Pádraig Brady <[email protected]> writes:
> Nice one.
>
> Note the usage() also states that both files being stdin
> is not supported, so we should remove the " (not both)" qualification.
Oh, nice catch. I didn't see that.
I also noticed just now that POSIX says this behavior is undefined [1]:
If both file1 and file2 refer to standard input or to the same FIFO
special, block special, or character special file, the results are
undefined.
I tested the following implementations that behave the same as us (minus
the closing standard input twice):
* FreeBSD 15.0
* MacOS 26
* OpenBSD 7.8
* NetBSD 10.1
* Solaris 11.4
* BusyBox
* Toybox
The one case I could find that didn't was AIX 7.3:
$ seq 9 | sed 'n;n;p' | /bin/comm - -
comm: 0653-039 File1 and File2 can not both refer to stdin.
Usage: comm [-123] File1 File2
I'm not too sure why anyone would invoke 'comm' like this, but I figured
the behavior was worth documenting in the manual. I didn't mention other
implementations since they may change in the future, who knows.
>> +# In coreutils 9.11 and earlier, running 'comm - -' would close
>> +# standard input twice, leading to an incorrect exit status.
>> +echo 'y' >exp || framework_failure_
>> +yes | head -n 1 | timeout 10 comm - - >out 2>err || fail=1
>
> I'm a bit confused with the use of yes(1) here.
> It might be better to use a more descriptive example
> showing the alternation of lines consumed from the same input:
>
> $ seq 9 | sed 'n;n;p' | src/comm - -
> 1
> 2
> 3
> 4
> 5
> 6
> 7
> 8
> 9
Yep, that is much better. The was testing something else and the
invocation was mostly copy pasted. I agree it looks a bit silly now.
Anyways, I pushed the attached patch which uses your improved test.
Collin
[1] https://pubs.opengroup.org/onlinepubs/9799919799/utilities/comm.html
>From c5ddd417aa8d8e1cf070445760e1747410fb42be Mon Sep 17 00:00:00 2001
Message-ID: <c5ddd417aa8d8e1cf070445760e1747410fb42be.1776911134.git.collin.fu...@gmail.com>
From: Collin Funk <[email protected]>
Date: Tue, 21 Apr 2026 20:10:55 -0700
Subject: [PATCH v2] comm: don't close standard input twice
* NEWS: Mention the bug fix.
* src/comm.c (usage): Remove mention that FILE1 and FILE2 cannot both be
standard input.
(compare_files): Only close standard input once.
* doc/coreutils.texi (comm invocation): Document the behavior of
'comm - -' which is not portable to all implementations.
* tests/comm/dash-dash.sh: New file.
* tests/misc/comm.pl: Move to tests/comm/comm.pl.
* tests/local.mk (all_tests): Add the new test. Rename the existing
test.
---
NEWS | 6 ++++++
doc/coreutils.texi | 6 ++++++
src/comm.c | 6 ++++--
tests/{misc => comm}/comm.pl | 0
tests/comm/dash-dash.sh | 31 +++++++++++++++++++++++++++++++
tests/local.mk | 3 ++-
6 files changed, 49 insertions(+), 3 deletions(-)
rename tests/{misc => comm}/comm.pl (100%)
create mode 100755 tests/comm/dash-dash.sh
diff --git a/NEWS b/NEWS
index 038a1b520..a827cf0d9 100644
--- a/NEWS
+++ b/NEWS
@@ -2,6 +2,12 @@ GNU coreutils NEWS -*- outline -*-
* Noteworthy changes in release ?.? (????-??-??) [?]
+** Bug fixes
+
+ 'comm - -' no longer closes standard input twice. Previously it would
+ mistakenly exit with a nonzero status.
+ [This bug was present in "the beginning".]
+
* Noteworthy changes in release 9.11 (2026-04-20) [stable]
diff --git a/doc/coreutils.texi b/doc/coreutils.texi
index 0f9d3e276..26d405b0b 100644
--- a/doc/coreutils.texi
+++ b/doc/coreutils.texi
@@ -5502,6 +5502,12 @@ @node comm invocation
@end table
+POSIX leaves the behavior undefined when @var{file1} and @var{file2}
+both refer to standard input, i.e., when both are @samp{-}. GNU
+@command{comm} reads the first line as if it came from @var{file1} and
+the second as if it were from @var{file2}, and continues alternating in
+this manner until the end of file is reached.
+
@node ptx invocation
@section @command{ptx}: Produce permuted indexes
diff --git a/src/comm.c b/src/comm.c
index 8e671f40d..b2ceca382 100644
--- a/src/comm.c
+++ b/src/comm.c
@@ -112,7 +112,7 @@ Compare sorted files FILE1 and FILE2 line by line.\n\
"), stdout);
fputs (_("\
\n\
-When FILE1 or FILE2 (not both) is -, read standard input.\n\
+When FILE1 or FILE2 is -, read standard input.\n\
"), stdout);
fputs (_("\
\n\
@@ -388,7 +388,9 @@ compare_files (char **infiles)
}
}
- for (int i = 0; i < 2; i++)
+ /* Avoid closing standard input twice when invoking 'comm - -'. */
+ const int n_streams = 2 - (streams[0] == streams[1]);
+ for (int i = 0; i < n_streams; i++)
if (fclose (streams[i]) != 0)
error (EXIT_FAILURE, errno, "%s", quotef (infiles[i]));
diff --git a/tests/misc/comm.pl b/tests/comm/comm.pl
similarity index 100%
rename from tests/misc/comm.pl
rename to tests/comm/comm.pl
diff --git a/tests/comm/dash-dash.sh b/tests/comm/dash-dash.sh
new file mode 100755
index 000000000..9d7dc6afc
--- /dev/null
+++ b/tests/comm/dash-dash.sh
@@ -0,0 +1,31 @@
+#!/bin/sh
+# Test that 'comm - -' works.
+
+# Copyright (C) 2026 Free Software Foundation, Inc.
+
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+# GNU General Public License for more details.
+
+# You should have received a copy of the GNU General Public License
+# along with this program. If not, see <https://www.gnu.org/licenses/>.
+
+. "${srcdir=.}/tests/init.sh"; path_prepend_ ./src
+print_ver_ comm
+
+printf '1\n\t2\n\t\t3\n4\n\t5\n\t\t6\n7\n\t8\n\t\t9\n' >exp \
+ || framework_failure_
+
+# In coreutils 9.11 and earlier, running 'comm - -' would close
+# standard input twice, leading to an incorrect exit status.
+seq 9 | sed 'n;n;p' | timeout 10 comm - - >out 2>err || fail=1
+compare exp out || fail=1
+compare /dev/null err || fail=1
+
+Exit $fail
diff --git a/tests/local.mk b/tests/local.mk
index 628e54436..fde3ea989 100644
--- a/tests/local.mk
+++ b/tests/local.mk
@@ -324,7 +324,8 @@ all_tests = \
tests/cksum/cksum-base64.pl \
tests/cksum/cksum-base64-untagged.sh \
tests/cksum/cksum-raw.sh \
- tests/misc/comm.pl \
+ tests/comm/comm.pl \
+ tests/comm/dash-dash.sh \
tests/csplit/csplit.sh \
tests/csplit/csplit-1000.sh \
tests/csplit/csplit-heap.sh \
--
2.53.0
