[Bug 40852] New: select(2): document action of close()'ing a fd during a select()

[Bug 40852] New: select(2): document action of close()'ing a fd during a select()

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

https://bugzilla.kernel.org/show_bug.cgi?id=40852

           Summary: select(2): document action of close()'ing a fd during
                    a select()
           Product: Documentation
           Version: unspecified
    Kernel Version: n/a
          Platform: All
        OS/Version: Linux
              Tree: Mainline
            Status: NEW
          Severity: normal
          Priority: P1
         Component: man-pages
        AssignedTo: documentation_man-pages-ztI5WcYan/vQLgFONoPN62D2FQJk+8+b@public.gmane.org
        ReportedBy: fillods-iA+eEnwkJgzk1uMJSBkQmQ@public.gmane.org
        Regression: No


I had a hard time figuring out what is the Linux's expected behavior of
select() syscall when a file descriptor is close()'d in the meantime. The man
page 3.27-1ubuntu2 lacks this piece of information. This is on Ubuntu 11.04.

Like many other OS, I would have expect select() to bail out on Linux, but it
is not the case. When some other thread close() a file descriptor currently
watched by select(), Linux does not make the select() syscall to return with
errno=EBADF, it simply ignores it and does not return.
Please note that this behavior is undefined in POSIX. 

IMO, this has to be appropriately documented in the select(2) man page.

Maybe something like "The select() system call never returns if a file
descriptor is closed in the meantime. This behavior is undefined in POSIX".[3]

Rem: this case is not to be confused with a file that has already been closed
before calling select().

Background:

[1]
http://stackoverflow.com/questions/543541/what-does-select2-do-if-you-close2-a-file-descriptor-in-a-separate-thread/543595#543595

[2] http://lkml.indiana.edu/hypermail/linux/kernel/0106.0/0768.html

[3]
https://lists.linux-foundation.org/pipermail/bugme-new/2003-April/008335.html

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[Bug 40852] select(2): document action of close()'ing a fd during a select()

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

https://bugzilla.kernel.org/show_bug.cgi?id=40852


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

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




--- Comment #1 from Michael Kerrisk <mtk.manpages-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>  2012-08-17 05:45:03 ---
Confirmed, with the program below.

#include <sys/time.h>
#include <pthread.h>
#include <sys/types.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>

#define errExit(msg)     do { perror(msg); exit(EXIT_FAILURE); \
            } while (0)

#define errExitEN(en, msg) \
                do { errno = en; perror(msg); \
                     exit(EXIT_FAILURE); } while (0)

static void *
threadFunc(void *arg)
{
    sleep(2);

    printf("Closing file descriptor\n");
    close(STDIN_FILENO);

    return NULL;
} /* threadFunc */

int
main(int argc, char *argv[])
{
    pthread_t t1;
    fd_set readfds;
    int ready, s;

    FD_ZERO(&readfds);
    FD_SET(STDIN_FILENO, &readfds);

    s = pthread_create(&t1, NULL, threadFunc, (void *) 1);
    if (s != 0)
    errExitEN(s, "pthread_create");

    printf("Main thread about to call select()\n");

    ready = select(STDIN_FILENO + 1, &readfds, NULL, NULL, NULL);
    if (ready == -1)
    errExit("select");

    s = pthread_join(t1, NULL);
    if (s != 0)
    errExitEN(s, "pthread_join");

    printf("Main thread exiting\n");

    exit(EXIT_SUCCESS);
}

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[Bug 40852] select(2): document action of close()'ing a fd during a select()

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

https://bugzilla.kernel.org/show_bug.cgi?id=40852





--- Comment #2 from Michael Kerrisk <mtk.manpages-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>  2012-08-17 06:30:36 ---
I added some text under NOTES in select(2).

--- a/man2/select.2
+++ b/man2/select.2
@@ -411,6 +411,22 @@ when
 .B _GNU_SOURCE
 is defined.
 Since glibc 2.2.2 the requirements are as shown in the SYNOPSIS.
+.SS Multithreaded applications
+If a file descriptor being monitored by
+.BR select ()
+is closed in another thread, the result is unspecified.
+On some UNIX systems,
+.BR select ()
+unblocks and returns, with an indication that the file descriptor is ready
+(a subsequent I/O operation will likely fail with an error,
+unless another the file descriptor reopened between the time
+.BR select ()
+returned and the I/O operations was performed).
+On Linux (and some other systems),
+closing the file descriptor in another thread has no effect on
+.BR select ().
+In summary, any application that relies on a particular behavior
+in this scenario must be considered buggy.
 .SS "Linux Notes"
 The
 .BR pselect ()

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[Bug 40852] select(2): document action of close()'ing a fd during a select()

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

https://bugzilla.kernel.org/show_bug.cgi?id=40852





--- Comment #3 from Michael Kerrisk <mtk.manpages-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>  2012-08-17 06:38:16 ---
Also added a note to poll(2) that points to the discussion in select(2).

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[Bug 40852] select(2): document action of close()'ing a fd during a select()

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

https://bugzilla.kernel.org/show_bug.cgi?id=40852


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

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




--- Comment #4 from Michael Kerrisk <mtk.manpages-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>  2012-08-17 06:42:06 ---
Also added a note to epoll_wait(2) that points to the discussion in select(2).

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