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POLL(3P) POSIX Programmer's Manual POLL(3P)
PROLOG
This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (con-
sult 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 struc-
tures within which fd specifies an open file descriptor and events and revents are bitmasks constructed by OR'ing a com-
bination 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 POLLRD-
NORM | 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
The device has been disconnected. 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-exclu-
sive. 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 millisec-
onds 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 inter-
rupted.
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 sup-
ported 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 con-
nections 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 pro-
ceeds 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 IEEE Std 1003.1-2001.
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
None.
FUTURE DIRECTIONS
None.
SEE ALSO
STREAMS, getmsg(), putmsg(), read(), select(), write(), the Base Definitions volume of IEEE Std 1003.1-2001, <poll.h>,
<stropts.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003 Edition, Standard for
Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 6, Copy-
right (C) 2001-2003 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.open-
group.org/unix/online.html .
IEEE/The Open Group 2003 POLL(3P)
