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

.\"	$NetBSD: select.2,v 1.5 1995/06/27 22:32:28 cgd Exp $
.\"
.\" Copyright (c) 1983, 1991, 1993
.\"	The Regents of the University of California.  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 the University of
.\"	California, Berkeley and its contributors.
.\" 4. Neither the name of the University 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 REGENTS 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 REGENTS 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.
.\"
.\"     @(#)select.2	8.2 (Berkeley) 3/25/94
.\"
.Dd March 25, 1994
.Dt SELECT 2
.Os BSD 4.2
.Sh NAME
.Nm select
.Nd synchronous I/O multiplexing
.Sh SYNOPSIS
.Fd #include <sys/select.h>
.D1 "- or -"
.Fd #include <sys/types.h>
.Fd #include <sys/time.h>
.Fd #include <unistd.h>
.Ft int
.Fn select "int nfds" "fd_set *readfds" "fd_set *writefds" "fd_set *exceptfds" "struct timeval *timeout"
.Fn FD_SET fd &fdset
.Fn FD_CLR fd &fdset
.Fn FD_ISSET fd &fdset
.Fn FD_COPY &fdset_orig &fdset_copy
.Fn FD_ZERO &fdset
.Sh DESCRIPTION
.Fn Select
examines the I/O descriptor sets whose addresses are passed in
.Fa readfds ,
.Fa writefds ,
and
.Fa exceptfds
to see if some of their descriptors
are ready for reading, are ready for writing, or have an exceptional
condition pending, respectively.
The first
.Fa nfds
descriptors are checked in each set;
i.e., the descriptors from 0 through
.Fa nfds Ns No -1
in the descriptor sets are examined.  (Example: If you have set two file descriptors "4" and "17",
.Fa nfds
should  not be "2", but rather "17 + 1" or "18".)
On return,
.Fn select
replaces the given descriptor sets
with subsets consisting of those descriptors that are ready
for the requested operation.
.Fn Select
returns the total number of ready descriptors in all the sets.
.Pp
The descriptor sets are stored as bit fields in arrays of integers.
The following macros are provided for manipulating such descriptor sets:
.Fn FD_ZERO &fdset
initializes a descriptor set
.Fa fdset
to the null set.
.Fn FD_SET fd &fdset
includes a particular descriptor
.Fa fd
in
.Fa fdset .
.Fn FD_CLR fd &fdset
removes
.Fa fd
from
.Fa fdset .
.Fn FD_ISSET fd &fdset
is non-zero if
.Fa fd
is a member of
.Fa fdset ,
zero otherwise.
.Fn FD_COPY &fdset_orig &fdset_copy
replaces an already allocated 
.Fa &fdset_copy
file descriptor set with a copy of
.Fa &fdset_orig .
The behavior of these macros is undefined if
a descriptor value is less than zero or greater than or equal to
.Dv FD_SETSIZE ,
which is normally at least equal
to the maximum number of descriptors supported by the system.
.Pp
If
.Fa timeout
is a non-nil pointer, it specifies a maximum interval to wait for the
selection to complete.  If 
.Fa timeout
is a nil pointer, the select blocks indefinitely.  To effect a poll, the
.Fa timeout
argument should be non-nil, pointing to a zero-valued timeval structure.
.Fa Timeout
is not changed by
.Fn select ,
and may be reused on subsequent calls, however it is good style to re-initialize
it before each invocation of
.Fn select .
.Pp
Any of
.Fa readfds ,
.Fa writefds ,
and
.Fa exceptfds
may be given as nil pointers if no descriptors are of interest.
.Sh RETURN VALUES
.Fn Select
returns the number of ready descriptors that are contained in
the descriptor sets,
or -1 if an error occurred.
If the time limit expires,
.Fn select
returns 0.
If
.Fn select
returns with an error,
including one due to an interrupted call,
the descriptor sets will be unmodified.
.Sh ERRORS
An error return from
.Fn select
indicates:
.Bl -tag -width Er
.It Bq Er EBADF
One of the descriptor sets specified an invalid descriptor.
.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 invalid.  One of its components is
negative or too large.
.El
.Sh SEE ALSO
.Xr accept 2 ,
.Xr connect 2 ,
.Xr getdtablesize 2 ,
.Xr gettimeofday 2 ,
.Xr read 2 ,
.Xr recv 2 ,
.Xr send 2 ,
.Xr write 2
.Sh BUGS
Although the provision of
.Xr getdtablesize 2
was intended to allow user programs to be written independent
of the kernel limit on the number of open files, the dimension
of a sufficiently large bit field for select remains a problem.
The default size
.Dv FD_SETSIZE
(currently 1024) is somewhat smaller than
the current kernel limit to the number of open files.
However, in order to accommodate programs which might potentially
use a larger number of open files with select, it is possible
to increase this size within a program by providing
a larger definition of
.Dv FD_SETSIZE
before the inclusion of
.Aq Pa sys/types.h .
.Pp
.Fn Select
should probably have been designed to return the time remaining from the
original timeout, if any, by modifying the time value in place.
However, it is unlikely this semantic will ever be implemented, as the
change would cause source code compatibility problems.
In general it is unwise to assume that the timeout value will be
unmodified by the
.Fn select
call, and the caller should reinitialize it on each invocation.
.Sh HISTORY
The
.Fn select
function call appeared in
.Bx 4.2 .
Commit	Line	Data
9bccf70c A	1	.\" $NetBSD: select.2,v 1.5 1995/06/27 22:32:28 cgd Exp $
	2	.\"
	3	.\" Copyright (c) 1983, 1991, 1993
	4	.\" The Regents of the University of California. All rights reserved.
	5	.\"
	6	.\" Redistribution and use in source and binary forms, with or without
	7	.\" modification, are permitted provided that the following conditions
	8	.\" are met:
	9	.\" 1. Redistributions of source code must retain the above copyright
	10	.\" notice, this list of conditions and the following disclaimer.
	11	.\" 2. Redistributions in binary form must reproduce the above copyright
	12	.\" notice, this list of conditions and the following disclaimer in the
	13	.\" documentation and/or other materials provided with the distribution.
	14	.\" 3. All advertising materials mentioning features or use of this software
	15	.\" must display the following acknowledgement:
	16	.\" This product includes software developed by the University of
	17	.\" California, Berkeley and its contributors.
	18	.\" 4. Neither the name of the University nor the names of its contributors
	19	.\" may be used to endorse or promote products derived from this software
	20	.\" without specific prior written permission.
	21	.\"
	22	.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
	23	.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
	24	.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
	25	.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
	26	.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
	27	.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
	28	.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
	29	.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
	30	.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
	31	.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
	32	.\" SUCH DAMAGE.
	33	.\"
	34	.\" @(#)select.2 8.2 (Berkeley) 3/25/94
	35	.\"
	36	.Dd March 25, 1994
	37	.Dt SELECT 2
	38	.Os BSD 4.2
	39	.Sh NAME
	40	.Nm select
	41	.Nd synchronous I/O multiplexing
	42	.Sh SYNOPSIS
55e303ae A	43	.Fd #include <sys/select.h>
55e303ae A	44	.D1 "- or -"
9bccf70c A	45	.Fd #include <sys/types.h>
	46	.Fd #include <sys/time.h>
	47	.Fd #include <unistd.h>
	48	.Ft int
	49	.Fn select "int nfds" "fd_set readfds" "fd_set writefds" "fd_set exceptfds" "struct timeval timeout"
	50	.Fn FD_SET fd &fdset
	51	.Fn FD_CLR fd &fdset
	52	.Fn FD_ISSET fd &fdset
91447636	53	.Fn FD_COPY &fdset_orig &fdset_copy
9bccf70c A	54	.Fn FD_ZERO &fdset
	55	.Sh DESCRIPTION
	56	.Fn Select
	57	examines the I/O descriptor sets whose addresses are passed in
	58	.Fa readfds ,
	59	.Fa writefds ,
	60	and
	61	.Fa exceptfds
	62	to see if some of their descriptors
	63	are ready for reading, are ready for writing, or have an exceptional
	64	condition pending, respectively.
	65	The first
	66	.Fa nfds
	67	descriptors are checked in each set;
	68	i.e., the descriptors from 0 through
	69	.Fa nfds Ns No -1
91447636 A	70	in the descriptor sets are examined. (Example: If you have set two file descriptors "4" and "17",
	71	.Fa nfds
	72	should not be "2", but rather "17 + 1" or "18".)
9bccf70c A	73	On return,
	74	.Fn select
	75	replaces the given descriptor sets
	76	with subsets consisting of those descriptors that are ready
	77	for the requested operation.
	78	.Fn Select
	79	returns the total number of ready descriptors in all the sets.
	80	.Pp
	81	The descriptor sets are stored as bit fields in arrays of integers.
	82	The following macros are provided for manipulating such descriptor sets:
	83	.Fn FD_ZERO &fdset
	84	initializes a descriptor set
	85	.Fa fdset
	86	to the null set.
	87	.Fn FD_SET fd &fdset
	88	includes a particular descriptor
	89	.Fa fd
	90	in
	91	.Fa fdset .
	92	.Fn FD_CLR fd &fdset
	93	removes
	94	.Fa fd
	95	from
	96	.Fa fdset .
	97	.Fn FD_ISSET fd &fdset
	98	is non-zero if
	99	.Fa fd
	100	is a member of
	101	.Fa fdset ,
	102	zero otherwise.
91447636 A	103	.Fn FD_COPY &fdset_orig &fdset_copy
	104	replaces an already allocated
	105	.Fa &fdset_copy
	106	file descriptor set with a copy of
	107	.Fa &fdset_orig .
9bccf70c A	108	The behavior of these macros is undefined if
	109	a descriptor value is less than zero or greater than or equal to
	110	.Dv FD_SETSIZE ,
	111	which is normally at least equal
	112	to the maximum number of descriptors supported by the system.
	113	.Pp
	114	If
	115	.Fa timeout
	116	is a non-nil pointer, it specifies a maximum interval to wait for the
	117	selection to complete. If
	118	.Fa timeout
	119	is a nil pointer, the select blocks indefinitely. To effect a poll, the
	120	.Fa timeout
	121	argument should be non-nil, pointing to a zero-valued timeval structure.
	122	.Fa Timeout
	123	is not changed by
	124	.Fn select ,
	125	and may be reused on subsequent calls, however it is good style to re-initialize
	126	it before each invocation of
	127	.Fn select .
	128	.Pp
	129	Any of
	130	.Fa readfds ,
	131	.Fa writefds ,
	132	and
	133	.Fa exceptfds
	134	may be given as nil pointers if no descriptors are of interest.
	135	.Sh RETURN VALUES
	136	.Fn Select
	137	returns the number of ready descriptors that are contained in
	138	the descriptor sets,
	139	or -1 if an error occurred.
	140	If the time limit expires,
	141	.Fn select
	142	returns 0.
	143	If
	144	.Fn select
	145	returns with an error,
	146	including one due to an interrupted call,
	147	the descriptor sets will be unmodified.
	148	.Sh ERRORS
	149	An error return from
	150	.Fn select
	151	indicates:
	152	.Bl -tag -width Er
	153	.It Bq Er EBADF
	154	One of the descriptor sets specified an invalid descriptor.
	155	.It Bq Er EINTR
	156	A signal was delivered before the time limit expired and
	157	before any of the selected events occurred.
	158	.It Bq Er EINVAL
	159	The specified time limit is invalid. One of its components is
	160	negative or too large.
	161	.El
	162	.Sh SEE ALSO
	163	.Xr accept 2 ,
	164	.Xr connect 2 ,
	165	.Xr getdtablesize 2 ,
	166	.Xr gettimeofday 2 ,
	167	.Xr read 2 ,
	168	.Xr recv 2 ,
	169	.Xr send 2 ,
	170	.Xr write 2
	171	.Sh BUGS
172	Although the provision of
173	.Xr getdtablesize 2
174	was intended to allow user programs to be written independent
175	of the kernel limit on the number of open files, the dimension
176	of a sufficiently large bit field for select remains a problem.
177	The default size
178	.Dv FD_SETSIZE
179	(currently 1024) is somewhat smaller than
180	the current kernel limit to the number of open files.
181	However, in order to accommodate programs which might potentially
182	use a larger number of open files with select, it is possible
183	to increase this size within a program by providing
184	a larger definition of
185	.Dv FD_SETSIZE
186	before the inclusion of
187	.Aq Pa sys/types.h .
188	.Pp
189	.Fn Select
190	should probably have been designed to return the time remaining from the
191	original timeout, if any, by modifying the time value in place.
192	However, it is unlikely this semantic will ever be implemented, as the
193	change would cause source code compatibility problems.
194	In general it is unwise to assume that the timeout value will be
195	unmodified by the
196	.Fn select
197	call, and the caller should reinitialize it on each invocation.
198	.Sh HISTORY
199	The
200	.Fn select
201	function call appeared in
202	.Bx 4.2 .