POLL(2)POLL(2)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 multi‐
plexing input/output over a set of file descriptors. For each member
of the array pointed to by fds, poll() examines the given file descrip‐
tor for the event(s) specified in events. The number of pollfd struc‐
tures in the fds array is specified by nfds. The poll() function iden‐
tifies 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, which contain the following mem‐
bers:
int fd; /* file descriptor */
short events; /* requested events */
short revents; /* returned events */
The fd member specifies an open file descriptor and the events and
revents members are bitmasks constructed by a logical OR operation of
any 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.
POLLRDNORM
Normal data (priority band equals 0) may be read without
blocking. For streams, this flag is set in revents even
if the message is of zero length.
POLLRDBAND
Data from a non-zero priority band may be read without
blocking. For streams, this flag is set in revents even
if the message is of zero length.
POLLPRI
High priority data may be received without blocking. For
streams, this flag is set in revents even if the message
is of zero length.
POLLOUT
Normal data (priority band equals 0) may be written with‐
out blocking.
POLLWRNORM
The same as POLLOUT.
POLLWRBAND
Priority data (priority band > 0) may be written. 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 is not used in
the events member.
POLLHUP
A hangup has occurred on the stream. 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 is not used in the events member.
POLLNVAL
The specified fd value does not belong to an open file.
This flag is only valid in the revents member; it is not
used in the events member.
If the value fd is less than 0, events is ignored and revents is set to
0 in that entry on return from poll().
The results of the poll() query are stored in the revents member in the
pollfd structure. Bits are set in the revents bitmask to indicate which
of the requested events are true. If none are true, none of the speci‐
fied bits are set in revents when the poll() call returns. The event
flags POLLHUP, POLLERR, and POLLNVAL are always set in revents if
the conditions they indicate are true; this occurs even though these
flags were not present in events.
If none of the defined events have occurred on any selected file
descriptor, poll() waits at least timeout milliseconds for an event to
occur on any of the selected file descriptors. On a computer where mil‐
lisecond timing accuracy is not available, timeout is rounded up to the
nearest legal value available on that system. If the value timeout is
0, poll() returns immediately. If the value of timeout is −1, poll()
blocks until a requested event occurs or until the call is interrupted.
The poll() function is not affected by the O_NDELAY and O_NONBLOCK
flags.
The poll() function supports regular files, terminal and pseudo-termi‐
nal devices, streams-based files, FIFOs and pipes. The behavior of
poll() on elements of fds that refer to other types of file is unspeci‐
fied.
The poll() function supports sockets.
A file descriptor for a socket that is listening for connections will
indicate that it is ready for reading, once connections are available.
A file descriptor for a socket that is connecting asynchronously will
indicate that it is ready for writing, once a connection has been
established.
Regular files always poll() TRUE for reading and writing.
RETURN VALUES
Upon successful completion, a non-negative value is returned. A posi‐
tive value indicates the total number of file descriptors that has 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, −1 is returned and errno
is set to indicate the error.
ERRORS
The poll() function will fail if:
EAGAIN
Allocation of internal data structures failed, but the
request may be attempted again.
EFAULT
Some argument points to an illegal address.
EINTR
A signal was caught during the poll() function.
EINVAL
The argument nfds 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.
ATTRIBUTES
See attributes(5) for descriptions of the following attributes:
┌────────────────────┬─────────────────┐
│ ATTRIBUTE TYPE │ ATTRIBUTE VALUE │
├────────────────────┼─────────────────┤
│Interface Stability │ Standard │
└────────────────────┴─────────────────┘
SEE ALSOIntro(2), getmsg(2), getrlimit(2), putmsg(2), read(2), write(2),
select(3C), attributes(5), standards(5), chpoll(9E)
STREAMS Programming Guide
NOTES
Non-STREAMS drivers use chpoll(9E) to implement poll() on these
devices.
Aug 23, 2001 POLL(2)
