Fork works by duplicating the process. The process is duplicated into
two (parent and child) unless it fails (returns -1). The parent fork
returns 0, and the child fork returns > 1. getpid() can
be used to find the child, and the child can get the parent’s pid with
getppid(). Fork also copies the entire process space with
Copy on Write semantics, so as long as the child process nor the parent
process write to their pages, they get fine performance.