int fork() turns a single process into 2 identical processes, known as the parent and the child. On success, fork() returns 0 to the child process and returns the process ID of the child process to the parent process. On failure, fork() returns -1 to the parent process, sets errno to indicate the error, and no child process is created.
NOTE: The child process will have its own unique PID.
The following program illustrates a simple use of fork, where two copies are made and run together (multitasking)
The Output of this would be:
NOTE: The processes have unique ID's which will be different at each run.
It also impossible to tell in advance which process will get to CPU's time - so one run may differ from the next.
When we spawn 2 processes we can easily detect (in each process) whether it is the child or parent since fork returns 0 to the child. We can trap any errors if fork returns a -1. i.e.: