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 ar-
       gument, 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 de-
       scriptor 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 be-

       The field revents is an output parameter, filled by the kernel with the
       events  that  actually  occurred.  The bits returned in revents can in-
       clude 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  oc-

       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 inter-
       val  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:

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

              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 cgroups(7)).

              Writing is now possible, though a write larger that  the  avail-
              able  space  in a socket or pipe will still block (unless O_NON-
              BLOCK 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 de-
              fined (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  rela-
       tionship between select(2) and pselect(2): like pselect(2), ppoll() al-
       lows an application to safely wait until either a file  descriptor  be-
       comes 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 infi-
       nite 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  ma-
       nipulation  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  indi-
       cates that the system call timed out before any file descriptors became

       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  pro-
              gram'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 (nega-

       ENOMEM Unable to allocate memory for kernel data structures.

       The poll() system call was introduced in Linux 2.1.23.  On  older  ker-
       nels that lack this system call, the glibc poll() wrapper function pro-
       vides 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 pro-
       vided in glibc.

       For a discussion of what may happen if a file  descriptor  being  moni-
       tored 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  vari-
       able 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 discus-
       sion on the differences between the kernel and the libc notion  of  the

       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 ar-
       guments  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:

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

       o if the file descriptor is readable, reads some data from it, and dis-
         plays that data on standard output; and

       o if  the  file  descriptors was not readable, but some other event oc-
         curred (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:

       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 pro-
         gram 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 available input.

       o On  the  final  return, poll() indicated only POLLHUP on the FIFO, at
         which point the file descriptor was closed  and  the  program  termi-

   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)

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