mirror of
https://github.com/freebsd/freebsd-src
synced 2024-09-06 17:18:32 +00:00
![Brooks Davis](/assets/img/avatar_default.png)
Remove core system call implementations and documentation to lib/libsys and lib/libsys/<arch> from lib/libc/sys and lib/libc/<arch>/<sys>. Update paths to allow libc to find them in their new home. Reviewed by: kib, emaste, imp Pull Request: https://github.com/freebsd/freebsd-src/pull/908
226 lines
5.8 KiB
Groff
226 lines
5.8 KiB
Groff
.\" 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. 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.
|
|
.\"
|
|
.Dd June 25, 2020
|
|
.Dt SELECT 2
|
|
.Os
|
|
.Sh NAME
|
|
.Nm select
|
|
.Nd synchronous I/O multiplexing
|
|
.Sh LIBRARY
|
|
.Lb libc
|
|
.Sh SYNOPSIS
|
|
.In sys/select.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
|
|
The
|
|
.Fn select
|
|
system call
|
|
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 only exceptional condition detectable is out-of-band
|
|
data received on a socket.
|
|
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.
|
|
The
|
|
.Fn select
|
|
system call
|
|
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.
|
|
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 not a null pointer, it specifies the maximum interval to wait for the
|
|
selection to complete.
|
|
System activity can lengthen the interval by
|
|
an indeterminate amount.
|
|
.Pp
|
|
If
|
|
.Fa timeout
|
|
is a null pointer, the select blocks indefinitely.
|
|
.Pp
|
|
To effect a poll, the
|
|
.Fa timeout
|
|
argument should not be a null pointer,
|
|
but it should point to a zero-valued timeval structure.
|
|
.Pp
|
|
Any of
|
|
.Fa readfds ,
|
|
.Fa writefds ,
|
|
and
|
|
.Fa exceptfds
|
|
may be given as null pointers if no descriptors are of interest.
|
|
.Sh RETURN VALUES
|
|
The
|
|
.Fn select
|
|
system call
|
|
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 system 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 EFAULT
|
|
One of the arguments
|
|
.Fa readfds , writefds , exceptfds ,
|
|
or
|
|
.Fa timeout
|
|
points to an invalid address.
|
|
.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.
|
|
.It Bq Er EINVAL
|
|
The
|
|
.Fa nfds
|
|
argument
|
|
was invalid.
|
|
.El
|
|
.Sh SEE ALSO
|
|
.Xr accept 2 ,
|
|
.Xr connect 2 ,
|
|
.Xr getdtablesize 2 ,
|
|
.Xr gettimeofday 2 ,
|
|
.Xr kqueue 2 ,
|
|
.Xr poll 2 ,
|
|
.Xr pselect 2 ,
|
|
.Xr read 2 ,
|
|
.Xr recv 2 ,
|
|
.Xr send 2 ,
|
|
.Xr write 2 ,
|
|
.Xr clocks 7
|
|
.Sh NOTES
|
|
The default size of
|
|
.Dv FD_SETSIZE
|
|
is currently 1024.
|
|
In order to accommodate programs which might potentially
|
|
use a larger number of open files with
|
|
.Fn select ,
|
|
it is possible
|
|
to increase this size by having the program define
|
|
.Dv FD_SETSIZE
|
|
before the inclusion of any header which includes
|
|
.In sys/types.h .
|
|
.Pp
|
|
If
|
|
.Fa nfds
|
|
is greater than the number of open files,
|
|
.Fn select
|
|
is not guaranteed to examine the unused file descriptors.
|
|
For historical
|
|
reasons,
|
|
.Fn select
|
|
will always examine the first 256 descriptors.
|
|
.Sh STANDARDS
|
|
The
|
|
.Fn select
|
|
system call and
|
|
.Fn FD_CLR ,
|
|
.Fn FD_ISSET ,
|
|
.Fn FD_SET ,
|
|
and
|
|
.Fn FD_ZERO
|
|
macros conform with
|
|
.St -p1003.1-2001 .
|
|
.Sh HISTORY
|
|
The
|
|
.Fn select
|
|
system call appeared in
|
|
.Bx 4.2 .
|
|
.Sh BUGS
|
|
.St -susv2
|
|
allows systems to modify the original timeout in place.
|
|
Thus, it is unwise to assume that the timeout value will be unmodified
|
|
by the
|
|
.Fn select
|
|
system call.
|
|
.Fx
|
|
does not modify the return value, which can cause problems for applications
|
|
ported from other systems.
|