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:

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:

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>

When compiling with _XOPEN_SOURCE defined, one also has the following, which convey no further information beyond the bits listed above:

Linux also knows about, but does not use POLLMSG.

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

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).

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:

Suppose we run the program in one terminal, asking it to open a FIFO:

In a second terminal window, we then open the FIFO for writing, write some data to it, and close the FIFO:

In the terminal where we are running the program, we would then see:

In the above output, we see that poll() returned three times:

/* 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)