Upon successful completion, rfork_thread returns the process ID of the child process to the parent process. Otherwise, a value of -1 is returned to the parent process, no child process is created, and the global variable errno is set to indicate the error.
The child process context is not aware of a return from the rfork_thread function as it begins executing directly with the supplied function.
See rfork(2) for error return codes.
fork(2), intro(2), minherit(2), rfork(2), vfork(2)