DCE Application Programming Guide

Building, Running, and Debugging NonStop DCE
Applications
HP NonStop DCE Application Programming Guide—429551-003
4-8
Debugging With Threads
Difficulty in Debugging Multiple Application Threads
Debugging multithreaded applications can be difficult because of the interaction and
synchronization of the threads.
The simplest multithreaded applications to debug are those that have no
interdependent threads. Many standard DCE servers are in this category (see the
discussion of Single Versus Multiple Application Threads on page 3-13). For these
applications, the application code can be tested using a single application thread. For
example, server manager code can be unit-tested by using a single client program that
issues one RPC at a time. Once the program passes its functional tests, it can be
tested using multiple concurrent clients and multiple application call threads.
Even in this simplest case, implicit dependencies can exist between the application
threads because they share the same process environment. These dependencies can
be subtle and might result in observable effects only under unusual conditions, such as
error handling. Find these problems during design and code reviews can be easier
than finding them during unit-testing or system-testing.
Implicit dependencies also occur when applications use libraries that return pointers to
static data or that maintain context across calls into the library. Application threads can
also interfere with each other implicitly as a result of using process-blocking calls, not
yielding the processor, or corrupting process memory. These issues are discussed at
length in Threading Considerations for NonStop DCE Applications on page 3-4.
Debugging a program becomes more complex as the amount of synchronization
increases. Types of application synchronization include using mutexes to reserve
access to a shared resource and using condition variables to wait for a shared
resource to be in a certain state. When these techniques are used, the programs must
be carefully designed with the threading model in mind.