    NAME
           select,  FD_CLR,  FD_ISSET,  FD_SET, FD_ZERO - synchronous
           I/O multiplexing

    SYNOPSIS
           #include <network.h>

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

           FD_CLR(int fd, fd_set *set);
           FD_ISSET(int fd, fd_set *set);
           FD_SET(int fd, fd_set *set);
           FD_ZERO(fd_set *set);

    DESCRIPTION
           select  waits  for  a number of file descriptors to change
           status.

           Three independent sets of descriptors are watched.   Those
           listed  in  readfds  will  be watched to see if characters
           become available for reading, those in  writefds  will  be
           watched  to  see if it is ok to immediately write on them,
           and those in exceptfds will be watched for exceptions.  On
           exit,  the  sets  are  modified in place to indicate which
           descriptors actually changed status.

           Four macros are provided to manipulate the sets.   FD_ZERO
           will clear a set.  FD_SET and FD_CLR add or remove a given
           descriptor from  a  set.   FD_ISSET  tests  to  see  if  a
           descriptor is part of the set; this is useful after select
           returns.

           n is the highest-numbered descriptor in any of  the  three
           sets, plus 1.

           timeout  is  an  upper bound on the amount of time elapsed
           before select returns. It may be zero, causing  select  to
           return  immediately.  If  timeout  is  NULL  (no timeout),
           select can block indefinitely.

    RETURN VALUE
           On success, select returns the number of descriptors  con-
           tained  in  the  descriptor sets, which may be zero if the
           timeout expires before anything interesting  happens.   On
           error, -1 is returned, and errno is set appropriately; the
           sets and timeout become undefined, so do not rely on their
           contents after an error.

    ERRORS
           EBADF   An invalid file descriptor was given in one of the
                   sets.

           EINTR   A non blocked signal was caught.

           EINVAL  n is negative.

           ENOMEM  select was unable to allocate memory for  internal
                   tables.

    NOTES
           Some  code calls select with all three sets empty, n zero,
           and a non-null timeout as a fairly portable way  to  sleep
           with subsecond precision.

    EXAMPLE
           #include <stdio.h>
           #include <sys/time.h>
           #include <sys/types.h>
           #include <unistd.h>

           int
           main(void)
           {
               fd_set rfds;
               struct timeval tv;
               int retval;

               /* Watch stdin (fd 0) to see when it has input. */
               FD_ZERO(&rfds);
               FD_SET(0, &rfds);
               /* Wait up to five seconds. */
               tv.tv_sec = 5;
               tv.tv_usec = 0;

               retval = select(1, &rfds, NULL, NULL, &tv);
               /* Don't rely on the value of tv now! */

               if (retval)
                   printf("Data is available now.\n");
                   /* FD_ISSET(0, &rfds) will be true. */
               else
                   printf("No data within five seconds.\n");

               exit(0);
           }

           Generally  portable  to/from  non-BSD  systems  supporting
           clones  of  the BSD socket layer (including System V vari-
           ants).  However, note that the System V variant  typically
           sets the timeout variable before exit, but the BSD variant
           does not.