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select(2)							     select(2)

Name
       select - synchronous I/O multiplexing

Syntax
       #include <sys/types.h>
       #include <sys/time.h>

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

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

Description
       The  system  call  examines the I/O descriptor sets whose addresses are
       passed in readfds, writefds, and exceptfds to  see  if  some  of	 their
       descriptors are ready for reading, ready for writing, or have an excep‐
       tion condition pending. The nfds parameter is the number of bits to  be
       checked in each bit mask that represent a file descriptor; the descrip‐
       tors from 0 through nfds-1 in the descriptor sets are  examined.	 Typi‐
       cally  nfds  has	 the  value returned by for the maximum number of file
       descriptors. On return, replaces the given descriptor sets with subsets
       consisting of those descriptors that are ready for the requested opera‐
       tion. The total	number	of  ready  descriptors	in  all	 the  sets  is
       returned.

       The descriptor sets are stored as bit fields in arrays of integers. The
       following macros are provided for manipulating such descriptor sets:

	    FD_ZERO(fdset)	Initializes  a descriptor  set	fdset  to  the
				null set.

	    FD_SET(fd, fdset)	Includes a particular descriptor fd in fdset.

	    FD_CLR(fd, fdset)	Removes fd from fdset.

	    FD_ISSET(fd, fdset) Is  nonzero if fd is a member of fdset; other‐
				wise the value is set to zero.

       The behavior of these macros is undefined if a descriptor value is less
       than zero or greater than or equal to FD_SETSIZE, which is equal to the
       maximum number of descriptors that can be supported by the system.

       If timeout is not a NULL pointer, it specifies a	 maximum  interval  to
       wait  for  the selection to complete. If timeout is a NULL pointer, the
       call blocks indefinitely. To effect a poll, the timeout argument should
       be a non-NULL pointer, pointing to a zero-valued timeval structure.

       Any  of	readfds, writefds, and exceptfds may be given as NULL pointers
       if no descriptors are of interest.

       Selecting true for reading on a socket descriptor upon which a call has
       been performed indicates that a subsequent call on that descriptor will
       not block.

Restrictions
       The call may indicate that a descriptor is ready for  writing  when  in
       fact  an	 attempt  to  write  would  block.  This  can happen if system
       resources necessary for a write are exhausted or otherwise unavailable.
       If  an  application  deems it critical that writes to a file descriptor
       not block, it should set the descriptor for non-blocking I/O using  the
       F_SETFL request to the call.

Return Values
       The  call  returns  a  non-negative value on success.  A positive value
       indicates the number of ready descriptors in the descriptor sets.  A  0
       indicates that the time limit referred to by timeout expired.  On fail‐
       ure, returns -1, sets errno to indicate the error, and does not	change
       the descriptor sets.

Diagnostics
       [EBADF]	   One of the descriptor sets specified an invalid descriptor.

       [EFAULT]	   One	of  the	 pointers given in the call referred to a non-
		   existent portion of the process's address space.

       [EINTR]	   A signal was delivered before any of	 the  selected	events
		   occurred, or before the time limit expired.

       [EINVAL]	   A  component	 of  the  pointed-to time limit is outside the
		   acceptable range.  The t_sec range must be  between	0  and
		   10^8, inclusive; t_usec must be greater than or equal to 0,
		   and less than 10^6.

See Also
       accept(2), connect(2), fcntl(2), gettimeofday(2),  listen(2),  read(2),
       recv(2), send(2), write(2), getdtablesize(2)

								     select(2)
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