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POLL(3P) POSIX Programmer's Manual POLL(3P)
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.
poll — input/output multiplexing
#include <poll.h>
int poll(struct pollfd fds[], nfds_t nfds, int timeout);
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.
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.
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.
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. */
...
}
}
}
None.
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.
None.
Section 2.6, STREAMS, getmsg(3p), pselect(3p), putmsg(3p), read(3p),
write(3p)
The Base Definitions volume of POSIX.1‐2008, poll.h(0p),
stropts.h(0p)
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 .
IEEE/The Open Group 2013 POLL(3P)
Pages that refer to this page: poll.h(0p), stropts.h(0p), connect(3p), getmsg(3p), ioctl(3p), pselect(3p), putmsg(3p), recv(3p), recvfrom(3p), recvmsg(3p), send(3p), sendmsg(3p), sendto(3p)