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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.
RETURN VALUE
Upon successful completion, poll() shall return a non-negative value. A positive value indicates the
total number of file descriptors that have been selected (that is, file descriptors 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.1‐2008, <poll.h>, <stropts.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2013 Edition,
Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, Copyright (C) 2013 by the Institute of Electrical and Electronics Engineers, Inc
and The Open Group. (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) 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.unix.org/online.html .
Any typographical or formatting errors that appear in this page are most likely to have been introduced
during the conversion of the source files to man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .