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NAME

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

LIBRARY

       Standard C library (libc, -lc)

SYNOPSIS

       #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);

DESCRIPTION

       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.

       Being  "ready"  means  that  the requested operation will not block; thus, poll()ing regular files, block
       devices, and other files with no reasonable polling semantic always returns instantly as  ready  to  read
       and write.

       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.

       POLLPRI
              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)).

       POLLOUT
              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.

       POLLERR
              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.

       POLLHUP
              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.

       POLLNVAL
              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:

       POLLRDNORM
              Equivalent to POLLIN.

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

       POLLWRNORM
              Equivalent to POLLOUT.

       POLLWRBAND
              Priority data may be written.

       Linux also knows about, but does not use POLLMSG.

   ppoll()
       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 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 timespec(3) structure.

       If tmo_p is specified as NULL, then ppoll() can block indefinitely.

RETURN VALUE

       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.

ERRORS

       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.

VERSIONS

       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.

   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.

STANDARDS

       poll() POSIX.1-2008.

       ppoll()
              Linux.

HISTORY

       poll() POSIX.1-2001.  Linux 2.1.23.

              On  older kernels that lack this system call, the glibc poll() wrapper function provides emulation
              using select(2).

       ppoll()
              Linux 2.6.16, glibc 2.4.

NOTES

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

       For a discussion of what may happen if a file descriptor being monitored by poll() is closed  in  another
       thread, see select(2).

BUGS

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

EXAMPLES

       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)

       int
       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]);
              exit(EXIT_FAILURE);
           }

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

           /* 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)
                   errExit("open");

               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)
                   errExit("poll");

               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)
                               errExit("read");
                           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)
                               errExit("close");
                           num_open_fds--;
                       }
                   }
               }
           }

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

SEE ALSO

       restart_syscall(2), select(2), select_tut(2), timespec(3), epoll(7), time(7)