A process may wish to receive information from a subset of the set of ports available for receipt. Systems differ in the amount of selectivity provided to a receiving process. Some allow a process to receive either from a particular port or all ports. Others allow a process to specify any subset of the set of all ports. Still others allow a process to peek at the value of incoming messages and make its decision to accept or reject messages based on the contents of messages.
Selection of a subset of ports is often specified as follows:
{\bf select}
{\bf receive} <port> ...
{\bf receive} <port> ...
...
{\bf receive} <port> ...
{\bf end}
(We are assuming RPC here, with synchronous send)
If none of the ports has a message,
then the process blocks.
If several ports have messages,
then one port is chosen non-deterministically.
If only one port has a message,
then the corresponding receive is executed.
Typically,
a select is enclosed within a loop.
Thus after servicing a request,
a process can service another request.
Often a system allows a guard to be attached to an arm of the select. The guard is a boolean expression and the associated receive occurs only if the condition is true. The following example shows guarded receives in a `bounded buffer' process:
{\bf loop}
{\bf select}
{\bf when} count > 0 {\bf receive} consume (...) ...
{\bf when} count < size {\bf receive} produce (...) ...
{\bf end}
{\bf end loop}.
Here count keeps the number of filled buffers.
The consuming process is blocked if the buffer is empty
and the producing process is blocked if it is full.
Notice a process executing such a loop statement is similar to a monitor.
Each receive in an arm of a select corresponds to an entry procedure
declaration in a Monitor.
A process services one receive at a time,
just as a monitor executes one entry procedure at a time.
The guards correspond to waits on conditions.
The Lauer and Needham paper contains a more detailed discussion on this topic.