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POLL

Section: Linux Programmer's Manual (2)
Updated: 2017-09-15
Index Return to Main Contents

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 <signal.h>
#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 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 that 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 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);

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, 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 and POSIX.1-2008. ppoll() is Linux-specific.

NOTES

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).

COLOPHON

This page is part of release 4.13 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/.