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 multiplexing input/output over a set of file descrip-
tors. For each member of the array pointed to by fds,
poll() examines 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 identi-
fies 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 exam-
ined 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 members:
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
without 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 specified 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 descrip-
tors. On a computer where millisecond 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-terminal devices, STREAMS-based files, FIFOs and
pipes. The behavior of poll() on elements of fds that refer
to other types of file is unspecified.
The poll() function supports sockets.
A file descriptor for a socket that is listening for connec-
tions will indicate that it is ready for reading, once con-
nections 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 positive 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.
SEE ALSO
intro(2), getmsg(2), getrlimit(2), putmsg(2), read(2),
write(2), select(3C), chpoll(9E)
STREAMS Programming Guide
NOTES
Non-STREAMS drivers use chpoll(9E) to implement poll() on
these devices.
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