The mode argument determines the action taken by the calling process before and during a spawn operation. The following values for mode are possible:
Overlays calling process with new process, destroying calling process (same effect as _exec calls).
Suspends calling thread until execution of new process is complete (synchronous _spawn).
Continues to execute calling process concurrently with new process (asynchronous _spawn).
Continues to execute calling process; new process is run in background with no access to console or keyboard. Calls to _cwait against new process will fail. This is an asynchronous _spawn.