[unix-history] / lib / libc / sys / select.2

.\" Copyright (c) 1983, 1991 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	6.8 (Berkeley) 3/10/91
.\"
.Dd March 10, 1991
.Dt SELECT 2
.Os BSD 4.2
.Sh NAME
.Nm select
.Nd synchronous I/O multiplexing
.Sh SYNOPSIS
.Fd #include <unistd.h>
.Fd #include <sys/types.h>
.Fd #include <sys/time.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_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.
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 &fdsetx
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.
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 affect a poll, the
.Fa timeout
argument should be non-nil, pointing to a zero-valued timeval structure.
.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 read 2 ,
.Xr write 2 ,
.Xr recv 2 ,
.Xr send 2 ,
.Xr getdtablesize 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 256) is somewhat larger 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 return the time remaining from the original timeout,
if any, by modifying the time value in place.
This may be implemented in future versions of the system.
Thus, it is unwise to assume that the timeout value will be unmodified
by the
.Fn select
call.
.Sh HISTORY
The
.Nm
function call appeared in
.Bx 4.2 .
Commit	Line	Data
15637ed4 RG	1	.\" Copyright (c) 1983, 1991 The Regents of the University of California.
	2	.\" All rights reserved.
	3	.\"
	4	.\" Redistribution and use in source and binary forms, with or without
	5	.\" modification, are permitted provided that the following conditions
	6	.\" are met:
	7	.\" 1. Redistributions of source code must retain the above copyright
	8	.\" notice, this list of conditions and the following disclaimer.
	9	.\" 2. Redistributions in binary form must reproduce the above copyright
	10	.\" notice, this list of conditions and the following disclaimer in the
	11	.\" documentation and/or other materials provided with the distribution.
	12	.\" 3. All advertising materials mentioning features or use of this software
	13	.\" must display the following acknowledgement:
	14	.\" This product includes software developed by the University of
	15	.\" California, Berkeley and its contributors.
	16	.\" 4. Neither the name of the University nor the names of its contributors
	17	.\" may be used to endorse or promote products derived from this software
	18	.\" without specific prior written permission.
	19	.\"
	20	.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
	21	.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
	22	.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
	23	.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
	24	.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
	25	.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
	26	.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
	27	.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
	28	.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
	29	.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
	30	.\" SUCH DAMAGE.
	31	.\"
	32	.\" @(#)select.2 6.8 (Berkeley) 3/10/91
	33	.\"
	34	.Dd March 10, 1991
	35	.Dt SELECT 2
	36	.Os BSD 4.2
	37	.Sh NAME
	38	.Nm select
	39	.Nd synchronous I/O multiplexing
	40	.Sh SYNOPSIS
	41	.Fd #include <unistd.h>
	42	.Fd #include <sys/types.h>
	43	.Fd #include <sys/time.h>
	44	.Ft int
	45	.Fn select "int nfds" "fd_set readfds" "fd_set writefds" "fd_set exceptfds" "struct timeval timeout"
	46	.Fn FD_SET fd &fdset
	47	.Fn FD_CLR fd &fdset
	48	.Fn FD_ISSET fd &fdset
	49	.Fn FD_ZERO &fdset
	50	.Sh DESCRIPTION
	51	.Fn Select
	52	examines the I/O descriptor sets whose addresses are passed in
	53	.Fa readfds ,
	54	.Fa writefds ,
	55	and
	56	.Fa exceptfds
	57	to see if some of their descriptors
	58	are ready for reading, are ready for writing, or have an exceptional
	59	condition pending, respectively.
	60	The first
	61	.Fa nfds
	62	descriptors are checked in each set;
	63	i.e., the descriptors from 0 through
	64	.Fa nfds Ns No -1
65	in the descriptor sets are examined.
66	On return,
67	.Fn select
68	replaces the given descriptor sets
69	with subsets consisting of those descriptors that are ready
70	for the requested operation.
71	.Fn Select
72	returns the total number of ready descriptors in all the sets.
73	.Pp
74	The descriptor sets are stored as bit fields in arrays of integers.
75	The following macros are provided for manipulating such descriptor sets:
76	.Fn FD_ZERO &fdsetx
77	initializes a descriptor set
78	.Fa fdset
79	to the null set.
80	.Fn FD_SET fd &fdset
81	includes a particular descriptor
82	.Fa fd
83	in
84	.Fa fdset .
85	.Fn FD_CLR fd &fdset
86	removes
87	.Fa fd
88	from
89	.Fa fdset .
90	.Fn FD_ISSET fd &fdset
91	is non-zero if
92	.Fa fd
93	is a member of
94	.Fa fdset ,
95	zero otherwise.
96	The behavior of these macros is undefined if
97	a descriptor value is less than zero or greater than or equal to
98	.Dv FD_SETSIZE ,
99	which is normally at least equal
100	to the maximum number of descriptors supported by the system.
101	.Pp
102	If
103	.Fa timeout
104	is a non-nil pointer, it specifies a maximum interval to wait for the
105	selection to complete. If
106	.Fa timeout
107	is a nil pointer, the select blocks indefinitely. To affect a poll, the
108	.Fa timeout
109	argument should be non-nil, pointing to a zero-valued timeval structure.
110	.Pp
111	Any of
112	.Fa readfds ,
113	.Fa writefds ,
114	and
115	.Fa exceptfds
116	may be given as nil pointers if no descriptors are of interest.
117	.Sh RETURN VALUES
118	.Fn Select
119	returns the number of ready descriptors that are contained in
120	the descriptor sets,
121	or -1 if an error occurred.
122	If the time limit expires,
123	.Fn select
124	returns 0.
125	If
126	.Fn select
127	returns with an error,
128	including one due to an interrupted call,
129	the descriptor sets will be unmodified.
130	.Sh ERRORS
131	An error return from
132	.Fn select
133	indicates:
134	.Bl -tag -width Er
135	.It Bq Er EBADF
136	One of the descriptor sets specified an invalid descriptor.
137	.It Bq Er EINTR
138	A signal was delivered before the time limit expired and
139	before any of the selected events occurred.
140	.It Bq Er EINVAL
141	The specified time limit is invalid. One of its components is
142	negative or too large.
143	.El
144	.Sh SEE ALSO
145	.Xr accept 2 ,
146	.Xr connect 2 ,
147	.Xr read 2 ,
148	.Xr write 2 ,
149	.Xr recv 2 ,
150	.Xr send 2 ,
151	.Xr getdtablesize 2
152	.Sh BUGS
153	Although the provision of
154	.Xr getdtablesize 2
155	was intended to allow user programs to be written independent
156	of the kernel limit on the number of open files, the dimension
157	of a sufficiently large bit field for select remains a problem.
158	The default size
159	.Dv FD_SETSIZE
160	(currently 256) is somewhat larger than
161	the current kernel limit to the number of open files.
162	However, in order to accommodate programs which might potentially
163	use a larger number of open files with select, it is possible
164	to increase this size within a program by providing
165	a larger definition of
166	.Dv FD_SETSIZE
167	before the inclusion of
168	.Aq Pa sys/types.h .
169	.Pp
170	.Fn Select
171	should probably return the time remaining from the original timeout,
172	if any, by modifying the time value in place.
173	This may be implemented in future versions of the system.
174	Thus, it is unwise to assume that the timeout value will be unmodified
175	by the
176	.Fn select
177	call.
178	.Sh HISTORY
179	The
180	.Nm
181	function call appeared in
182	.Bx 4.2 .