POLL
Section: POSIX Programmer's Manual (3P)
Updated: 2017
Page Index
PROLOG
This manual page is part of the POSIX Programmer's Manual.
The Linux implementation of this interface may differ (consult
the corresponding Linux manual page for details of Linux behavior),
or the interface may not be implemented on Linux.
NAME
poll
--- input/output multiplexing
SYNOPSIS
#include <poll.h>
int poll(struct pollfd fds[], nfds_t nfds, int timeout);
DESCRIPTION
The
poll()
function provides applications with a mechanism for multiplexing
input/output over a set of file descriptors. For each member of the
array pointed to by
fds,
poll()
shall examine the given file descriptor for the event(s) specified in
events.
The number of
pollfd
structures in the
fds
array is specified by
nfds.
The
poll()
function shall identify those file descriptors on which an application
can read or write data, or on which certain events have occurred.
The
fds
argument specifies the file descriptors to be examined
and the events of interest for each file descriptor. It is a pointer to
an array with one member for each open file descriptor of interest. The
array's members are
pollfd
structures within which
fd
specifies an open file descriptor and
events
and
revents
are bitmasks constructed by OR'ing a combination of the following event
flags:
- POLLIN
-
Data other than high-priority data may be read without blocking.
-
For STREAMS, this flag is set in
revents
even if the message is of zero length. This flag shall be equivalent
to POLLRDNORM | POLLRDBAND.
- POLLRDNORM
-
Normal data may be read without blocking.
-
For STREAMS, data on priority band 0 may be read without blocking. This
flag is set in
revents
even if the message is of zero length.
- POLLRDBAND
-
Priority data may be read without blocking.
-
For STREAMS, data on priority bands greater than 0 may be read without
blocking. This flag is set in
revents
even if the message is of zero length.
- POLLPRI
-
High-priority data may be read without blocking.
-
For STREAMS, this flag is set in
revents
even if the message is of zero length.
- POLLOUT
-
Normal data may be written without blocking.
-
For STREAMS, data on priority band 0 may be written without blocking.
- POLLWRNORM
-
Equivalent to POLLOUT.
- POLLWRBAND
-
Priority data may be written.
-
For STREAMS, data on priority bands greater than 0 may be written
without blocking. If any priority band has been written to on this
STREAM, this event only examines bands that have been written
to at least once.
- POLLERR
-
An error has occurred on the device or stream. This flag is only valid
in the
revents
bitmask; it shall be ignored in the
events
member.
- POLLHUP
-
A device has been disconnected, or a pipe or FIFO has been closed by the
last process that had it open for writing. Once set, the hangup state of a
FIFO shall persist until some process opens the FIFO for writing or until
all read-only file descriptors for the FIFO are closed. This event and
POLLOUT are mutually-exclusive; a stream can never be writable if a hangup
has occurred. However, this event and POLLIN, POLLRDNORM, POLLRDBAND,
or POLLPRI are not mutually-exclusive. This flag is only valid in the
revents
bitmask; it shall be ignored in the
events
member.
- POLLNVAL
-
The specified
fd
value is invalid. This flag is only valid in the
revents
member; it shall ignored in the
events
member.
The significance and semantics of normal, priority, and high-priority
data are file and device-specific.
If the value of
fd
is less than 0,
events
shall be ignored, and
revents
shall be set to 0 in that entry on return from
poll().
In each
pollfd
structure,
poll()
shall clear the
revents
member, except that where the application requested a report on a
condition by setting one of the bits of
events
listed above,
poll()
shall set the corresponding bit in
revents
if the requested condition is true. In addition,
poll()
shall set the POLLHUP, POLLERR, and POLLNVAL flag in
revents
if the condition is true, even if the application did not set the
corresponding bit in
events.
If none of the defined events have occurred on any selected file
descriptor,
poll()
shall wait at least
timeout
milliseconds for an event to occur on any of the selected file
descriptors. If the value of
timeout
is 0,
poll()
shall return immediately. If the value of
timeout
is -1,
poll()
shall block until a requested event occurs or until the call is
interrupted.
Implementations may place limitations on the granularity of timeout
intervals. If the requested timeout interval requires a finer
granularity than the implementation supports, the actual timeout
interval shall be rounded up to the next supported value.
The
poll()
function shall not be affected by the O_NONBLOCK flag.
The
poll()
function shall support regular files, terminal and pseudo-terminal
devices, FIFOs, pipes, sockets and
STREAMS-based files.
The behavior of
poll()
on elements of
fds
that refer to other types of file is unspecified.
Regular files shall always poll TRUE for reading and writing.
A file descriptor for a socket that is listening for connections shall
indicate that it is ready for reading, once connections are available.
A file descriptor for a socket that is connecting asynchronously shall
indicate that it is ready for writing, once a connection has been
established.
Provided the application does not perform any action that results in
unspecified or undefined behavior, the value of the
fd
and
events
members of each element of
fds
shall not be modified by
poll().
RETURN VALUE
Upon successful completion,
poll()
shall return a non-negative value. A positive value indicates the total
number of
pollfd
structures that have selected events (that is, those for which the
revents
member is non-zero). A value of 0 indicates that the call timed out and
no file descriptors have been selected. Upon failure,
poll()
shall return -1 and set
errno
to indicate the error.
ERRORS
The
poll()
function shall fail if:
- EAGAIN
-
The allocation of internal data structures failed but a subsequent
request may succeed.
- EINTR
-
A signal was caught during
poll().
- EINVAL
-
The
nfds
argument is greater than
{OPEN_MAX},
or one of the
fd
members refers to a STREAM or multiplexer that is linked (directly or
indirectly) downstream from a multiplexer.
The following sections are informative.
EXAMPLES
Checking for Events on a Stream
The following example opens a pair of STREAMS devices and then waits
for either one to become writable. This example proceeds as follows:
- 1.
-
Sets the
timeout
parameter to 500 milliseconds.
- 2.
-
Opens the STREAMS devices
/dev/dev0
and
/dev/dev1,
and then polls them, specifying POLLOUT and POLLWRBAND as the events of
interest.
-
The STREAMS device names
/dev/dev0
and
/dev/dev1
are only examples of how STREAMS devices can be named; STREAMS naming
conventions may vary among systems conforming to the POSIX.1-2008.
- 3.
-
Uses the
ret
variable to determine whether an event has occurred on either of the
two STREAMS. The
poll()
function is given 500 milliseconds to wait for an event to occur (if it
has not occurred prior to the
poll()
call).
- 4.
-
Checks the returned value of
ret.
If a positive value is returned, one of the following can be done:
-
- a.
-
Priority data can be written to the open STREAM on priority bands
greater than 0, because the POLLWRBAND event occurred on the open
STREAM (fds[0]
or
fds[1]).
- b.
-
Data can be written to the open STREAM on priority-band 0, because the
POLLOUT event occurred on the open STREAM (fds[0]
or
fds[1]).
- 5.
-
If the returned value is not a positive value, permission to write data
to the open STREAM (on any priority band) is denied.
- 6.
-
If the POLLHUP event occurs on the open STREAM (fds[0]
or
fds[1]),
the device on the open STREAM has disconnected.
-
#include <stropts.h>
#include <poll.h>
...
struct pollfd fds[2];
int timeout_msecs = 500;
int ret;
int i;
/* Open STREAMS device. */
fds[0].fd = open("/dev/dev0", ...);
fds[1].fd = open("/dev/dev1", ...);
fds[0].events = POLLOUT | POLLWRBAND;
fds[1].events = POLLOUT | POLLWRBAND;
ret = poll(fds, 2, timeout_msecs);
if (ret > 0) {
/* An event on one of the fds has occurred. */
for (i=0; i<2; i++) {
if (fds[i].revents & POLLWRBAND) {
/* Priority data may be written on device number i. */
...
}
if (fds[i].revents & POLLOUT) {
/* Data may be written on device number i. */
...
}
if (fds[i].revents & POLLHUP) {
/* A hangup has occurred on device number i. */
...
}
}
}
APPLICATION USAGE
None.
RATIONALE
The POLLHUP event does not occur for FIFOs just because the FIFO is not
open for writing. It only occurs when the FIFO is closed by the last
writer and persists until some process opens the FIFO for writing or
until all read-only file descriptors for the FIFO are closed.
FUTURE DIRECTIONS
None.
SEE ALSO
Section 2.6,
STREAMS,
getmsg(),
pselect(),
putmsg(),
read(),
write()
The Base Definitions volume of POSIX.1-2017,
<poll.h>,
<stropts.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1-2017, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, 2018 Edition,
Copyright (C) 2018 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group.
In the event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online at
http://www.opengroup.org/unix/online.html .
Any typographical or formatting errors that appear
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https://www.kernel.org/doc/man-pages/reporting_bugs.html .