Fork/Exec is only good if you are going to run a different program then you are currently executing.

If you want a series of worker threads that are effectively the same as the master/initial thread. The proper way to do this clone(2) not fork, or fork/exec.

When used properly clone allows your group of threads to share a single PID, and TGID (Thread Group IDentifier). This cuts down on kernel resources your process(es) are using. They can natively share file descriptors and memory between each other (improving cache coherence). Also their signals are handled globally for all threads at once, not each thread managing it's own signals like Fork/Exec will result in.