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SELECT(2)		    BSD	System Calls Manual		     SELECT(2)

     select -- synchronous I/O multiplexing

     Standard C	Library	(libc, -lc)

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

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

     FD_SET(fd,	_fdset);

     FD_CLR(fd,	_fdset);

     FD_ISSET(fd, _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 condi-
     tion pending, respectively.  The only exceptional condition detectable is
     out-of-band data received on a socket.  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 de-
     scriptor sets with	subsets	consisting of those descriptors	that are ready
     for the requested operation.  Select() returns the	total number of	ready
     descriptors in all	the sets.

     The descriptor sets are stored as bit fields in arrays of integers.  The
     following macros are provided for manipulating such 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	non-
     zero 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 max-
     imum number of descriptors	supported by the system.

     If	timeout	is a non-nil pointer, it specifies the maximum interval	to
     wait for the selection to complete.  System activity can lengthen the in-
     terval by an indeterminate	amount.

     If	timeout	is a nil pointer, the select blocks indefinitely.

     To	effect a poll, the timeout argument should be non-nil, pointing	to a
     zero-valued timeval structure.

     Any of readfds, writefds, and exceptfds may be given as nil 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 ex-
     pires, select() returns 0.	 If select() returns with an error, including
     one due to	an interrupted call, the descriptor sets will be unmodified.

     An	error return from select() indicates:

     [EBADF]		One of the descriptor sets specified an	invalid	de-

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

     [EINVAL]		The specified time limit is invalid.  One of its com-
			ponents	is negative or too large.

     [EINVAL]		nfds was invalid.

     accept(2),	connect(2), getdtablesize(2), gettimeofday(2), read(2),
     recv(2), send(2), write(2), clocks(7)

     The default size of FD_SETSIZE is currently 1024.	In order to accommo-
     date programs which might potentially use a larger	number of open files
     with select(), it is possible to increase this size by having the program
     define FD_SETSIZE before the inclusion of any header which	includes

     If	nfds is	greater	than the number	of open	files, select()	is not guaran-
     teed to examine the unused	file descriptors.   For	historical reasons,
     select() will always examine the first 256	descriptors.

     Version 2 of the Single UNIX Specification	("SUSv2") allows systems to
     modify the	original timeout in place.  Thus, it is	unwise to assume that
     the timeout value will be unmodified by the select() call.

     The select() function call	appeared in 4.2BSD.

BSD				March 25, 1994				   BSD


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