POLL(2)                   (2020-04-11)                    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, how-
          ever, that this technique can't be used to ignore file
          descriptor 0.)

          The field events is an input parameter, a bit mask specify-
          ing 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 ker-
          nel with the events that actually occurred.  The bits

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          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:

          +o a file descriptor becomes ready;

          +o the call is interrupted by a signal handler; or

          +o 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>:

               There is data to read.

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

               +o There is out-of-band data on a TCP socket (see

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

               +o A cgroup.events file has been modified (see

               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)

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               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

               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 sig-
          nal is caught.

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

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

          is nearly equivalent to atomically executing the following

              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 indef-

          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.

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

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               in the calling program's address space.

               A signal occurred before any requested event; see

               The nfds value exceeds the RLIMIT_NOFILE value.

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

               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-specific.

          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

        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.

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          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 discus-
          sion 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 descrip-
          tors, printing the number of ready file descriptors on
          return.  For each ready file descriptor, the program:

          +o displays the returned revents field in a human-readable

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

          +o if the file descriptors was not readable, but some other
            event occurred (presumably POLLHUP), closes the file

          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 writ-
          ing, 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

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                  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

          +o 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.

          +o The second return from poll() also indicated POLLIN and
            POLLHUP; the program then consumed the last of the avail-
            able input.

          +o 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)

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

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

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

              /* Open each file on command line, and add it aqpfdsaq 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)

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

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

          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/.

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