C Programming: Debugging with pthreads

Valgrind is an excellent tool to find race conditions and pthreads API misuses. It keeps a model of program memory (and perhaps of shared resources) accesses and will detect missing locks even when the bug is benign (which of course means that it will completely unexpectedly become less benign at some later point).

To use it, you invoke valgrind --tool=helgrind, here is its manual. Also, there is valgrind --tool=drd (manual). Helgrind and DRD use different models so they detect overlapping but possibly different set of bugs. False positives also may occur.

Anyway, valgrind has saved countless hours of debugging (not all of them though :) for me.

One of the things that will suprise you about debugging threaded programs is that you will often find the bug changes, or even goes away when you add printf's or run the program in the debugger (colloquially known as a Heisenbug).

In a threaded program, a Heisenbug usually means you have a race condition. A good programmer will look for shared variables or resources that are order-dependent. A crappy programmer will try to blindly fix it with sleep() statements.

Debugging a multithreaded application is difficult. A good debugger such as GDB (with optional DDD front end) for the *nix environment or the one that comes with Visual Studio on windows will help tremendously.