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

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

       #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);

       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

       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

       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

       POLLIN There is data to read.

              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)).

              Writing is now possible, though a write larger that 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.

              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.

              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.

              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:

              Equivalent to POLLIN.

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

              Equivalent to POLLOUT.

              Priority data may be written.

       Linux also knows about, but does not use POLLMSG.

       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

       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.

       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

       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

       ENOMEM Unable to allocate memory for kernel data structures.

       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.

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

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

       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.

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

       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

   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)

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

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

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

           /* 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)

               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)

               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)
                           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)

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

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

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Linux                             2020-04-11                           POLL(2)