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


       Standard C library (libc, -lc)


       #include <poll.h>

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

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

       int ppoll(struct pollfd *fds, nfds_t nfds,
                 const struct timespec *_Nullable tmo_p,
                 const sigset_t *_Nullable 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  set
       the fd field to its bitwise complement.)

       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.

              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

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

              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.

              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()

           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

       The tmo_p argument specifies an upper limit on the amount of time that ppoll() will block.
       This argument is a pointer to a timespec(3) structure.

       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 ready.

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


       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 *tmo_p is invalid (negative).

       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 in Linux 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 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

       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


       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 descriptor 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 <fcntl.h>
       #include <poll.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            ready;
           char           buf[10];
           nfds_t         num_open_fds, nfds;
           ssize_t        s;
           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 to 'pfds' array. */

           for (nfds_t 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) {
               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 (nfds_t j = 0; j < nfds; j++) {
                   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) {
                           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), timespec(3), epoll(7), time(7)