| SELECT(2) | System Calls Manual | SELECT(2) | 
select, pselect,
  FD_SET, FD_CLR,
  FD_ISSET, FD_ZERO —
#include <sys/select.h>
int
  
  select(int
    nfds, fd_set * restrict
    readfds, fd_set *
    restrict writefds, fd_set
    * restrict exceptfds,
    struct timeval * restrict
    timeout);
int
  
  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 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 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
    select().
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.
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 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>
int
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 */
	FD_ZERO(&read_set);
	/* Add file descriptor 1 to read_set */
	FD_SET(fd, &read_set);
	/*
	 * Check if data is ready to be read 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");
		break;
	case 0:
		printf("Timeout, no data received.\n");
		break;
	default:
		printf("Data received on %d file descriptor(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);
			}
		}
		break;
	}
	return 0;
}
select() indicates:
EBADF]EFAULT]EINTR]EINVAL]select() function call appeared in
  4.2BSD.
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 userland libraries:
	fd_set *fdsr;
	int max = fd;
	fdsr = (fd_set *)calloc(howmany(max+1, NFDBITS),
	    sizeof(fd_mask));
	if (fdsr == NULL) {
		...
		return (-1);
	}
	FD_SET(fd, fdsr);
	n = select(max+1, fdsr, NULL, NULL, &tv);
	...
	free(fdsr);
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 descriptor.
| November 28, 2013 | NetBSD 9.3 |