[Bug 95191] New: document behavior of open(2) when path names a fifo with no readers

[Bug 95191] New: document behavior of open(2) when path names a fifo with no readers

[bugzilla-daemon-590EEB7GvNiWaY/ihj7yzEB+6BGkLq7r]

https://bugzilla.kernel.org/show_bug.cgi?id=95191

            Bug ID: 95191
           Summary: document behavior of open(2) when path names a  fifo
                    with no readers
           Product: Documentation
           Version: unspecified
          Hardware: All
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P1
         Component: man-pages
          Assignee: documentation_man-pages-ztI5WcYan/vQLgFONoPN62D2FQJk+8+b@public.gmane.org
          Reporter: jason.vas.dias-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org
        Regression: No

linux 3.13 open() , if the path refers to a fifo, and O_WRONLY is set in
its flags , but not O_NONBLOCK, then it does not return until a reader has
connected to the output end of the pipe.

This fact is not documented anywhere in the open(2) manual page - in fact,
it only mentions :

    ENXIO  O_NONBLOCK | O_WRONLY is set, the named file is a FIFO and no
process 
           has the file open for reading.  Or, the file is a device special
file 
           and no corresponding device exists.

You can easily verify this behaviour with bash :

  $ mkfifo /tmp/a.fifo
  $ strace -f bash -c 'echo 1 >/tmp/a.fifo' &
  ...
  open("/tmp/a.fifo", O_WRONLY|O_CREAT|O_TRUNC, 0666
  $ read res </tmp/a.fifo
) = 3
fcntl(1, F_GETFD)                       = 0
fcntl(1, F_DUPFD, 10)                   = 10
fcntl(1, F_GETFD)                       = 0
fcntl(10, F_SETFD, FD_CLOEXEC)          = 0
dup2(3, 1)                              = 1
close(3)                                = 0
write(1, "1\n", 2)                      = 2
dup2(10, 1)                             = 1
fcntl(10, F_GETFD)                      = 0x1 (flags FD_CLOEXEC)
close(10)                               = 0
rt_sigprocmask(SIG_BLOCK, [CHLD], [], 8) = 0
rt_sigprocmask(SIG_SETMASK, [], NULL, 8) = 0
exit_group(0)                           = ?
+++ exited with 0 +++

Please add words to the effect :

  "If the path refers to a named pipe (FIFO), and O_NONBLOCK is not set,
   open() will block until a reader is connected to the write end of the pipe.
  "
to the open(2) manual page.

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[Bug 95191] document behavior of open(2) when path names a fifo with no readers

[bugzilla-daemon-590EEB7GvNiWaY/ihj7yzEB+6BGkLq7r]

https://bugzilla.kernel.org/show_bug.cgi?id=95191

--- Comment #1 from Jason Vas Dias <jason.vas.dias-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> ---
I think the fcntl documentation is also particularly misleading when it
suggests
that output file descriptors can be enabled to have SIGIO sent for them when 
only O_ASYNC and not O_NONBLOCK bits are set in the FD flags .
I cannot get the attached program to work if I do not open() the fifo file -
it will not do to simply fcntl(fd,F_SETFL,previous_flags|O_ASYNC) and 
fcntl(fd,F_SETOWN_EX,{ .type = F_OWNER_TID, .pid  = gettid() }) and
 sigio_sa = (struct sigaction)
      { .sa_sigaction = sigio_handler,
        .sa_flags = SA_NODEFER | SA_SIGINFO 
      };  
  if( sigaction(SIGIO, &sigio_sa, &sigio_prev_sa)
and then expect the process /  thread to receive SIGIO
where si->si_fd == 1 for stdout - the process never gets a SIGIO signal.
Is this a kernel bug ? if not why is this not documented ?

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[Bug 95191] document behavior of open(2) when path names a fifo with no readers

[bugzilla-daemon-590EEB7GvNiWaY/ihj7yzEB+6BGkLq7r]

https://bugzilla.kernel.org/show_bug.cgi?id=95191

--- Comment #2 from Jason Vas Dias <jason.vas.dias-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> ---
Created attachment 171551
  --> https://bugzilla.kernel.org/attachment.cgi?id=171551&action=edit
illustrates problem - #1 working version that uses open

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[Bug 95191] document behavior of open(2) when path names a fifo with no readers

[bugzilla-daemon-590EEB7GvNiWaY/ihj7yzEB+6BGkLq7r]

https://bugzilla.kernel.org/show_bug.cgi?id=95191

--- Comment #3 from Jason Vas Dias <jason.vas.dias-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> ---
Created attachment 171561
  --> https://bugzilla.kernel.org/attachment.cgi?id=171561&action=edit
illustration #2: program that attempts to enable SIGIO on output fd 1 only -
fails to recieve SIGIO

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[Bug 95191] document behavior of open(2) when path names a fifo with no readers

[bugzilla-daemon-590EEB7GvNiWaY/ihj7yzEB+6BGkLq7r]

https://bugzilla.kernel.org/show_bug.cgi?id=95191

--- Comment #4 from Jason Vas Dias <jason.vas.dias-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> ---
Should have added:
To test: 
  Save attachment #1 as t_sigio_writer.c
  Save attachment #2 as t_sigio_writer_no_open.c
  $ gcc -o t_sigio_writer t_sigio_writer.c
  $ mkfifo /tmp/a.fifo
  $ ./t_sigio_writer /tmp/a.fifo &
  $ read -d $'\004' res < /tmp/a.fifo && echo "RES: $res"
  RES:
  $
  $ gcc -o t_sigio_writer_no_open t_sigio_writer_no_open.c
  $ ./t_sigio_writer_no_open </tmp/a.fifo &
  $ read -d $'\004' res < /tmp/a.fifo && echo "RES: $res"
  ^C

An strace of 2nd process, which only does fcntl(1,F_SETFL,flags|O_ASYNC),
and does identical signal handling, that had to be killed,  shows it only
gets a SIGIO on FD 0, not on FD 1, after <CTRL+C> is pressed.

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[Bug 95191] document behavior of open(2) when path names a fifo with no readers

[bugzilla-daemon-590EEB7GvNiWaY/ihj7yzEB+6BGkLq7r]

https://bugzilla.kernel.org/show_bug.cgi?id=95191

--- Comment #5 from Jason Vas Dias <jason.vas.dias-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> ---
Oops :
$ ./t_sigio_writer_no_open </tmp/a.fifo &
should have been:
$ ./t_sigio_writer_no_open >/tmp/a.fifo &

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[Bug 95191] document behavior of open(2) when path names a fifo with no readers

[bugzilla-daemon-590EEB7GvNiWaY/ihj7yzEB+6BGkLq7r]

https://bugzilla.kernel.org/show_bug.cgi?id=95191

--- Comment #6 from Jason Vas Dias <jason.vas.dias-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> ---
Oops! Actually, all tests pass if compiled with '-pthread' .

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[Bug 95191] document behavior of open(2) when path names a fifo with no readers

[bugzilla-daemon-590EEB7GvNiWaY/ihj7yzEB+6BGkLq7r]

https://bugzilla.kernel.org/show_bug.cgi?id=95191

--- Comment #7 from Jason Vas Dias <jason.vas.dias-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> ---
Created attachment 171581
  --> https://bugzilla.kernel.org/attachment.cgi?id=171581&action=edit
better version of t_sigio_write_no_open

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[Bug 95191] document behavior of open(2) when path names a fifo with no readers

[bugzilla-daemon-590EEB7GvNiWaY/ihj7yzEB+6BGkLq7r]

https://bugzilla.kernel.org/show_bug.cgi?id=95191

Michael Kerrisk <mtk.manpages-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |mtk.manpages-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org

--- Comment #8 from Michael Kerrisk <mtk.manpages-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> ---
Hello Jason,

(In reply to Jason Vas Dias from comment #0)
> linux 3.13 open() , if the path refers to a fifo, and O_WRONLY is set in
> its flags , but not O_NONBLOCK, then it does not return until a reader has
> connected to the output end of the pipe.
> 
> This fact is not documented anywhere in the open(2) manual page

[...]

> Please add words to the effect :
> 
>   "If the path refers to a named pipe (FIFO), and O_NONBLOCK is not set,
>    open() will block until a reader is connected to the write end of the
> pipe.
>   "
> to the open(2) manual page.

Well, there is already this text in the man page:

    O_NONBLOCK or O_NDELAY
        .... For the handling of FIFOs (named pipes), see also fifo(7).

(Were you aware of the fifo(7) page?)

But it's true that something could be said more prominently in 
open(2), I think. I added the following under NOTES:

   FIFOs
       Opening  the read or write end of a FIFO blocks until the other
       end is also opened (by another process or thread).  See fifo(7)
       for further details.

Thanks,

Michael

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[Bug 95191] document behavior of open(2) when path names a fifo with no readers

[bugzilla-daemon-590EEB7GvNiWaY/ihj7yzEB+6BGkLq7r]

https://bugzilla.kernel.org/show_bug.cgi?id=95191

Michael Kerrisk <mtk.manpages-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|---                         |CODE_FIX

--- Comment #9 from Michael Kerrisk <mtk.manpages-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> ---
(In reply to Jason Vas Dias from comment #1)
> I think the fcntl documentation is also particularly misleading when it
> suggests
> that output file descriptors can be enabled to have SIGIO sent for them when 
> only O_ASYNC and not O_NONBLOCK bits are set in the FD flags .

Hello Jason,

I don't understand what you mean here. But, also, I would really prefer a
separate bug report, since this seems to be a separate issue.

I'm going to close this bug. Could you raise the above as a separate topics,
and repost example programs. But please: make the example programs as simple as
possible. I see extraneous stuff in the code you posted already, which just
makes it harder for me to work out what's going on.

Thanks,

Michael

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[Bug 95191] document behavior of open(2) when path names a fifo with no readers

[bugzilla-daemon-590EEB7GvNiWaY/ihj7yzEB+6BGkLq7r]

https://bugzilla.kernel.org/show_bug.cgi?id=95191

--- Comment #10 from Jason Vas Dias <jason.vas.dias-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> ---
Thanks for taking a look at this, Micheal. As you suggested, I opened
another bug: Bug #95331 - please take a look.

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