As mentioned, this is sometimes due to a resource limitation, but you would be seeing a long list of "child" processes in your "ps" command if that were the case. Memory leaks can exhaust available RAM, which can also result in fork failures. The "second option" mentioned here is often done through use of IPC's such as semaphore's. We also use these for management of threads. Threads are more efficient because it does not cause a full process duplication. There are pthread.h management facilities available also, very cool to use and to have in your code.

The question gets down to whether one is multi-processing with the same code, or doing a fork/exec in order to spawn a completely separate process.

Does this help?