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SELECT(2)							     SELECT(2)

NAME    [Toc]    [Back]

     select - synchronous I/O multiplexing

SYNOPSIS    [Toc]    [Back]

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

     int 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)
     int fd;
     fd_set fdset;

DESCRIPTION    [Toc]    [Back]

     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 file descriptors are
     checked in	each set; i.e. the file	descriptors from 0 through nfds	- 1
     will be examined (see getdtablehi(3) for largest open descriptor).	 On
     return, select replaces the given descriptor sets with subsets consisting
     of	those descriptors that are ready for the requested operation.  The
     total number of ready descriptors in all the sets is the return value.

     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
     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 effect 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.

RETURN VALUE    [Toc]    [Back]

     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.  If	select returns with an error, including	one

									Page 1

SELECT(2)							     SELECT(2)

     due to an interrupted call, the descriptor	sets will be unmodified.

ERRORS    [Toc]    [Back]

     An	error return from select indicates:

     [EBADF]	    One	of the descriptor sets specified an invalid

     [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
		    components is negative or too large.

SEE ALSO    [Toc]    [Back]

     poll(2), accept(2), connect(2), read(2), write(2),	recv(2), send(2),

NOTES    [Toc]    [Back]

     Some devices do not support polling via the select(2) and poll(2) system
     calls.  Doing a select or poll on a file descriptor associated with an
     "un-pollable" device will cause the select	or poll	to return immediately
     with a success value of 0 and the with the	corresponding file descriptor
     events of queried set true.  For instance,	if a select or poll is
     performed on a read file descriptor associated with an un-pollable
     device, the call would return immediately,	even though there may be
     nothing to	read on	the device.  A subsequent read(2) in this situation
     might return with a "bytes-read" count of 0 or might block	if the device
     supports read blocking.  Devices which exhibit this behavior (especially
     those from	third-party vendors) should be suspected as not	supporting

BUGS    [Toc]    [Back]

     Select should probably return the time remaining from the original
     timeout, if any, by modifying the time value in place.  This may be
     implemented in future versions of the system.  Thus, it is	unwise to
     assume that the timeout value will	be unmodified by the select call.

									PPPPaaaaggggeeee 2222
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