close STDERR considered puzzling
in [Perl]
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From: Ilya Zakharevich on 5 Jul 2010 06:13 perl -wle "close STDERR; open F, q(>), q(/dev/tty) or print 11; print fileno F; warn 22; print STDERR 33; print $x+77" Try to guess the output (use /dev/con on DOSISH, ' otherwise). Solution below: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . perl -wle "close STDERR; open F, q(>), q(/dev/tty) or print 11; print fileno F; warn 22; print STDERR 33; print $x+77" Name "main::x" used only once: possible typo at -e line 1. 2 22 at -e line 1. print() on closed filehandle STDERR at -e line 1. Use of uninitialized value in addition (+) at -e line 1. 77 ======================================================= Of course, 2=`fileno F' is output, as well as 77. And 33 is not. However, `warn 22' IS output, as well as the warning for `undefined $x'. Well, at least it was not what I expected... Did anybody succeed? Which experiences of yours helped you to guess it? Thanks, Ilya
From: Ben Morrow on 5 Jul 2010 07:41 Quoth Ilya Zakharevich <nospam-abuse(a)ilyaz.org>: > > perl -wle "close STDERR; open F, q(>), q(/dev/tty) or print 11; > print fileno F; warn 22; print STDERR 33; print $x+77" > Name "main::x" used only once: possible typo at -e line 1. > 2 > 22 at -e line 1. > print() on closed filehandle STDERR at -e line 1. > Use of uninitialized value in addition (+) at -e line 1. > 77 > > ======================================================= > > Of course, 2=`fileno F' is output, as well as 77. And 33 is not. > > However, `warn 22' IS output, as well as the warning for `undefined $x'. > > Well, at least it was not what I expected... Did anybody succeed? > Which experiences of yours helped you to guess it? open(2) is documented as opening the lowest-numbered currently-closed file descriptor. If you close STDERR, and thus close fd 2, then the next-opened file handle will be associated with fd 2. If (the Perl-level) STDERR is currently closed, warn will write its output to (the C-level) stderr, which means fd 2. As a rule you should never close STD{IN,OUT,ERR}. If you must, reopen them to or from File::Spec->devnull. Ben
From: Ilya Zakharevich on 5 Jul 2010 09:26 On 2010-07-05, Ben Morrow <ben(a)morrow.me.uk> wrote: > If (the Perl-level) STDERR is currently closed, warn will write its > output to (the C-level) stderr, which means fd 2. It is THIS behaviour which causes unexpected results. Is it documented anywhere? > As a rule you should never close STD{IN,OUT,ERR}. If you must, reopen > them to or from File::Spec->devnull. I MUST close them (in all possible combinations) to find bugs in some UNRELATEDLY obscure ;-) part of perl. And inspecting the output leads to some deep surprises... Thanks, Ilya
From: Ben Morrow on 5 Jul 2010 16:48 Quoth Ilya Zakharevich <nospam-abuse(a)ilyaz.org>: > On 2010-07-05, Ben Morrow <ben(a)morrow.me.uk> wrote: > > > If (the Perl-level) STDERR is currently closed, warn will write its > > output to (the C-level) stderr, which means fd 2. > > It is THIS behaviour which causes unexpected results. Is it > documented anywhere? I don't know. I don't think so. > > As a rule you should never close STD{IN,OUT,ERR}. If you must, reopen > > them to or from File::Spec->devnull. > > I MUST close them (in all possible combinations) to find bugs in some > UNRELATEDLY obscure ;-) part of perl. And inspecting the output leads to > some deep surprises... I see. Ben
From: Ilya Zakharevich on 6 Jul 2010 05:23
On 2010-07-05, Ben Morrow <ben(a)morrow.me.uk> wrote: >> > If (the Perl-level) STDERR is currently closed, warn will write its >> > output to (the C-level) stderr, which means fd 2. >> >> It is THIS behaviour which causes unexpected results. Is it >> documented anywhere? > I don't know. I don't think so. Which does not make the behaviour less puzzling... [ I note that I have seen this behaviour explicitly mentioned on p5p in 90s. But at that time I did not realize how puzzling this behaviour may be. ] BTW, is there any backdoor way to redirect STDERR to some other fd than 2? Thanks again, Ilya |