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SELECT(2)                     System Calls Manual                    SELECT(2)

     select, pselect, FD_SET, FD_CLR, FD_ISSET, FD_ZERO - synchronous I/O

     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 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.

     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
     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 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");

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

                     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);

             return 0;

     An error return from select() indicates:

     [EBADF]            One of the descriptor sets specified an invalid

     [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), 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
     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

     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
     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.

NetBSD 9.99                    November 28, 2013                   NetBSD 9.99