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

     select, pselect --	synchronous I/O	multiplexing

     Standard C	Library	(libc, -lc)

     #include <sys/select.h>

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

     pselect(int nfds, fd_set *	restrict readfds, fd_set * restrict writefds,
	 fd_set	* restrict exceptfds, const struct timespec *restrict timeout,
	 const sigset_t	* restrict sigmask);

     FD_SET(int	fd, fd_set *fdset);

     FD_CLR(int	fd, fd_set *fdset);

     FD_ISSET(int fd, fd_set *fdset);

     FD_ZERO(fd_set *fdset);

     select() and pselect() examine 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 first nfds	descriptors
     are checked in each set; i.e., the	descriptors from 0 through nfds-1 in
     the descriptor sets are examined.	This means that	nfds must be set to
     the highest file descriptor of the	three sets, plus one.  On return,
     select() and pselect() replace the	given descriptor sets with subsets
     consisting	of those descriptors that are ready for	the requested opera-
     tion.  select() and pselect() return the total number of ready descrip-
     tors 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 pointed to by 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 maximum number	of descriptors supported by the	system.

     If	timeout	is a non-null pointer, it specifies a maximum interval to wait
     for the selection to complete.  If	timeout	is a null pointer, the select
     blocks indefinitely.  To poll without blocking, the timeout argument
     should be non-null, pointing to a zero-valued timeval or timespec struc-
     ture, as appropriate.  timeout is not changed by select(),	and may	be
     reused on subsequent calls; however, it is	good style to re-initialize it
     before each invocation of select().

     If	sigmask	is a non-null pointer, then the	pselect() function shall re-
     place the signal mask of the caller by the	set of signals pointed to by
     sigmask before examining the descriptors, and shall restore the signal
     mask of the calling thread	before returning.

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

     It	is recommended to use the poll(2) interface instead, which tends to be
     more portable and efficient.

     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.

     #include <stdio.h>
     #include <stdlib.h>
     #include <unistd.h>
     #include <string.h>
     #include <err.h>
     #include <errno.h>
     #include <sys/types.h>
     #include <sys/time.h>

     main(int argc, char **argv)
	     fd_set read_set;
	     struct timeval timeout;
	     int ret, fd, i;

	     /*	file descriptor	1 is stdout */
	     fd	= 1;

	     /*	Wait for ten seconds. */
	     timeout.tv_sec = 10;
	     timeout.tv_usec = 0;

	     /*	Initialize the read set	to null	*/

	     /*	Add file descriptor 1 to read_set */
	     FD_SET(fd,	&read_set);

	      *	Check if data is ready to be readen on
	      *	file descriptor	1, give	up after 10 seconds.
	     ret = select(fd + 1, &read_set, NULL, NULL, &timeout);

	      *	Returned value is the number of	file
	      *	descriptors ready for I/O, or -1 on error.
	     switch (ret) {
	     case -1:
		     err(EXIT_FAILURE, "select() failed");

	     case 0:
		     printf("Timeout, no data received.\n");

		     printf("Data received on %d file desciptor(s)\n", ret);

		      *	select(2) hands	back a file descriptor set where
		      *	only descriptors ready for I/O are set.	These can
		      *	be tested using	FD_ISSET
		     for (i = 0; i <= fd; i++) {
			     if	(FD_ISSET(i, &read_set)) {
				     printf("Data on file descriptor %d\n", i);
				     /*	Remove the file	descriptor from	the set	*/
				     FD_CLR(fd,	&read_set);

	     return 0;

     An	error return from select() indicates:

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

     [EFAULT]		One or more of readfds,	writefds, or exceptfds points
			outside	the process's allocated	address	space.

     [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.

     accept(2),	connect(2), gettimeofday(2), poll(2), read(2), recv(2),
     send(2), write(2),	getdtablesize(3)

     The select() function call	appeared in 4.2BSD.

     Although the provision of getdtablesize(3)	was intended to	allow user
     programs to be written independent	of the kernel limit on the number of
     open files, the dimension of a sufficiently large bit field for select
     remains a problem.	 The default bit size of fd_set	is based on the	symbol
     FD_SETSIZE	(currently 256), but that is somewhat smaller than the current
     kernel limit to the number	of open	files.	However, 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 within a	program	by
     providing a larger	definition of FD_SETSIZE before	the inclusion of
     <sys/types.h>.  The kernel	will cope, and the userland libraries provided
     with the system are also ready for	large numbers of file descriptors.

     Note: rpc(3) library uses fd_set with the default FD_SETSIZE as part of
     its ABI.  Therefore, programs that	use rpc(3) routines cannot change

     Alternatively, to be really safe, it is possible to allocate fd_set bit-
     arrays dynamically.  The idea is to permit	a program to work properly
     even if it	is execve(2)'d with 4000 file descriptors pre-allocated.  The
     following illustrates the technique which is used by userland libraries:

		   fd_set *fdsr;
		   int max = fd;

		   fdsr	= (fd_set *)calloc(howmany(max+1, NFDBITS),
		   if (fdsr == NULL) {
			   return (-1);
		   FD_SET(fd, fdsr);
		   n = select(max+1, fdsr, NULL, NULL, &tv);

     select() should probably have been	designed to return the time remaining
     from the original timeout,	if any,	by modifying the time value in place.
     Even though some systems stupidly act in this different way, it is	un-
     likely this semantic will ever be commonly	implemented, as	the change
     causes massive source code	compatibility problems.	 Furthermore, recent
     new standards have	dictated the current behaviour.	 In general, due to
     the existence of those non-conforming systems, it is unwise to assume
     that the timeout value will be unmodified by the select() call, and the
     caller should reinitialize	it on each invocation.	Calculating the	delta
     is	easily done by calling gettimeofday(2) before and after	the call to
     select(), and using timersub() (as	described in getitimer(2)).

     Internally	to the kernel, select()	works poorly if	multiple processes
     wait on the same file descriptor.

BSD			       October 18, 2008				   BSD


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