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 *_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.

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.

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.

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);
}

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