Getting Started with QNX Neutrino - QNX Software Systems

© 2009, QNX Software Systems GmbH & Co. KG. More on synchronization
More on synchronization
To sum up: know what the trade-offs are, and use what works for your design.
We’ve already seen:
• mutexes
• semaphores
• barriers
Readers/writer locks
Let’s now finish up our discussion of synchronization by talking about:
• readers/writer locks
• sleepon locks
• condition variables
• additional Neutrino services
Readers and writer locks are used for exactly what their name implies: multiple
readers can be using a resource, with no writers, or one writer can be using a resource
with no other writers or readers.
This situation occurs often enough to warrant a special kind of synchronization
primitive devoted exclusively to that purpose.
Often you’ll have a data structure that’s shared by a bunch of threads. Obviously, only
one thread can be writing to the data structure at a time. If more than one thread was
writing, then the threads could potentially overwrite each other’s data. To prevent this
from happening, the writing thread would obtain the “rwlock” (the readers/writer lock)
in an exclusive manner, meaning that it and only it has access to the data structure.
Note that the exclusivity of the access is controlled strictly by voluntary means. It’s up
to you, the system designer, to ensure that all threads that touch the data area
synchronize by using the rwlocks.
The opposite occurs with readers. Since reading a data area is a non-destructive
operation, any number of threads can be reading the data (even if it’s the same piece of
data that another thread is reading). An implicit point here is that no threads can be
writing to the data area while any thread or threads are reading from it. Otherwise, the
reading threads may be confused by reading a part of the data, getting preempted by a
writing thread, and then, when the reading thread resumes, continue reading data, but
from a newer “update” of the data. A data inconsistency would then result.
Let’s look at the calls that you’d use with rwlocks.
The first two calls are used to initialize the library’s internal storage areas for the
rwlocks:
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