xnu-3248.20.55.tar.gz

[apple/xnu.git] / bsd / man / man2 / poll.2

.\"
.\" Copyright (c) 2005 Apple Computer, Inc. All rights reserved.
.\"
.\" @APPLE_LICENSE_HEADER_START@
.\"
.\" This file contains Original Code and/or Modifications of Original Code
.\" as defined in and that are subject to the Apple Public Source License
.\" Version 2.0 (the 'License'). You may not use this file except in
.\" compliance with the License. Please obtain a copy of the License at
.\" http://www.opensource.apple.com/apsl/ and read it before using this
.\" file.
.\"
.\" The Original Code and all software distributed under the License are
.\" distributed on an 'AS IS' basis, WITHOUT WARRANTY OF ANY KIND, EITHER
.\" EXPRESS OR IMPLIED, AND APPLE HEREBY DISCLAIMS ALL SUCH WARRANTIES,
.\" INCLUDING WITHOUT LIMITATION, ANY WARRANTIES OF MERCHANTABILITY,
.\" FITNESS FOR A PARTICULAR PURPOSE, QUIET ENJOYMENT OR NON-INFRINGEMENT.
.\" Please see the License for the specific language governing rights and
.\" limitations under the License.
.\"
.\" @APPLE_LICENSE_HEADER_END@
.\"
.\"
.\" Copyright (c) 1996 Charles M. Hannum.  All rights reserved.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\" 1. Redistributions of source code must retain the above copyright
.\"    notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\"    notice, this list of conditions and the following disclaimer in the
.\"    documentation and/or other materials provided with the distribution.
.\" 3. All advertising materials mentioning features or use of this software
.\"    must display the following acknowledgement:
.\"	This product includes software developed by Charles M. Hannum.
.\" 4. The name of the author may not be used to endorse or promote products
.\"    derived from this software without specific prior written permission.
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
.\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
.\"
.Dd March 18, 2015
.Dt POLL 2
.Os
.Sh NAME
.Nm poll
.Nd synchronous I/O multiplexing
.Sh SYNOPSIS
.In poll.h
.Ft int
.Fo poll
.Fa "struct pollfd fds[]"
.Fa "nfds_t nfds"
.Fa "int timeout"
.Fc
.Sh DESCRIPTION
.Fn poll
examines a set of file descriptors
to see if some of them are ready for I/O
or if certain events have occurred on them.
The
.Fa fds
argument is a pointer to an array of pollfd structures,
as defined in
.Aq Pa poll.h
(shown below).  The
.Fa nfds
argument specifies the size of the
.Fa fds
array.
.Bd -literal
struct pollfd {
    int    fd;       /* file descriptor */
    short  events;   /* events to look for */
    short  revents;  /* events returned */
};
.Ed
.Pp
The fields of
.Fa struct pollfd
are as follows:
.Bl -tag -width XXXPOLLWRNORM
.It fd
File descriptor to poll.
.It events
Events to poll for.  (See below.)
.It revents
Events which may occur or have occurred.  (See below.)
.El
.Pp
The event bitmasks in
.Fa events
and
.Fa revents
have the following bits:
.Bl -tag -width XXXPOLLWRNORM
.\" ===========
.It POLLERR
An exceptional condition has occurred on the device or socket.
This flag is output only, and ignored if present in the input
.Fa events
bitmask.
.\" ===========
.It POLLHUP
The device or socket has been disconnected.
This flag is output only,
and ignored if present in the input
.Fa events
bitmask.
Note that POLLHUP and POLLOUT
are mutually exclusive and should never be present in the
.Fa revents
bitmask at the same time.
.\" ===========
.It POLLIN
Data other than high priority data may be read without blocking.
This is equivalent to ( POLLRDNORM | POLLRDBAND ).
.\" ===========
.It POLLNVAL
The file descriptor is not open.
This flag is output only, and ignored if present in the input
.Fa events
bitmask.
.\" ===========
.It POLLOUT
Normal data may be written without blocking.
This is equivalent to POLLWRNORM.
.\" ===========
.It POLLPRI
High priority data may be read without blocking.
.\" ===========
.It POLLRDBAND
Priority data may be read without blocking.
.\" ===========
.It POLLRDNORM
Normal data may be read without blocking.
.\" ===========
.It POLLWRBAND
Priority data may be written without blocking.
.\" ===========
.It POLLWRNORM
Normal data may be written without blocking.
.El
.Pp
The distinction between normal, priority, and high-priority data
is specific to particular file types or devices.
.Pp
If
.Fa timeout
is greater than zero,
it specifies a maximum interval (in milliseconds)
to wait for any file descriptor to become ready.
If
.Fa timeout
is zero, then
.Fn poll
will return without blocking. If the value of
.Fa timeout
is -1, the poll blocks indefinitely.
.Sh RETURN VALUES
.Fn poll
returns the number of descriptors that are ready for I/O,
or -1 if an error occurred.
If the time limit expires,
.Fn poll
returns 0.
If
.Fn poll
returns with an error,
including one due to an interrupted call,
the
.Fa fds
array will be unmodified and the global variable
.Va errno
will be set to indicate the error.
.Sh ERRORS
.Fn poll
will fail if:
.Bl -tag -width Er
.\" ===========
.It Bq Er EAGAIN
Allocation of internal data structures fails.
A subsequent request may succeed.
.\" ===========
.It Bq Er EFAULT
.Fa Fds
points outside the process's allocated address space.
.\" ===========
.It Bq Er EINTR
A signal is delivered before the time limit expires
and before any of the selected events occurs.
.\" ===========
.It Bq Er EINVAL
The
.Fa nfds
argument is greater than OPEN_MAX or the
.Fa timeout
argument is less than -1.
.El
.Sh BUGS
The
.Fn poll
system call currently does not support devices.
.Sh SEE ALSO
.Xr accept 2 ,
.Xr connect 2 ,
.Xr connectx 2 ,
.Xr kevent 2 ,
.Xr read 2 ,
.Xr recv 2 ,
.Xr select 2 ,
.Xr send 2 ,
.Xr write 2
.Sh HISTORY
The
.Fn poll
function call appeared in
.At V .