Getting Started with QNX Neutrino - QNX Software Systems

© 2009, QNX Software Systems GmbH & Co. KG. More on synchronization
sleepons, the underlying condvars (on top of which sleepons are implemented) are
allocated dynamically as threads wait for a particular object. Therefore, using sleepons
with M objects and N threads blocked, you’d have (at most) N condvars (instead of
M).
However, condvars are more flexible than sleepons, because:
1 Sleepons are built on top of condvars anyway.
2 Sleepons have the mutex buried in the library; condvars allow you to specify it
explicitly.
The first point might just be viewed as being argumentative. :-) The second point,
however, is significant. When the mutex is buried in the library, this means that there
can be only one per process — regardless of the number of threads in that process, or
the number of different “sets” of data variables. This can be a very limiting factor,
especially when you consider that you must use the one and only mutex to access any
and all data variables that any thread in the process needs to touch!
A much better design is to use multiple mutexes, one for each data set, and explicitly
combine them with condition variables as required. The true power and danger of this
approach is that there is absolutely no compile time or run time checking to make sure
that you:
• have locked the mutex before manipulating a variable
• are using the correct mutex for the particular variable
• are using the correct condvar with the appropriate mutex and variable
The easiest way around these problems is to have a good design and design review,
and also to borrow techniques from object-oriented programming (like having the
mutex contained in a data structure, having routines to access the data structure, etc.).
Of course, how much of one or both you apply depends not only on your personal
style, but also on performance requirements.
The key points to remember when using condvars are:
1 The mutex is to be used for testing and accessing the variables.
2 The condvar is to be used as a rendezvous point.
Here’s a picture:
April 30, 2009 Chapter 1 • Processes and Threads 67