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select
======

Syntax
------

     #include <time.h>
     
     int
     select(int nfds,
            fd_set *readfds,
            fd_set *writefds,
            fd_set *exceptfds,
            struct timeval *timeout)

Description
-----------

This function waits for files to be ready for input or output, or for a
timeout.  Each fd_set represents a set of bits representing file
descriptors.  The following macros shall be used to deal with these
sets:

`FD_ZERO(p)'
     Initialize the set to all zeros.

`FD_SET(n, p)'
     Set member N in set P.

`FD_CLR(n, p)'
     Clear member N in set P.

`FD_ISSET(n, p)'
     Return the value of member N in set P.

The TIMEOUT value may be a NULL pointer (no timeout), a pointer to a
zero-value structure (poll mode), or a pointer to an interval-filled
structure (timeout).

Return Value
------------

The number of files ready.  The input sets are replaced with sets that
describe which files are ready for which operations.