Smalltalk and Object Orientation: an Introduction - Free

31.3 Processes in Smalltalk
For further details on processes in Smalltalk see the appropriate system documentation, for example in
VisualWorks see Chapter 8 in the Smalltalk User's Guide, Release 4.1 and pages 25 -40 of [Lalonde and
Pugh 1991b]. In this section we shall only provide a brief introduction to this subject.
A Smalltalk process is a non -preemptive light-weight process. That i s, a Smalltalk process will run
to completion unless a higher priority process attempts to gain control Smalltalk does not attempt to
share the processor time amongst processes of the same priority. When the highest priority is held by
multiple processes, the active process can be bumped to the back of the line with the expression:
Processoryield - otherwise it will run until it is suspended or terminated before giving up the processor.
A process that has been bumped will regain control before a process of lower priority.
This of course implies that every process has an associated priority. By default a process inherits the
same priority as the process which spawned it. The priority of a process can be changed using the
priority: message.
The process which is currently being executed by the processor is termed the active process. A
process can also be waiting to use the processor or stopped, waiting for some resource. There are a
number of message related to the state of a process these are:
• aProcess resume. This schedules a process.
• aProcess suspend. This stops the process from executing.
• aProcess terminate. This unschedules the process permanently.
There are a number of messages which a re sent directly to the process scheduler which can be useful in
working with active processes. These are:
• Processor activeProcess . This returns the process which is currently executing.
• Processor activePriority . This returns the priority of the active process; i.e. the
priority of the process which is currently executing.
• Processor terminateActivity . Permanently removes the active process from
execution.
Note that the messages are sent to a global variable Processor, which holds the instance of
ProcessorScheduler.
For a process to be spawned from the current process there must be some way of creating a new
process. This is done using one of four messages to a block. These messages are:
aBlock fork
aBlock forkAt:
aPriority
This creates and schedules a process which will execute
the block. The priority of this process is inherited from the
parent process.
This creates and schedules a process executing the block at
a specific priority.
aBlock newProcess This creates a new pro cess. It does not schedule the
process. The process is created with the same priority as
the parent block. It can be scheduled using the resume
message.
aBlock newProcessWith:
anArrayOfParameters
This message creates a new process with the same priority
as the parent process. An array of parameters is passed to
the block.
31.3.1 Semaphores
Semaphores provide a (simple) means of synchronization between multiple processes. For example, if
two processes are executing and one must not pass a certain point until the other has completed some
operation, then a semaphore between the two processes can be used as a flag to indicate to the first
process that the operation has been completed.
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