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SELECT(2)		  FreeBSD System Calls Manual		     SELECT(2)

     select -- synchronous I/O multiplexing

     Standard C	Library	(libc, -lc)

     #include <sys/select.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);


     The select() system call 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 ex-
     ceptional condition pending, respectively.	 The only exceptional condi-
     tion 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 descriptor sets with subsets consisting of those	descriptors
     that are ready for	the requested operation.  The select() system call re-
     turns 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 not a null 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 null pointer, the select blocks indefinitely.

     To	effect a poll, the timeout argument should not be a null pointer, but
     it	should point to	a zero-valued timeval structure.

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

     The select() system call returns the number of ready descriptors that are
     contained in the descriptor sets, or -1 if	an error occurred.  If the
     time limit	expires, select() returns 0.  If select() returns with an er-
     ror, including one	due to an interrupted system call, the descriptor sets
     will be unmodified.

     An	error return from select() indicates:

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

     [EFAULT]		One of the arguments readfds, writefds,	exceptfds, or
			timeout	points to an invalid address.

     [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]		The nfds argument was invalid.

     accept(2),	connect(2), getdtablesize(2), gettimeofday(2), kqueue(2),
     poll(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.

     The select() system call and FD_CLR(), FD_ISSET(),	FD_SET(), and
     FD_ZERO() macros conform with IEEE	Std 1003.1-2001	("POSIX.1").

     The select() system call appeared in 4.2BSD.

     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() system call.
     FreeBSD does not modify the return	value, which can cause problems	for
     applications ported from other systems.

FreeBSD	13.0		       November	17, 2002		  FreeBSD 13.0


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