Function fork 

pub fn fork() -> Result<Fork, i32>

Create a new child process see fork(2)

Upon successful completion, fork() returns a value of 0 to the child process and 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.

Example:

use fork::{fork, Fork};

match fork() {
   Ok(Fork::Parent(child)) => {
       println!("Continuing execution in parent process, new child has pid: {}", child);
   }
   Ok(Fork::Child) => println!("I'm a new child process"),
   Err(_) => println!("Fork failed"),
}

This will print something like the following (order indeterministic).

Continuing execution in parent process, new child has pid: 1234
I'm a new child process

The thing to note is that you end up with two processes continuing execution immediately after the fork call but with different match arms.

nix::unistd::fork

The example has been taken from the nix::unistd::fork, please check the Safety section

Errors

returns -1 if error