.\" Copyright (c) 1983 The Regents of the University of California. .\" All rights reserved. .\" .\" Redistribution and use in source and binary forms are permitted provided .\" that: (1) source distributions retain this entire copyright notice and .\" comment, and (2) distributions including binaries display the following .\" acknowledgement: ``This product includes software developed by the .\" University of California, Berkeley and its contributors'' in the .\" documentation or other materials provided with the distribution and in .\" all advertising materials mentioning features or use of this software. .\" 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 ``AS IS'' AND WITHOUT ANY EXPRESS OR IMPLIED .\" WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED WARRANTIES OF .\" MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. .\" .\" @(#)select.2 6.7 (Berkeley) 6/23/90 .\" .TH SELECT 2 "June 23, 1990" .UC 5 .SH NAME select \- synchronous I/O multiplexing .SH SYNOPSIS .nf .ft B #include <sys/types.h> #include <sys/time.h> .PP .ft B nfound = select(nfds, readfds, writefds, exceptfds, timeout) int nfound, nfds; fd_set *readfds, *writefds, *exceptfds; struct timeval *timeout; .PP .ft B FD_SET(fd, &fdset) FD_CLR(fd, &fdset) FD_ISSET(fd, &fdset) FD_ZERO(&fdset) int fd; fd_set fdset; .fi .SH DESCRIPTION .I Select examines the I/O descriptor sets whose addresses are passed in .IR readfds , .IR writefds , and .I 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 .I nfds descriptors are checked in each set; i.e. the descriptors from 0 through .IR nfds -1 in the descriptor sets are examined. On return, .I select replaces the given descriptor sets with subsets consisting of those descriptors that are ready for the requested operation. The total number of ready descriptors in all the sets is returned in .IR nfound . .PP The descriptor sets are stored as bit fields in arrays of integers. The following macros are provided for manipulating such descriptor sets: .I "FD_ZERO(&fdset)" initializes a descriptor set .I fdset to the null set. .I "FD_SET(fd, &fdset)" includes a particular descriptor .I fd in .IR fdset . .I "FD_CLR(fd, &fdset)" removes .I fd from .IR fdset . .I "FD_ISSET(fd, &fdset)" is nonzero if .I fd is a member of .IR fdset , zero otherwise. The behavior of these macros is undefined if a descriptor value is less than zero or greater than or equal to .IR FD_SETSIZE , which is normally at least equal to the maximum number of descriptors supported by the system. .PP If .I timeout is a non-zero pointer, it specifies a maximum interval to wait for the selection to complete. If .I timeout is a zero pointer, the select blocks indefinitely. To affect a poll, the .I timeout argument should be non-zero, pointing to a zero-valued timeval structure. .PP Any of .IR readfds , .IR writefds , and .I exceptfds may be given as zero pointers if no descriptors are of interest. .SH "RETURN VALUE .I 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 then .I select returns 0. If .I select returns with an error, including one due to an interrupted call, the descriptor sets will be unmodified. .SH "ERRORS An error return from \fIselect\fP indicates: .TP 15 [EBADF] One of the descriptor sets specified an invalid descriptor. .TP 15 [EINTR] A signal was delivered before the time limit expired and before any of the selected events occurred. .TP 15 [EINVAL] The specified time limit is invalid. One of its components is negative or too large. .SH SEE ALSO accept(2), connect(2), read(2), write(2), recv(2), send(2), getdtablesize(2) .SH BUGS Although the provision of .IR 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 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 FD_SETSIZE before the inclusion of <sys/types.h>. .PP .I 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 .I select call.