NetBSD/lib/libc/sys/poll.2

.\"	$NetBSD: poll.2,v 1.23 2005/03/10 16:51:42 wiz Exp $
.\"
.\" Copyright (c) 1998 The NetBSD Foundation, Inc.
.\" All rights reserved.
.\"
.\" This code is derived from software contributed to The NetBSD Foundation
.\" by Charles M. Hannum.
.\"
.\" 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 the NetBSD
.\"        Foundation, Inc. and its contributors.
.\" 4. Neither the name of The NetBSD Foundation nor the names of its
.\"    contributors may be used to endorse or promote products derived
.\"    from this software without specific prior written permission.
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS
.\" ``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 FOUNDATION OR CONTRIBUTORS
.\" 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 10, 2005
.Dt POLL 2
.Os
.Sh NAME
.Nm poll, pollts
.Nd synchronous I/O multiplexing
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In poll.h
.Ft int
.Fn poll "struct pollfd *fds" "nfds_t nfds" "int timeout"
.In poll.h
.In signal.h
.In time.h
.Ft int
.Fn pollts "struct pollfd * restrict fds" "nfds_t nfds" "const struct timespec * restrict ts" "const sigset_t * restrict sigmask"
.Sh DESCRIPTION
.Fn poll
and
.Fn pollts
examine a set of file descriptors to see if some of them are ready for
I/O.
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 determines 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 XXXrevents
.It fd
File descriptor to poll.
If the value in
.Em fd
is negative, the file descriptor is ignored and
.Em revents
is set to 0.
.It events
Events to poll for.
(See below.)
.It revents
Events which may occur.
(See below.)
.El
.Pp
The event bitmasks in
.Fa events
and
.Fa revents
have the following bits:
.Bl -tag -width XXXPOLLWRNORM
.It POLLIN
Data other than high priority data may be read without blocking.
.It POLLRDNORM
Normal data may be read without blocking.
.It POLLRDBAND
Data with a non-zero priority may be read without blocking.
.It POLLPRI
High priority data may be read without blocking.
.It POLLOUT
.It POLLWRNORM
Normal data may be written without blocking.
.It POLLWRBAND
Data with a non-zero priority may be written without blocking.
.It POLLERR
An exceptional condition has occurred on the device or socket.
This flag is always checked, even if not present in the
.Fa events
bitmask.
.It POLLHUP
The device or socket has been disconnected.
This flag is always
checked, even if not present in the
.Fa events
bitmask.
Note that
POLLHUP
and
POLLOUT
should never be present in the
.Fa revents
bitmask at the same time.
If the remote end of a socket is closed,
.Fn poll
returns a
POLLIN
event, rather than a
POLLHUP.
.It POLLNVAL
The file descriptor is not open.
This flag is always checked, even
if not present in the
.Fa events
bitmask.
.El
.Pp
If
.Fa timeout
is neither zero nor INFTIM (\-1), it specifies a maximum interval to
wait for any file descriptor to become ready, in milliseconds.
If
.Fa timeout
is INFTIM (\-1), the poll blocks indefinitely.
If
.Fa timeout
is zero, then
.Fn poll
will return without blocking.
.Pp
If
.Fa ts
is a non-null pointer, it references a timespec structure which specifies a
maximum interval to wait for any file descriptor to become ready.
If
.Fa ts
is a null pointer,
.Fn pollts
blocks indefinitely.
If
.Fa ts
is a non-null pointer, referencing a zero-valued timespec structure, then
.Fn pollts
will return without blocking.
.Pp
If
.Fa sigmask
is a non-null pointer, then the
.Fn pollts
function shall replace the signal mask of the caller by the set of
signals pointed to by
.Fa sigmask
before examining the descriptors, and shall restore the signal mask
of the caller before returning.
.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.
.Sh COMPATIBILITY
This implementation differs from the historical one in that a given
file descriptor may not cause
.Fn poll
to return with an error.
In cases where this would have happened in the historical implementation
(e.g. trying to poll a
.Xr revoke 2 Ns d
descriptor), this implementation instead copies the
.Fa events
bitmask to the
.Fa revents
bitmask.
Attempting to perform I/O on this descriptor will then return an error.
This behaviour is believed to be more useful.
.Sh ERRORS
An error return from
.Fn poll
indicates:
.Bl -tag -width Er
.It Bq Er EFAULT
.Fa fds
points outside the process's allocated address space.
.It Bq Er EINTR
A signal was delivered before the time limit expired and
before any of the selected events occurred.
.It Bq Er EINVAL
The specified time limit is negative.
.El
.Sh SEE ALSO
.Xr accept 2 ,
.Xr connect 2 ,
.Xr read 2 ,
.Xr recv 2 ,
.Xr select 2 ,
.Xr send 2 ,
.Xr write 2
.Sh HISTORY
The
.Fn poll
function appeared in
.At V.3 .
The
.Fn pollts
function first appeared in
.Nx 3.0 .
.Sh BUGS
The distinction between some of the fields in the
.Fa events
and
.Fa revents
bitmasks is really not useful without STREAMS.
The fields are defined for compatibility with existing software.