Copyright (c) 1983 Regents of the University of California.
All rights reserved. The Berkeley software License Agreement
specifies the terms and conditions for redistribution.

@(#)select.2 6.3 (Berkeley) 02/03/86

C 5 select - synchronous i/o multiplexing

#include <sys/types.h>
#include <sys/time.h>

nfound = select(nfds, readfds, writefds, exceptfds, timeout)
int nfound, nfds;
fd_set *readfds, *writefds, *exceptfds;
struct timeval *timeout;

int fd;
fd_set fdset;
FD_SET(fd, &fdset) 
FD_CLR(fd, &fdset) 
FD_ISSET(fd, &fdset) 
FD_ZERO(&fdset) 

Select examines the i/o descriptor sets whose addresses are passed in readfds , writefds , and 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 nfds descriptors are checked in each set; i.e. the descriptors from 0 through nfds -1 in the descriptor sets are examined. On return, select replaces the given descriptor sets with subsets consisting of those descriptors that are ready. The total number of ready descriptors in all the sets is returned in nfound .

Some macros are provided for manipulating descriptor sets. "FD_ZERO(&fdset)" initializes a descriptor set fdset to the null set. "FD_SET(fd, &fdset)" includes a particular descriptor fd in fdset . "FD_CLR(fd, &fdset)" removes fd from fdset . "FD_ISSET(fd, &fdset)" is nonzero if fd is a member of fdset , zero otherwise. The behavior of these macros is undefined if a descriptor value is less than zero or greater than or equal to FD_SETSIZE , which is normally at least equal to the maximum number of descriptors supported by the system.

If timeout is a non-zero pointer, it specifies a maximum interval to wait for the selection to complete. If timeout is a zero pointer, the select blocks indefinitely. To affect a poll, the timeout argument should be non-zero, pointing to a zero valued timeval structure.

Any of readfds , writefds , and exceptfds may be given as zero pointers if no descriptors are of interest.

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 select returns 0. An error return from select indicates:

15 [EBADF] One of the descriptor sets specified an invalid descriptor.

15 [EINTR] A signal was delivered before the time limit expired and before any of the selected events occurred.

15 [EINVAL] The specified time limit is invalid. One of its components is negative or too large.

accept(2), connect(2), read(2), write(2), recv(2), send(2), getdtablesize(2) The descriptor sets are always modified on return, even if the call returns as the result of the timeout.