ultrix man page for select

select(2)							System Calls Manual							 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 exceptional condition pending. The nfds parameter is the number of bits to
       be  checked  in	each bit mask that represent a file descriptor; the descriptors from 0 through nfds-1 in the descriptor sets are examined.
       Typically 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 operation. 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(fds  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, zero otherwise. 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
       select  blocks  indefinitely. To effect a poll, the timeout argument should be a non-NULL pointer, pointing to a zero-valued timeval struc-
       ture.

       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 descrip-
       tor 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 failure, returns -1, sets errno to indicate the error, and the descriptor
       sets are not changed.

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' 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; t_sec 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)