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       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 <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

       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 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 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 *ip 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 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 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  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)

       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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