poll -- input/output multiplexing


#include <stropts.h>
#include <poll.h>

int poll(struct pollfd *fds, nfds_t nfds, int timeout);


poll provides users with a mechanism for multiplexing input/output over a set of file descriptors that reference open files. poll identifies those files on which a user can send or receive messages, or on which certain events have occurred.

fds 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 element for each open file descriptor of interest. The array's elements are pollfd structures, which contain the following members:

   int fd;	/* file descriptor */
   short events;	/* requested events */
   short revents;	/* returned events */

fd specifies an open file descriptor and ``events'' and ``revents'' are bitmasks constructed by an OR of any combination of the following event flags:

Data other than high priority data may be read without blocking. For STREAMS, this flag is set even if the message is of zero length.

Normal data (priority band = 0) may be read without blocking. For STREAMS, this flag is set even if the message is of zero length.

Data from a non-zero priority band may be read without blocking For STREAMS, this flag is set even if the message is of zero length.

High priority data may be received without blocking. For STREAMS, this flag is set even if the message is of zero length.

Normal data may be written without blocking.

The same as POLLOUT.

Priority data (priority band > 0) may be written. This event only examines bands that have been written to at least once.

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'' field.

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'' field.

The specified fd value does not belong to an open file. This flag is only valid in the ``revents'' field; it is not used in the ``events'' field.

For each element of the array pointed to by fds, poll examines the given file descriptor for the event(s) specified in ``events''. The number of file descriptors to be examined is specified by nfds.

If the value fd is less than zero, ``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'' field 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 descriptors. 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 INFTIM (or -1), poll blocks until a requested event occurs or until the call is interrupted. poll is not affected by the O_NDELAY and O_NONBLOCK flags.

A file descriptor for a socket that is listening for connections will indicate that 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.

Return values

On success, poll returns a non-negative value. A positive value indicates the total number of file descriptors that has been selected (that is, file descriptors for which the ``revents'' field is non-zero). A value of 0 indicates that the call timed out and no file descriptors have been selected.

On failure, poll returns -1 and sets errno to identify the error.


In the following conditions, poll fails and sets errno to:

Allocation of internal data structures failed, but the request may be attempted again.

Some argument points outside the allocated address space.

A signal was caught during the poll system call.

The argument nfds is greater than the maximum number of open files allowed; see getrlimit(2).


intro(2), getmsg(2), getrlimit(2), poll(7), putmsg(2), read(2), write(2)


Considerations for threads programming

Open file descriptors are a process resource and available to any sibling thread; if used concurrently, actions by one thread can interfere with those of a sibling.

While one thread is blocked, siblings might still be executing.

© 2004 The SCO Group, Inc. All rights reserved.
UnixWare 7 Release 7.1.4 - 25 April 2004