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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.