.\" Copyright (C) 1997 Andries Brouwer (aeb@cwi.nl)

.\" and Copyright (C) 2006, Michael Kerrisk <mtk.manpages@gmail.com>

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.\" Additions from Richard Gooch <rgooch@atnf.CSIRO.AU> and aeb, 971207

.\" 2006-03-13, mtk, Added ppoll() + various other rewordings

.\" 2006-07-01, mtk, Added POLLRDHUP + various other wording and

.\"	formatting changes.

.\"

.TH POLL 2 2017-09-15 "Linux" "Linux Programmer's Manual"

.SH NAME

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

.SH SYNOPSIS

.nf

.B #include <poll.h>

.PP

.BI "int poll(struct pollfd *" fds ", nfds_t " nfds ", int " timeout );

.BR "#define _GNU_SOURCE" "         /* See feature_test_macros(7) */"

.B #include <signal.h>

.B #include <poll.h>

.PP

.BI "int ppoll(struct pollfd *" fds ", nfds_t " nfds ", "

.BI "        const struct timespec *" tmo_p ", const sigset_t *" sigmask );

.fi

.SH DESCRIPTION

.BR poll ()

performs a similar task to

.BR select (2):

it waits for one of a set of file descriptors to become ready

to perform I/O.

.PP

The set of file descriptors to be monitored is specified in the

.I fds

argument, which is an array of structures of the following form:

.PP

.in +4n

.EX

struct pollfd {

    int   fd;         /* file descriptor */

    short events;     /* requested events */

    short revents;    /* returned events */

};

.EE

.in

.PP

The caller should specify the number of items in the

.I fds

array in

.IR nfds .

.PP

The field

.I fd

contains a file descriptor for an open file.

If this field is negative, then the corresponding

.I events

field is ignored and the

.I revents

field returns zero.

(This provides an easy way of ignoring a

file descriptor for a single

.BR poll ()

call: simply negate the

.I fd

field.

Note, however, that this technique can't be used to ignore file descriptor 0.)

.PP

The field

.I events

is an input parameter, a bit mask specifying the events the application

is interested in for the file descriptor

.IR fd .

This field may be specified as zero,

in which case the only events that can be returned in

.I revents

are

.BR POLLHUP ,

.BR POLLERR ,

and

.B POLLNVAL

(see below).

.PP

The field

.I revents

is an output parameter, filled by the kernel with the events that

actually occurred.

The bits returned in

.I revents

can include any of those specified in

.IR events ,

or one of the values

.BR POLLERR ,

.BR POLLHUP ,

or

.BR POLLNVAL .

(These three bits are meaningless in the

.I events

field, and will be set in the

.I revents

field whenever the corresponding condition is true.)

.PP

If none of the events requested (and no error) has occurred for any

of the file descriptors, then

.BR poll ()

blocks until one of the events occurs.

.PP

The

.I timeout

argument specifies the number of milliseconds that

.BR poll ()

should block waiting for a file descriptor to become ready.

The call will block until either:

.IP * 3

a file descriptor becomes ready;

.IP *

the call is interrupted by a signal handler; or

.IP *

the timeout expires.

.PP

Note that the

.I 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

.I timeout

means an infinite timeout.

Specifying a

.I timeout

of zero causes

.BR poll ()

to return immediately, even if no file descriptors are ready.

.PP

The bits that may be set/returned in

.I events

and

.I revents

are defined in \fI<poll.h>\fP:

.TP

.B POLLIN

There is data to read.

.TP

.B POLLPRI

There is some exceptional condition on the file descriptor.

Possibilities include:

.RS

.IP * 3

There is out-of-band data on a TCP socket (see

.BR tcp (7)).

.IP *

A pseudoterminal master in packet mode has seen a state change on the slave

(see

.BR ioctl_tty (2)).

.IP *

A

.I cgroup.events

file has been modified (see

.BR cgroups (7)).

.RE

.TP

.B POLLOUT

Writing is now possible, though a write larger that the available space

in a socket or pipe will still block (unless

.B O_NONBLOCK

is set).

.TP

.BR POLLRDHUP " (since Linux 2.6.17)"

Stream socket peer closed connection,

or shut down writing half of connection.

The

.B _GNU_SOURCE

feature test macro must be defined

(before including

.I any

header files)

in order to obtain this definition.

.TP

.B POLLERR

Error condition (only returned in

.IR revents ;

ignored in

.IR 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.

.TP

.B POLLHUP

Hang up (only returned in

.IR revents ;

ignored in

.IR 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.

.TP

.B POLLNVAL

Invalid request:

.I fd

not open (only returned in

.IR revents ;

ignored in

.IR events ).

.PP

When compiling with

.B _XOPEN_SOURCE

defined, one also has the following,

which convey no further information beyond the bits listed above:

.TP

.B POLLRDNORM

Equivalent to

.BR POLLIN .

.TP

.B POLLRDBAND

Priority band data can be read (generally unused on Linux).

.\" POLLRDBAND is used in the DECnet protocol.

.TP

.B POLLWRNORM

Equivalent to

.BR POLLOUT .

.TP

.B POLLWRBAND

Priority data may be written.

.PP

Linux also knows about, but does not use

.BR POLLMSG .

.SS ppoll()

The relationship between

.BR poll ()

and

.BR ppoll ()

is analogous to the relationship between

.BR select (2)

and

.BR pselect (2):

like

.BR pselect (2),

.BR ppoll ()

allows an application to safely wait until either a file descriptor

becomes ready or until a signal is caught.

.PP

Other than the difference in the precision of the

.I timeout

argument, the following

.BR ppoll ()

call:

.PP

.in +4n

.EX

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

.EE

.in

.PP

is equivalent to

.I atomically

executing the following calls:

.PP

.in +4n

.EX

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);

.EE

.in

.PP

See the description of

.BR pselect (2)

for an explanation of why

.BR ppoll ()

is necessary.

.PP

If the

.I sigmask

argument is specified as NULL, then

no signal mask manipulation is performed

(and thus

.BR ppoll ()

differs from

.BR poll ()

only in the precision of the

.I timeout

argument).

.PP

The

.I tmo_p

argument specifies an upper limit on the amount of time that

.BR ppoll ()

will block.

This argument is a pointer to a structure of the following form:

.PP

.in +4n

.EX

struct timespec {

    long    tv_sec;         /* seconds */

    long    tv_nsec;        /* nanoseconds */

};

.EE

.in

.PP

If

.I tmo_p

is specified as NULL, then

.BR ppoll ()

can block indefinitely.

.SH RETURN VALUE

On success, a positive number is returned; this is

the number of structures which have nonzero

.I 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

.I errno

is set appropriately.

.SH ERRORS

.TP

.B EFAULT

The array given as argument was not contained in the calling program's

address space.

.TP

.B EINTR

A signal occurred before any requested event; see

.BR signal (7).

.TP

.B EINVAL

The

.I nfds

value exceeds the

.B RLIMIT_NOFILE

value.

.TP

.B ENOMEM

There was no space to allocate file descriptor tables.

.SH VERSIONS

The

.BR poll ()

system call was introduced in Linux 2.1.23.

On older kernels that lack this system call,

.\" library call was introduced in libc 5.4.28

the glibc (and the old Linux libc)

.BR poll ()

wrapper function provides emulation using

.BR select (2).

.PP

The

.BR ppoll ()

system call was added to Linux in kernel 2.6.16.

The

.BR ppoll ()

library call was added in glibc 2.4.

.SH CONFORMING TO

.BR poll ()

conforms to POSIX.1-2001 and POSIX.1-2008.

.BR ppoll ()

is Linux-specific.

.\" NetBSD 3.0 has a pollts() which is like Linux ppoll().

.SH NOTES

On some other UNIX systems,

.\" Darwin, according to a report by Jeremy Sequoia, relayed by Josh Triplett

.BR poll ()

can fail with the error

.B EAGAIN

if the system fails to allocate kernel-internal resources, rather than

.B ENOMEM

as Linux does.

POSIX permits this behavior.

Portable programs may wish to check for

.B EAGAIN

and loop, just as with

.BR EINTR .

.PP

Some implementations define the nonstandard constant

.B INFTIM

with the value \-1 for use as a

.IR timeout

for

.BR poll ().

This constant is not provided in glibc.

.PP

For a discussion of what may happen if a file descriptor being monitored by

.BR poll ()

is closed in another thread, see

.BR select (2).

.SS C library/kernel differences

The Linux

.BR ppoll ()

system call modifies its

.I 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

.BR ppoll ()

function does not modify its

.I tmo_p

argument.

.PP

The raw

.BR ppoll ()

system call has a fifth argument,

.IR "size_t sigsetsize" ,

which specifies the size in bytes of the

.IR sigmask

argument.

The glibc

.BR ppoll ()

wrapper function specifies this argument as a fixed value

(equal to

.IR sizeof(kernel_sigset_t) ).

See

.BR sigprocmask (2)

for a discussion on the differences between the kernel and the libc

notion of the sigset.

.SH BUGS

See the discussion of spurious readiness notifications under the

BUGS section of

.BR select (2).

.SH SEE ALSO

.BR restart_syscall (2),

.BR select (2),

.BR select_tut (2),

.BR epoll (7),

.BR time (7)

.SH COLOPHON

This page is part of release 4.15 of the Linux

.I 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/.