ppoll

POLL(2)                     Linux Programmer's Manual                    POLL(2)



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

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 *tmo_p, const sigset_t *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 negate the fd field.
       Note, however, that this technique can't be used to ignore file
       descriptor 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 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.

       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 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.

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 read.

       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 *ip is invalid
              (negative).

       ENOMEM Unable to allocate memory for kernel data structures.

VERSIONS
       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.

CONFORMING TO
       poll() conforms to POSIX.1-2001 and POSIX.1-2008.  ppoll() is Linux-
       specific.

NOTES
       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 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.

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 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)

       int
       main(int argc, char *argv[])
       {
           int nfds, num_open_fds;
           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 'pfds' array. */

           for (int 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) {
               int ready;

               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 (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)
                               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), epoll(7), time(7)

COLOPHON
       This page is part of release 5.13 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/.




Linux                              2021-03-22                            POLL(2)