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


     #include <sys/time.h>
     #include <sys/types.h>
     #include <unistd.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));


     select waits for a number  of  file  descriptors  to  change

     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  descrip-
     tor 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.


     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.


     EBADF   An invalid file descriptor was given in one  of  the

     EINTR   A non blocked signal was caught.

     EINVAL  n is negative.

     ENOMEM  select was unable to allocate  memory  for  internal


     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.

     On Linux, timeout is modified to reflect the amount of  time
     not  slept; most other implementations do not do this.  This
     causes problems both when Linux code which reads timeout  is
     ported  to  other operating systems, and when code is ported
     to Linux that reuses a struct timeval for  multiple  selects
     in a loop without reinitializing it.  Consider timeout to be
     undefined after select returns.


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

         fd_set rfds;
         struct timeval tv;
         int retval;

         /* Watch stdin (fd 0) to see when it has input. */
         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. */
             printf("No data within five seconds.\n");




     4.4BSD (the select function first appeared in 4.2BSD).  Gen-
     erally portable to/from non-BSD systems supporting clones of
     the BSD socket layer (including System  V  variants).   How-
     ever,  note  that  the  System  V variant typically sets the
     timeout variable before exit, but the BSD variant does not.


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