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 *timeout_ts, 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 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.)

The field events is an input parameter, a bit mask specifying the events the application is interested in for the file descriptor fd. If this field is specified as zero, then all events are ignored for fd and revents returns zero.

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 minimum number of milliseconds that poll() will block. (This 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.

    ready = ppoll(&fds, nfds, timeout_ts, &sigmask);

    sigset_t origmask;


    int timeout;


    timeout = (timeout_ts == NULL) ? -1 :


              (timeout_ts.tv_sec * 1000 + timeout_ts.tv_nsec / 1000000);


    sigprocmask(SIG_SETMASK, &sigmask, &origmask);


    ready = poll(&fds, nfds, timeout);


    sigprocmask(SIG_SETMASK, &origmask, NULL);

struct timespec {


    long    tv_sec;         /* seconds */


    long    tv_nsec;        /* nanoseconds */
};

RETURN VALUE¶

On success, a positive number is returned; this is the number of structures which have nonzero revents fields (in other words, those descriptors with events or errors reported). A value of 0 indicates that the call timed out and no file descriptors were ready. On error, -1 is returned, and errno is set appropriately.

ERRORS¶

EFAULT

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.

ENOMEM

There was no space to allocate file descriptor tables.

VERSIONS¶

The poll() system call was introduced in Linux 2.1.23. On older kernels that lack this system call, the glibc (and the old Linux libc) 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. ppoll() is Linux-specific.

NOTES¶

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).

SEE ALSO¶

restart_syscall(2), select(2), select_tut(2), time(7)

COLOPHON¶

This page is part of release 3.53 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.