So what should you use?

Obviously, if you're porting existing code, you'll want to use whatever the existing code uses. For new code, you should avoid fork() if at all possible. Here's why:

• Although fork() works with multiple threads, you need to register a pthread_atfork() handler and lock every single mutex before you fork, complicating the design.
• The child of fork() duplicates all open file descriptors. As we'll see in the Resource Manager chapter later, this causes a lot of work — most of which will be unnecessary if the child then immediately does an exec() and closes all the file descriptors anyway.

The choice between vfork() and the spawn() family boils down to portability, and what you want the child and parent to be doing. The vfork() function will pause until the child calls exec() or exits, whereas the spawn() family of functions can allow both to run concurrently. The vfork() function, however, is subtly different between operating systems.