Ok, consider that, in Linux, fork can be called inside a thread. However, you have to keep in mind some important considerations. When fork is called in a multi-threaded program, only the calling thread is duplicated in the child process, while all other threads are not. This can lead to issues if the other threads were holding locks or were in the middle of critical operations. And posix_spawn is designed to be safer because it avoid the pitfalls associated with fork. Check it up: pthread, posix_spawn and related topics.