Ada '83 Quality and Style:

Guidelines for Professional Programmers

Copyright 1989, 1991,1992 Software Productivity Consortium, Inc., Herndon, Virginia.

CHAPTER 6: Concurrency

Chapter 6 Summary

6.1 tasking

• Use tasks to model asynchronous entities within the problem domain.
• Use tasks to control or synchronize access to tasks or devices.
• Use tasks to define concurrent algorithms.
• Use anonymous task types for single instances.
• Justify the use of dynamically allocated task objects.
• Avoid disassociating a dynamic task from all names.
• Do not rely on pragma Priority to prioritize the service of entries.
• Arrange task bodies in order of their priorities (if possible).
• Do not depend on a particular delay being achievable.
• Do not use a busy waiting loop instead of a delay.
• Design to limit polling to those cases where absolutely necessary.
• Do not use knowledge of the execution pattern of tasks to achieve timing requirements.

6.2 communication

• Minimize the work performed during a rendezvous.
• Minimize the work performed in the selective wait loop of a task.
• Provide a handler for exception Program_Error whenever you cannot avoid a selective wait statement whose alternates can all be closed.
• Make systematic use of handlers for Tasking_Error.
• Be prepared to handle exceptions during a rendezvous.
• Do not depend on the values of the task attributes 'Callable or 'Terminated.
• Do not depend on attributes to avoid Tasking_Error on an entry call.
• Do not depend on the value of the entry attribute 'Count.
• Use the rendezvous mechanism, not shared variables, to pass data between tasks.
• Do not use shared variables as a task synchronization device.
• Do not reference nonlocal variables in a guard.
• Use caution with conditional entry calls.
• Use caution with selective waits with else parts.
• Do not depend upon a particular delay in timed entry calls.
• Do not depend upon a particular delay in selective waits with delay alternatives.
• Minimize the number of accept and select statements per task.
• Minimize the number of accept statements per entry.

6.3 termination

• Place an exception handler for a rendezvous within the main tasking loop.
• Do not create non-terminating tasks unintentionally.
• Explicitly shut down tasks dependent on library packages.
• Use a select statement rather than an accept statement alone.
• Provide a terminate alternative for every selective wait that does not require an else part or a delay.
• Avoid using the abort statement.
• Place an exception handler for others at the end of a task body.
• Have each exception handler at the end of a task body report the task's demise.