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Diffstat (limited to 'lib/libc/sys/select.2')
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diff --git a/lib/libc/sys/select.2 b/lib/libc/sys/select.2 new file mode 100644 index 0000000..2435e37 --- /dev/null +++ b/lib/libc/sys/select.2 @@ -0,0 +1,227 @@ +.\" 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. +.\" 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 +.\" $FreeBSD$ +.\" +.Dd November 17, 2002 +.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 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. |