNIX, fd #3 is open to the controlling terminal by convention. But, hey, if you want to close an fd and re-use it for other purposes, feel free.