.\"	$OpenBSD: select.2,v 1.20 2001/09/04 08:04:08 deraadt 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. 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.Sh NAME.Nm select.Nd synchronous I/O multiplexing.Sh SYNOPSIS.Fd #include <sys/types.h>.Fd #include <sys/time.h>.Fd #include <unistd.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.Fn selectexamines the I/O descriptor sets whose addresses are passed in.Fa readfds ,.Fa writefds ,and.Fa exceptfdsto see if some of their descriptorsare ready for reading, are ready for writing, or have an exceptionalcondition pending, respectively.The first.Fa nfdsdescriptors are checked in each set;i.e., the descriptors from 0 through.Fa nfds Ns No -1in the descriptor sets are examined.On return,.Fn selectreplaces the given descriptor setswith subsets consisting of those descriptors that are readyfor the requested operation..Fn selectreturns the total number of ready descriptors in all the sets..PpThe descriptor sets are stored as bit fields in arrays of integers.The following macros are provided for manipulating such descriptor sets:.Fn FD_ZERO &fdsetinitializes a descriptor set.Fa fdsetto the null set..Fn FD_SET fd &fdsetincludes a particular descriptor.Fa fdin.Fa fdset ..Fn FD_CLR fd &fdsetremoves.Fa fdfrom.Fa fdset ..Fn FD_ISSET fd &fdsetis non-zero if.Fa fdis a member of.Fa fdset ,zero otherwise.The behavior of these macros is undefined ifa descriptor value is less than zero or greater than or equal to.Dv FD_SETSIZE ,which is normally at least equalto the maximum number of descriptors supported by the system..PpIf.Fa timeoutis a non-null pointer, it specifies a maximum interval to wait for theselection to complete.If.Fa timeoutis a null pointer, the select blocks indefinitely.To effect a poll, the.Fa timeoutargument should be non-null, pointing to a zero-valued timeval structure..Fa timeoutis not changed by.Fn select ,and may be reused on subsequent calls; however, it is good style tore-initialize it before each invocation of.Fn select ..PpAny of.Fa readfds ,.Fa writefds ,and.Fa exceptfdsmay be given as null pointers if no descriptors are of interest..Sh RETURN VALUES.Fn selectreturns the number of ready descriptors that are contained inthe descriptor sets, or \-1 is an error occurred.If the time limit expires,.Fn selectreturns 0.If.Fn selectreturns with an error, including one due to an interrupted call,the descriptor sets will be unmodified..Sh ERRORSAn error return from.Fn selectindicates:.Bl -tag -width Er.It Bq Er EFAULTOne or more of.Fa readfds ,.Fa writefds ,or.Fa exceptfdspoints outside the process's allocated address space..It Bq Er EBADFOne of the descriptor sets specified an invalid descriptor..It Bq Er EINTRA signal was delivered before the time limit expired andbefore any of the selected events occurred..It Bq Er EINVALThe 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 BUGSAlthough the provision of.Xr getdtablesize 3was intended to allow user programs to be written independentof the kernel limit on the number of open files, the dimensionof a sufficiently large bit field for select remains a problem.The default bit size of.Ft fd_setis based on the symbol.Dv FD_SETSIZE(currently 256),but that is somewhat smaller than the current kernel limitto the number of open files.However, in order to accommodate programs which might potentiallyuse a larger number of open files with select, it is possibleto increase this size within a program by providinga larger definition of.Dv FD_SETSIZEbefore the inclusion of.Aq Pa sys/types.h .The kernel will cope, and the userland libraries provided with thesystem are also ready for large numbers of file descriptors..PpAlternatively, to be really safe, it is possible to allocate.Ft fd_setbit-arrays dynamically.The idea is to permit a program to work properly even if it is.Xr execve 2 Ns 'dwith 4000 file descriptors pre-allocated.The following illustrates the technique which is used byuserland 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.PpAlternatively, it is possible to use the.Xr poll 2interface..Xr poll 2is more efficient when the size of.Fn select Ns 's.Ft fd_setbit-arrays are very large, and for fixed numbers offile descriptors one need not size and dynamically allocate amemory object..Pp.Fn selectshould probably have been designed to return the time remaining from theoriginal timeout, if any, by modifying the time value in place.Even though some systems stupidly act in this different way, it isunlikely this semantic will ever be commonly implemented, as thechange causes massive source code compatibility problems.Furthermore, recent new standards have dictated the current behaviour.In general, due to the existence of those brain-damagednon-conforming systems, it is unwise to assume that the timeoutvalue will be unmodified by the.Fn selectcall, and the caller should reinitialize it on each invocation.Calculating the delta is easily done by calling.Xr gettimeofday 2before and after the call to.Fn select Ns ,and using.Fn timersub(as described in.Xr getitimer 2 ) ..PpInternally to the kernel,.Fn selectworks poorly if multiple processes wait on the same file descriptor.Given that, it is rather surprising to see that many daemons arewritten that way (i.e.,.Xr httpd 8 ) ..Sh HISTORYThe.Fn selectfunction call appeared in.Bx 4.2 .