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<html>
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<body>
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<pre>
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NAME
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select, FD_CLR, FD_ISSET, FD_SET, FD_ZERO - synchronous
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I/O multiplexing
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SYNOPSIS
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#include <network.h>
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int select(int n, fd_set *readfds, fd_set *writefds,
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fd_set *exceptfds, struct timeval *timeout);
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FD_CLR(int fd, fd_set *set);
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FD_ISSET(int fd, fd_set *set);
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FD_SET(int fd, fd_set *set);
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FD_ZERO(fd_set *set);
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DESCRIPTION
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select waits for a number of file descriptors to change
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status.
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Three independent sets of descriptors are watched. Those
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listed in readfds will be watched to see if characters
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become available for reading, those in writefds will be
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watched to see if it is ok to immediately write on them,
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and those in exceptfds will be watched for exceptions. On
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exit, the sets are modified in place to indicate which
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descriptors actually changed status.
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Four macros are provided to manipulate the sets. FD_ZERO
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will clear a set. FD_SET and FD_CLR add or remove a given
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descriptor from a set. FD_ISSET tests to see if a
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descriptor is part of the set; this is useful after select
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returns.
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n is the highest-numbered descriptor in any of the three
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sets, plus 1.
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timeout is an upper bound on the amount of time elapsed
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before select returns. It may be zero, causing select to
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return immediately. If timeout is NULL (no timeout),
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select can block indefinitely.
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RETURN VALUE
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On success, select returns the number of descriptors con-
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tained in the descriptor sets, which may be zero if the
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timeout expires before anything interesting happens. On
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error, -1 is returned, and errno is set appropriately; the
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sets and timeout become undefined, so do not rely on their
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contents after an error.
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ERRORS
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EBADF An invalid file descriptor was given in one of the
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sets.
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EINTR A non blocked signal was caught.
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EINVAL n is negative.
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ENOMEM select was unable to allocate memory for internal
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tables.
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NOTES
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Some code calls select with all three sets empty, n zero,
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and a non-null timeout as a fairly portable way to sleep
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with subsecond precision.
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EXAMPLE
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#include <stdio.h>
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#include <sys/time.h>
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#include <sys/types.h>
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#include <unistd.h>
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int
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main(void)
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{
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fd_set rfds;
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struct timeval tv;
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int retval;
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/* Watch stdin (fd 0) to see when it has input. */
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FD_ZERO(&rfds);
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FD_SET(0, &rfds);
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/* Wait up to five seconds. */
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tv.tv_sec = 5;
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tv.tv_usec = 0;
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retval = select(1, &rfds, NULL, NULL, &tv);
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/* Don't rely on the value of tv now! */
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if (retval)
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printf("Data is available now.\n");
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/* FD_ISSET(0, &rfds) will be true. */
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else
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printf("No data within five seconds.\n");
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exit(0);
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}
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Generally portable to/from non-BSD systems supporting
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clones of the BSD socket layer (including System V vari-
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ants). However, note that the System V variant typically
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sets the timeout variable before exit, but the BSD variant
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does not.
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</pre>
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</body>
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</html>
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