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PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
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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.
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().
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.
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.1‐2017, poll.h(0p),
stropts.h(0p)
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 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 2017 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)
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HTML rendering created 2025-02-02 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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