Testing sockets

Read, write, and exception operations can be tested. The SELECT macro monitors activity on selected sockets to determine whether:

A buffer for the specified sockets contains input data. If input data is available for a given socket, a read operation on that socket does not block.
TCP/IP can accommodate additional output data. If TCP/IP can accept additional output for a socket, a write operation on the socket does not block.
An exceptional condition occurs on a socket.
A timeout occurs on the SELECT macro itself. A TIMEOUT period can be specified when the SELECT macro is issued.

Each socket descriptor is represented by a bit in a bit string. The length of this bit-mask array is dependent on the value in MAXSOC and must be a multiple of 4 bytes. See Selecting requests in a concurrent server program for more information.