I would suggest that you check if you have enough file descriptors available for the operation.

If memory serves me well, ulimit will help you here. Check your ulimit documentation on how to see (and change) this limit.

An easy test, would be to do a batch of one or two processes to validate your code.

You're using waitpid(), but could also do a wait() as well, as you want the call to block there.

Good luck.