poll(2) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | CONFORMING TO | NOTES | BUGS | EXAMPLES | SEE ALSO | COLOPHON

POLL(2)                 Linux Programmer's Manual                POLL(2)

NAME         top

       poll, ppoll - wait for some event on a file descriptor

SYNOPSIS         top

       #include <poll.h>

       int poll(struct pollfd *fds, nfds_t nfds, int timeout);

       #define _GNU_SOURCE         /* See feature_test_macros(7) */
       #include <signal.h>
       #include <poll.h>

       int ppoll(struct pollfd *fds, nfds_t nfds,
               const struct timespec *tmo_p, const sigset_t *sigmask);

DESCRIPTION         top

       poll() performs a similar task to select(2): it waits for one of
       a set of file descriptors to become ready to perform I/O.  The
       Linux-specific epoll(7) API performs a similar task, but offers
       features beyond those found in poll().

       The set of file descriptors to be monitored is specified in the
       fds argument, which is an array of structures of the following
       form:

           struct pollfd {
               int   fd;         /* file descriptor */
               short events;     /* requested events */
               short revents;    /* returned events */
           };

       The caller should specify the number of items in the fds array in
       nfds.

       The field fd contains a file descriptor for an open file.  If
       this field is negative, then the corresponding events field is
       ignored and the revents field returns zero.  (This provides an
       easy way of ignoring a file descriptor for a single poll() call:
       simply negate the fd field.  Note, however, that this technique
       can't be used to ignore file descriptor 0.)

       The field events is an input parameter, a bit mask specifying the
       events the application is interested in for the file descriptor
       fd.  This field may be specified as zero, in which case the only
       events that can be returned in revents are POLLHUP, POLLERR, and
       POLLNVAL (see below).

       The field revents is an output parameter, filled by the kernel
       with the events that actually occurred.  The bits returned in
       revents can include any of those specified in events, or one of
       the values POLLERR, POLLHUP, or POLLNVAL.  (These three bits are
       meaningless in the events field, and will be set in the revents
       field whenever the corresponding condition is true.)

       If none of the events requested (and no error) has occurred for
       any of the file descriptors, then poll() blocks until one of the
       events occurs.

       The timeout argument specifies the number of milliseconds that
       poll() should block waiting for a file descriptor to become
       ready.  The call will block until either:

       • a file descriptor becomes ready;

       • the call is interrupted by a signal handler; or

       • the timeout expires.

       Note that the timeout interval will be rounded up to the system
       clock granularity, and kernel scheduling delays mean that the
       blocking interval may overrun by a small amount.  Specifying a
       negative value in timeout means an infinite timeout.  Specifying
       a timeout of zero causes poll() to return immediately, even if no
       file descriptors are ready.

       The bits that may be set/returned in events and revents are
       defined in <poll.h>:

       POLLIN There is data to read.

       POLLPRI
              There is some exceptional condition on the file
              descriptor.  Possibilities include:

              • There is out-of-band data on a TCP socket (see tcp(7)).

              • A pseudoterminal master in packet mode has seen a state
                change on the slave (see ioctl_tty(2)).

              • A cgroup.events file has been modified (see cgroups(7)).

       POLLOUT
              Writing is now possible, though a write larger than the
              available space in a socket or pipe will still block
              (unless O_NONBLOCK is set).

       POLLRDHUP (since Linux 2.6.17)
              Stream socket peer closed connection, or shut down writing
              half of connection.  The _GNU_SOURCE feature test macro
              must be defined (before including any header files) in
              order to obtain this definition.

       POLLERR
              Error condition (only returned in revents; ignored in
              events).  This bit is also set for a file descriptor
              referring to the write end of a pipe when the read end has
              been closed.

       POLLHUP
              Hang up (only returned in revents; ignored in events).
              Note that when reading from a channel such as a pipe or a
              stream socket, this event merely indicates that the peer
              closed its end of the channel.  Subsequent reads from the
              channel will return 0 (end of file) only after all
              outstanding data in the channel has been consumed.

       POLLNVAL
              Invalid request: fd not open (only returned in revents;
              ignored in events).

       When compiling with _XOPEN_SOURCE defined, one also has the
       following, which convey no further information beyond the bits
       listed above:

       POLLRDNORM
              Equivalent to POLLIN.

       POLLRDBAND
              Priority band data can be read (generally unused on
              Linux).

       POLLWRNORM
              Equivalent to POLLOUT.

       POLLWRBAND
              Priority data may be written.

       Linux also knows about, but does not use POLLMSG.

   ppoll()
       The relationship between poll() and ppoll() is analogous to the
       relationship between select(2) and pselect(2): like pselect(2),
       ppoll() allows an application to safely wait until either a file
       descriptor becomes ready or until a signal is caught.

       Other than the difference in the precision of the timeout
       argument, the following ppoll() call:

           ready = ppoll(&fds, nfds, tmo_p, &sigmask);

       is nearly equivalent to atomically executing the following calls:

           sigset_t origmask;
           int timeout;

           timeout = (tmo_p == NULL) ? -1 :
                     (tmo_p->tv_sec * 1000 + tmo_p->tv_nsec / 1000000);
           pthread_sigmask(SIG_SETMASK, &sigmask, &origmask);
           ready = poll(&fds, nfds, timeout);
           pthread_sigmask(SIG_SETMASK, &origmask, NULL);

       The above code segment is described as nearly equivalent because
       whereas a negative timeout value for poll() is interpreted as an
       infinite timeout, a negative value expressed in *tmo_p results in
       an error from ppoll().

       See the description of pselect(2) for an explanation of why
       ppoll() is necessary.

       If the sigmask argument is specified as NULL, then no signal mask
       manipulation is performed (and thus ppoll() differs from poll()
       only in the precision of the timeout argument).

       The tmo_p argument specifies an upper limit on the amount of time
       that ppoll() will block.  This argument is a pointer to a
       structure of the following form:

           struct timespec {
               long    tv_sec;         /* seconds */
               long    tv_nsec;        /* nanoseconds */
           };

       If tmo_p is specified as NULL, then ppoll() can block
       indefinitely.

RETURN VALUE         top

       On success, poll() returns a nonnegative value which is the
       number of elements in the pollfds whose revents fields have been
       set to a nonzero value (indicating an event or an error).  A
       return value of zero indicates that the system call timed out
       before any file descriptors became read.

       On error, -1 is returned, and errno is set to indicate the cause
       of the error.

ERRORS         top

       EFAULT fds points outside the process's accessible address space.
              The array given as argument was not contained in the
              calling program's address space.

       EINTR  A signal occurred before any requested event; see
              signal(7).

       EINVAL The nfds value exceeds the RLIMIT_NOFILE value.

       EINVAL (ppoll()) The timeout value expressed in *ip is invalid
              (negative).

       ENOMEM Unable to allocate memory for kernel data structures.

VERSIONS         top

       The poll() system call was introduced in Linux 2.1.23.  On older
       kernels that lack this system call, the glibc poll() wrapper
       function provides emulation using select(2).

       The ppoll() system call was added to Linux in kernel 2.6.16.  The
       ppoll() library call was added in glibc 2.4.

CONFORMING TO         top

       poll() conforms to POSIX.1-2001 and POSIX.1-2008.  ppoll() is
       Linux-specific.

NOTES         top

       The operation of poll() and ppoll() is not affected by the
       O_NONBLOCK flag.

       On some other UNIX systems, poll() can fail with the error EAGAIN
       if the system fails to allocate kernel-internal resources, rather
       than ENOMEM as Linux does.  POSIX permits this behavior.
       Portable programs may wish to check for EAGAIN and loop, just as
       with EINTR.

       Some implementations define the nonstandard constant INFTIM with
       the value -1 for use as a timeout for poll().  This constant is
       not provided in glibc.

       For a discussion of what may happen if a file descriptor being
       monitored by poll() is closed in another thread, see select(2).

   C library/kernel differences
       The Linux ppoll() system call modifies its tmo_p argument.
       However, the glibc wrapper function hides this behavior by using
       a local variable for the timeout argument that is passed to the
       system call.  Thus, the glibc ppoll() function does not modify
       its tmo_p argument.

       The raw ppoll() system call has a fifth argument, size_t
       sigsetsize, which specifies the size in bytes of the sigmask
       argument.  The glibc ppoll() wrapper function specifies this
       argument as a fixed value (equal to sizeof(kernel_sigset_t)).
       See sigprocmask(2) for a discussion on the differences between
       the kernel and the libc notion of the sigset.

BUGS         top

       See the discussion of spurious readiness notifications under the
       BUGS section of select(2).

EXAMPLES         top

       The program below opens each of the files named in its command-
       line arguments and monitors the resulting file descriptors for
       readiness to read (POLLIN).  The program loops, repeatedly using
       poll() to monitor the file descriptors, printing the number of
       ready file descriptors on return.  For each ready file
       descriptor, the program:

       • displays the returned revents field in a human-readable form;

       • if the file descriptor is readable, reads some data from it,
         and displays that data on standard output; and

       • if the file descriptors was not readable, but some other event
         occurred (presumably POLLHUP), closes the file descriptor.

       Suppose we run the program in one terminal, asking it to open a
       FIFO:

           $ mkfifo myfifo
           $ ./poll_input myfifo

       In a second terminal window, we then open the FIFO for writing,
       write some data to it, and close the FIFO:

           $ echo aaaaabbbbbccccc > myfifo

       In the terminal where we are running the program, we would then
       see:

           Opened "myfifo" on fd 3
           About to poll()
           Ready: 1
             fd=3; events: POLLIN POLLHUP
               read 10 bytes: aaaaabbbbb
           About to poll()
           Ready: 1
             fd=3; events: POLLIN POLLHUP
               read 6 bytes: ccccc

           About to poll()
           Ready: 1
             fd=3; events: POLLHUP
               closing fd 3
           All file descriptors closed; bye

       In the above output, we see that poll() returned three times:

       • On the first return, the bits returned in the revents field
         were POLLIN, indicating that the file descriptor is readable,
         and POLLHUP, indicating that the other end of the FIFO has been
         closed.  The program then consumed some of the available input.

       • The second return from poll() also indicated POLLIN and
         POLLHUP; the program then consumed the last of the available
         input.

       • On the final return, poll() indicated only POLLHUP on the FIFO,
         at which point the file descriptor was closed and the program
         terminated.

   Program source

       /* poll_input.c

          Licensed under GNU General Public License v2 or later.
       */
       #include <poll.h>
       #include <fcntl.h>
       #include <sys/types.h>
       #include <stdio.h>
       #include <stdlib.h>
       #include <unistd.h>

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

       int
       main(int argc, char *argv[])
       {
           int nfds, num_open_fds;
           struct pollfd *pfds;

           if (argc < 2) {
              fprintf(stderr, "Usage: %s file...\n", argv[0]);
              exit(EXIT_FAILURE);
           }

           num_open_fds = nfds = argc - 1;
           pfds = calloc(nfds, sizeof(struct pollfd));
           if (pfds == NULL)
               errExit("malloc");

           /* Open each file on command line, and add it 'pfds' array */

           for (int j = 0; j < nfds; j++) {
               pfds[j].fd = open(argv[j + 1], O_RDONLY);
               if (pfds[j].fd == -1)
                   errExit("open");

               printf("Opened \"%s\" on fd %d\n", argv[j + 1], pfds[j].fd);

               pfds[j].events = POLLIN;
           }

           /* Keep calling poll() as long as at least one file descriptor is
              open */

           while (num_open_fds > 0) {
               int ready;

               printf("About to poll()\n");
               ready = poll(pfds, nfds, -1);
               if (ready == -1)
                   errExit("poll");

               printf("Ready: %d\n", ready);

               /* Deal with array returned by poll() */

               for (int j = 0; j < nfds; j++) {
                   char buf[10];

                   if (pfds[j].revents != 0) {
                       printf("  fd=%d; events: %s%s%s\n", pfds[j].fd,
                               (pfds[j].revents & POLLIN)  ? "POLLIN "  : "",
                               (pfds[j].revents & POLLHUP) ? "POLLHUP " : "",
                               (pfds[j].revents & POLLERR) ? "POLLERR " : "");

                       if (pfds[j].revents & POLLIN) {
                           ssize_t s = read(pfds[j].fd, buf, sizeof(buf));
                           if (s == -1)
                               errExit("read");
                           printf("    read %zd bytes: %.*s\n",
                                   s, (int) s, buf);
                       } else {                /* POLLERR | POLLHUP */
                           printf("    closing fd %d\n", pfds[j].fd);
                           if (close(pfds[j].fd) == -1)
                               errExit("close");
                           num_open_fds--;
                       }
                   }
               }
           }

           printf("All file descriptors closed; bye\n");
           exit(EXIT_SUCCESS);
       }

SEE ALSO         top

       restart_syscall(2), select(2), select_tut(2), epoll(7), time(7)

COLOPHON         top

       This page is part of release 5.10 of the Linux man-pages project.
       A description of the project, information about reporting bugs,
       and the latest version of this page, can be found at
       https://www.kernel.org/doc/man-pages/.

Linux                          2020-04-11                        POLL(2)

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