C Programming: Debugging with pthreads

Question

One of the hardest things for me to initially adjust to was my first intense experience programming with pthreads in C. I was used to knowing exactly what the next line of c

Answer 1

When I started doing multithreaded programming I... stopped using debuggers. For me the key point is good program decomposition and encapsulation.

Monitors are the easiest way of error-free multithreaded programming. If you cannot avoid complex lock dependencies then it is easy to check if they are cyclic - wait until program hangs ans check the stacktraces using 'pstack'. You can break cyclic locks by introducing some new threads and asynchronous communication buffers.

Use assertions, and make sure to write singlethreaded unittests for particular components of your software - you can then run them in debugger if you want.

Answer 2

My approach to multi-threaded debugging is similar to single-threaded, but more time is usually spent in the thinking phase:

1. Develop a theory as to what could be causing the problem.

2. Determine what kind of results could be expected if the theory is true.

3. If necessary, add code that can disprove or verify your results and theory.

4. If your theory is true, fix the problem.

Often, the 'experiment' that proves the theory is the addition of a critical section or mutex around suspect code. I will then try to narrow down the problem by systematically shrinking the critical section. Critical sections are not always the best fix (though can often be the quick fix). However, they're useful for pinpointing the 'smoking gun'.

Like I said, the same steps apply to single-threaded debugging, though it is far too easy to just jump into a debugger and have at it. Multi-threaded debugging requires a much stronger understanding of the code, as I usually find the running multi-threaded code through a debugger doesn't yield anything useful.

Also, hellgrind is a great tool. Intel's Thread Checker performs a similar function for Windows, but costs a lot more than hellgrind.