poll() returns -1 but errno not set!

From: Raj Kotaru (raj_kotaru_at_hotmail.com)
Date: 10/29/04

  • Next message: Casper H.S. ***: "Re: poll() returns -1 but errno not set!" 
    Date: 28 Oct 2004 15:20:55 -0700

    Hello all,

    I am running a program compiled on a Sun Solaris machine with SunOS
    5.7 using the GNU egcs1.1.2 g++ compiler. The program opens a socket
    and uses the poll() system call to see if data is available for
    reading.

    I specify INFTIM as the timeout parameter to poll() to cause it to
    hang until data is available. I am also guarding against poll() being
    interrupted by a signal by checking errno against EINTR and re-issuing
    the poll() call if a signal interrupts it.

    If I send an interrupt to the program, I find that poll returns -1 but
    errno is not set. A print statement shows that errno remains 0 after
    the poll() returns.
    truss output shows that within poll() errno is being temporarily set
    to ERESTART, but when poll() returns errno is being reset to 0.

    According to the Solaris man page on poll(), errno will be set to a
    non-zero value any time poll() returns -1. But this is not happening.
     The really strange part is that even though errno remains set to 0, a
    call to perror() results in "Interrupted system call", implying that
    errno must be EINTR. Has anyone encountered this bug? Is there a
    patch fix for it?

    Thanks
    Raj

    See code snippet below:

        static char framebuf [MAX_FRAME_SIZE];
        struct pollfd fds[1];
        char *frameptr = framebuf;

        int bytes_read = 0;
        int status = 0;
        int frame_error_flag = 0;

        fds[0].fd = _sock;
        fds[0].events = POLLIN | POLLHUP | POLLPRI | POLLRDBAND;

        for (;;)
        {
            /*
             * use poll () to sleep until there's something to read or
    hangup
             */
            fds[0].revents = 0;

            while (((status = poll (fds, 1, INFTIM)) < 0) && (errno ==
    EINTR))
            {

               cout << "******* Inside while **********" << endl;
               cout << "status = " << status << endl;
               cout << "errno = " << errno << endl;
               cout << "fds[0].fd = " << fds[0].fd << endl;
               cout << "fds[0].events = " << fds[0].events << endl;
               cout << "fds[0].revents = " << fds[0].revents << endl;
            }

            cout << "@@@@@@@@@ Outside while @@@@@@@@@@" << endl;
               cout << "status = " << status << endl;
               cout << "errno = " << errno << endl;
               cout << "fds[0].fd = " << fds[0].fd << endl;
               cout << "fds[0].events = " << fds[0].events << endl;
               cout << "fds[0].revents = " << fds[0].revents << endl;
            if (status < 0)
            {
                perror ("poll()");
                terminate (-17);
            }

    See program output below:

    ^C@@@@@@@@@ Outside while @@@@@@@@@@
    status = -1
    errno = 0
    fds[0].fd = 6
    fds[0].events = 147
    fds[0].revents = 0
    poll(): Interrupted system call

    See truss output below:

    door_return(0x00000000, 0, 0x00000000, 0) (sleeping...)
    lwp_sema_wait(0xFF24DF08) (sleeping...)
    signotifywait() = 2
    lwp_sigredirect(6, SIGINT) = 0
        Received signal #2, SIGINT, in poll() [caught]
    poll(0xFF0879B8, 1, -1) Err#91 ERESTART
    sysconfig(_CONFIG_SIGRT_MIN) = 38
    sigprocmask(SIG_SETMASK, 0xFF24D520, 0x00000000) = 0
    sigaction(SIGINT, 0xFF087280, 0xFF087420) = 0
    sigaction(SIGINT, 0xFF087280, 0xFF087420) = 0
    sysconfig(_CONFIG_SIGRT_MIN) = 38
    setcontext(0xFF087380)
    write(1, " @ @ @ @ @ @ @ @ @ O u".., 35) = 35
    write(1, " s t a t u s = - 1\n", 12) = 12
    write(1, " e r r n o = 0\n", 10) = 10
    write(1, " f d s [ 0 ] . f d = ".., 14) = 14
    write(1, " f d s [ 0 ] . e v e n t".., 20) = 20
    write(1, " f d s [ 0 ] . r e v e n".., 19) = 19
    write(2, " p o l l ( )", 6) = 6
    write(2, " : ", 2) = 2
    write(2, " I n t e r r u p t e d ".., 23) = 23

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