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 6.1 (Berkeley) 05/15/85

C 5 select - synchronous i/o multiplexing

#include <sys/time.h>

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

Select examines the i/o descriptors specified by the bit masks readfds , writefds , and exceptfds to see if they are ready for reading, writing, or have an exceptional condition pending, respectively. File descriptor f is represented by the bit \*(lq1<<f\*(rq in the mask. Nfds desciptors are checked, i.e. the bits from 0 through nfds -1 in the masks are examined. Select returns, in place, a mask of those descriptors which are ready. The total number of bits set in all the masks is returned in nfound .

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 0 if no descriptors are of interest.

Select returns the number of 'one' bits which are contained in the bit masks, 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 bit masks specified an invalid descriptor.

15 [EINTR] An signal was delivered before any of the selected for events occurred or the time limit expired.

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