The following method is suggested for debugging MPI programs. First, if possible, write the program as a serial program. This will allow you to debug most syntax, logic, and indexing errors.
Then, modify the program and run it with 2--4 processes on the same machine. This step will allow you to catch syntax and logic errors concerning intertask communication. A common error found at this point is the use of non-unique message tags. The final step in debugging your application is to run the same processes on different machines. This will check for synchronization errors caused by message delays on the network.
You should first try to find the bug by using a few printf statements. If this does not work, then you may want to try running the program under a debugger such as dbx or gdb which will start the first process under the debugger where possible. See man dbx or man gdb for information on these debuggers and man mpirun for information on adding them to your command line arguments. Note that the debuggers are not parallel versions, but are only being used by the first process. Also, their interaction with parallel programs was not tested, therefore use at your own risk.