[committed] libstdc++: Fix tests that open FIFOs with O_RDWR [PR127424]
Tomasz Kaminski
tkaminsk@redhat.com
Fri Sep 18 08:00:32 GMT 2026
On Thu, Sep 17, 2026 at 8:01 PM Jonathan Wakely <jwakely@redhat.com> wrote:
> From: Gökbey Keskin <gokbeykeskin@gmail.com>
>
> Several tests under 27_io open a FIFO with ios_base::in|ios_base::out,
> which
> fopen_mode() maps to "r+", i.e. O_RDWR. POSIX leaves this undefined:
>
> O_RDWR
> Open for reading and writing. The result is undefined if this flag
> is
> applied to a FIFO.
>
> Linux permits it, so the problem is invisible there, but it is not
> required.
> On a target where opening a FIFO O_RDWR fails, the affected tests deadlock:
> the process that issues the O_RDWR open fails to open the FIFO, so its peer
> blocks forever in a blocking open() for the opposite direction, because the
> FIFO never acquires a second endpoint.
>
> None of these tests needs read-write access. In every case the process
> opening in|out only ever writes. Opening write-only replaces the O_RDWR
> open
> with the ordinary FIFO rendezvous (child O_WRONLY, parent O_RDONLY, or vice
> versa), which is fully specified and introduces no ordering hazard.
>
> Second, separate issue: close/char/9964.cc has not tested PR 9964 since
> 2004.
>
> PR libstdc++/9964 was fixed by Petur Runolfsson in
> r0-48647-g0c45b8e0733816 so that basic_filebuf::close() always closes
> the file even if it returns null to indicate an error.
>
> The test as originally committed opened the parent end write-only and
> asserted
> exactly that:
>
> filebuf* ret = fb.open(name, ios_base::out | ios_base::trunc);
> ...
> ret = fb.close();
> VERIFY( ret == NULL );
> VERIFY( !fb.is_open() );
>
> The write-only open is what makes the flush fail: the child closes the sole
> read end, the parent's buffered 'a' meets a FIFO with no readers, SIGPIPE
> is
> ignored, and write() fails with EPIPE.
>
> r0-56237-g6a734d618f8 (2004-02-04, "Correct flags to filebuf::open calls",
> the commit that introduced fopen_mode) changed both the mode and the
> expectation:
>
> - filebuf* ret = fb.open(name, ios_base::out | ios_base::trunc);
> + filebuf* ret = fb.open(name, ios_base::in | ios_base::out);
> ...
> - VERIFY( ret == NULL );
> + VERIFY( ret != NULL );
>
> With O_RDWR the parent holds its own read end, so the write at close
> succeeds
> and the failure path is never entered. The test now passes whether or not
> the
> PR 9964 fix is present, i.e. it is a dead regression test.
>
> So the fix for this file is to restore the original write-only open and the
> original close() expectation, which also removes its O_RDWR dependency.
>
> libstdc++-v3/ChangeLog:
>
> PR libstdc++/127424
> * testsuite/27_io/basic_filebuf/close/char/9964.cc: Only open
> filebuf for output and expect close() to return null.
> * testsuite/27_io/basic_filebuf/seekoff/char/26777.cc: Only open
> filebuf for output.
> * testsuite/27_io/basic_filebuf/showmanyc/char/9533-1.cc:
> Likewise.
> * testsuite/27_io/basic_filebuf/underflow/char/10097.cc:
> Likewise.
> * testsuite/27_io/objects/char/7.cc: Likewise.
> * testsuite/27_io/objects/char/9661-1.cc: Only open FILE for
> output.
> ---
>
> Tested x86_64-linux. Pushed to trunk.
>
> I'm ambivalent whether to backport this one.
>
The reporter does not seem to need it, so I think there is no need.
>
> .../testsuite/27_io/basic_filebuf/close/char/9964.cc | 7 +++++--
> .../testsuite/27_io/basic_filebuf/seekoff/char/26777.cc | 2 +-
> .../testsuite/27_io/basic_filebuf/showmanyc/char/9533-1.cc | 2 +-
> .../testsuite/27_io/basic_filebuf/underflow/char/10097.cc | 2 +-
> libstdc++-v3/testsuite/27_io/objects/char/7.cc | 2 +-
> libstdc++-v3/testsuite/27_io/objects/char/9661-1.cc | 2 +-
> 6 files changed, 10 insertions(+), 7 deletions(-)
>
> diff --git a/libstdc++-v3/testsuite/27_io/basic_filebuf/close/char/9964.cc
> b/libstdc++-v3/testsuite/27_io/basic_filebuf/close/char/9964.cc
> index 84b406ac0727..e34eea5923b8 100644
> --- a/libstdc++-v3/testsuite/27_io/basic_filebuf/close/char/9964.cc
> +++ b/libstdc++-v3/testsuite/27_io/basic_filebuf/close/char/9964.cc
> @@ -64,15 +64,18 @@ bool test_07()
> }
>
> filebuf fb;
> - filebuf* ret = fb.open(name, ios_base::in | ios_base::out);
> + filebuf* ret = fb.open(name, ios_base::out);
> test &= bool( ret != 0 );
> test &= bool( fb.is_open() );
> s1.signal();
> s2.wait();
> fb.sputc('a');
>
> + // The child has closed the only read end of the fifo, so flushing the
> + // pending 'a' fails with EPIPE. close() must report that failure by
> + // returning null, but must close the file all the same.
> ret = fb.close();
> - test &= bool( ret != 0 );
> + test &= bool( ret == 0 );
> test &= bool( !fb.is_open() );
>
> return test;
> diff --git
> a/libstdc++-v3/testsuite/27_io/basic_filebuf/seekoff/char/26777.cc
> b/libstdc++-v3/testsuite/27_io/basic_filebuf/seekoff/char/26777.cc
> index 1931db09ad04..c7c091e4999e 100644
> --- a/libstdc++-v3/testsuite/27_io/basic_filebuf/seekoff/char/26777.cc
> +++ b/libstdc++-v3/testsuite/27_io/basic_filebuf/seekoff/char/26777.cc
> @@ -54,7 +54,7 @@ bool test01()
> if (child == 0)
> {
> filebuf fbout;
> - fbout.open(name, ios_base::in | ios_base::out);
> + fbout.open(name, ios_base::out);
> test &= bool( fbout.is_open() );
> fbout.sputn("Whatever", 8);
> fbout.pubsync();
> diff --git
> a/libstdc++-v3/testsuite/27_io/basic_filebuf/showmanyc/char/9533-1.cc
> b/libstdc++-v3/testsuite/27_io/basic_filebuf/showmanyc/char/9533-1.cc
> index 57b234515f09..9a42cadacc11 100644
> --- a/libstdc++-v3/testsuite/27_io/basic_filebuf/showmanyc/char/9533-1.cc
> +++ b/libstdc++-v3/testsuite/27_io/basic_filebuf/showmanyc/char/9533-1.cc
> @@ -55,7 +55,7 @@ void test_01()
> else if (fval == 0)
> {
> filebuf ofbuf;
> - ofbuf.open(name, ios_base::in|ios_base::out);
> + ofbuf.open(name, ios_base::out);
> VERIFY( ofbuf.is_open() );
> sleep(1);
>
> diff --git
> a/libstdc++-v3/testsuite/27_io/basic_filebuf/underflow/char/10097.cc
> b/libstdc++-v3/testsuite/27_io/basic_filebuf/underflow/char/10097.cc
> index c320b99920bd..eb117bf736ed 100644
> --- a/libstdc++-v3/testsuite/27_io/basic_filebuf/underflow/char/10097.cc
> +++ b/libstdc++-v3/testsuite/27_io/basic_filebuf/underflow/char/10097.cc
> @@ -74,7 +74,7 @@ bool test16()
> else if (fval == 0)
> {
> filebuf fbout;
> - fbout.open(name, ios_base::in|ios_base::out);
> + fbout.open(name, ios_base::out);
> test &= bool( fbout.is_open() );
> fbout.sputn("0123456789", 10);
> fbout.pubsync();
> diff --git a/libstdc++-v3/testsuite/27_io/objects/char/7.cc
> b/libstdc++-v3/testsuite/27_io/objects/char/7.cc
> index 2edcf56f7889..5a60e1df2b21 100644
> --- a/libstdc++-v3/testsuite/27_io/objects/char/7.cc
> +++ b/libstdc++-v3/testsuite/27_io/objects/char/7.cc
> @@ -55,7 +55,7 @@ bool test07()
> if (child == 0)
> {
> filebuf fbout;
> - fbout.open(name, ios_base::in|ios_base::out);
> + fbout.open(name, ios_base::out);
> test &= bool( fbout.is_open() );
> s1.wait();
> cout.rdbuf(&fbout);
> diff --git a/libstdc++-v3/testsuite/27_io/objects/char/9661-1.cc
> b/libstdc++-v3/testsuite/27_io/objects/char/9661-1.cc
> index 9374685969db..ba7f55c3bf33 100644
> --- a/libstdc++-v3/testsuite/27_io/objects/char/9661-1.cc
> +++ b/libstdc++-v3/testsuite/27_io/objects/char/9661-1.cc
> @@ -55,7 +55,7 @@ bool test01()
>
> if (child == 0)
> {
> - FILE* file = fopen(name, "r+");
> + FILE* file = fopen(name, "w");
> test &= bool( file != 0 );
> fputs("Whatever\n", file);
> fflush(file);
> --
> 2.55.0
>
>
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