.\" $OpenBSD: select.2,v 1.25 2004/01/23 23:08:46 jmc Exp $ .\" $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. 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 .Sh NAME .Nm select .Nd synchronous I/O multiplexing .Sh SYNOPSIS .Fd #include .Fd #include .Fd #include .Fd #include .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 -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 &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 a non-null pointer, it specifies a maximum interval to wait for the selection to complete. If .Fa timeout is a null pointer, the select blocks indefinitely. To effect a poll, the .Fa timeout argument should be non-null, 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 null 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 EFAULT One or more of .Fa readfds , .Fa writefds , or .Fa exceptfds points outside the process's allocated address space. .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 gettimeofday 2 , .Xr poll 2 , .Xr read 2 , .Xr recv 2 , .Xr send 2 , .Xr write 2 , .Xr getdtablesize 3 .Sh HISTORY The .Fn select function call appeared in .Bx 4.2 . .Sh BUGS Although the provision of .Xr getdtablesize 3 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 bit size of .Ft fd_set is based on the symbol .Dv FD_SETSIZE (currently 256), but that 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 . The kernel will cope, and the userland libraries provided with the system are also ready for large numbers of file descriptors. .Pp Alternatively, to be really safe, it is possible to allocate .Ft fd_set bit-arrays dynamically. The idea is to permit a program to work properly even if it is .Xr execve 2 Ns 'd with 4000 file descriptors pre-allocated. The following illustrates the technique which is used by userland libraries: .Pp .Bd -literal -offset indent -compact fd_set *fdsr; int max = fd; fdsr = (fd_set *)calloc(howmany(max+1, NFDBITS), sizeof(fd_mask)); if (fdsr == NULL) { ... return (-1); } FD_SET(fd, fdsr); n = select(max+1, fdsr, NULL, NULL, &tv); ... free(fdsr); .Ed .Pp Alternatively, it is possible to use the .Xr poll 2 interface. .Xr poll 2 is more efficient when the size of .Fn select Ns 's .Ft fd_set bit-arrays are very large, and for fixed numbers of file descriptors one need not size and dynamically allocate a memory object. .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. Even though some systems stupidly act in this different way, it is unlikely this semantic will ever be commonly implemented, as the change causes massive source code compatibility problems. Furthermore, recent new standards have dictated the current behaviour. In general, due to the existence of those brain-damaged non-conforming systems, 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. Calculating the delta is easily done by calling .Xr gettimeofday 2 before and after the call to .Fn select , and using .Fn timersub (as described in .Xr getitimer 2 ) . .Pp Internally to the kernel, .Fn select works poorly if multiple processes wait on the same file descriptor. Given that, it is rather surprising to see that many daemons are written that way (i.e., .Xr httpd 8 ) .