.\" $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 FD_CLR , .Nm FD_COPY , .Nm FD_ISSET , .Nm FD_SET , .Nm FD_ZERO , .Nm select .Nd synchronous I/O multiplexing .Sh SYNOPSIS .Fd #include <sys/select.h> .\" .Ft void .Fo FD_CLR .Fa fd .Fa "fd_set *fdset" .Fc .Ft void .Fo FD_COPY .Fa "fd_set *fdset_orig" .Fa "fd_set *fdset_copy" .Fc .Ft int .Fo FD_ISSET .Fa fd .Fa "fd_set *fdset" .Fc .Ft void .Fo FD_SET .Fa fd .Fa "fd_set *fdset" .Fc .Ft void .Fo FD_ZERO .Fa "fd_set *fdset" .Fc .Ft int .Fo select .Fa "int nfds" .Fa "fd_set *restrict readfds" .Fa "fd_set *restrict writefds" .Fa "fd_set *restrict errorfds" .Fa "struct timeval *restrict timeout" .Fc .Sh DESCRIPTION .Fn Select examines the I/O descriptor sets whose addresses are passed in .Fa readfds , .Fa writefds , and .Fa errorfds 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 errorfds 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 and the global variable .Va errno will be set to indicate the error. .Sh ERRORS An error return from .Fn select indicates: .Bl -tag -width Er .\" =========== .It Bq Er EAGAIN The kernel was (perhaps temporarily) unable to allocate the requested number of file descriptors. .\" =========== .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. .\" =========== .It Bq Er EINVAL .Fa ndfs is greater than FD_SETSIZE and _DARWIN_UNLIMITED_SELECT is not defined. .El .Sh LEGACY SYNOPSIS .Fd #include <sys/select.h> .D1 "- or -" .Fd #include <sys/types.h> .Fd #include <sys/time.h> .Fd #include <unistd.h> .Pp .Fo FD_SET .Fa fd .Fa &fdset .Fc ; .Pp .Fo FD_CLR .Fa fd .Fa &fdset .Fc ; .Pp .Fo FD_ISSET .Fa fd .Fa &fdset .Fc ; .Pp .Fo FD_COPY .Fa &fdset_orig .Fa &fdset_copy .Fc ; .Pp .Fo FD_ZERO .Fa &fdset .Fc ; .Sh COMPATIBILITY .Fn select now returns with .Va errno set to EINVAL when .Fa nfds is greater than FD_SETSIZE. Use a smaller value for .Fa nfds or compile with -D_DARWIN_UNLIMITED_SELECT. .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 , .Xr compat 5 .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 .