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NetBSD 6.1.5 - man page for select (netbsd section 2)

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 exceptional 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 descrip-
     tor 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 operation.  select() and pselect() return 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 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 structure, 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

     If sigmask is a non-null pointer, then the pselect() function shall replace 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

     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, select() returns 0.  If select()
     returns with an error, including one due to an interrupted call, the descriptor sets will be

     #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 descriptor.

     [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 components is negative
			or too large.

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

     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 accommodate 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 FD_SETSIZE.

     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 user-
     land 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 unlikely 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

BSD					 October 18, 2008				      BSD

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