Double fork

Roughly 4 years ago we learned about fork() function. Sometimes when we want the process to become a daemon, i.e. to run in background by detaching from a controlling terminal without a possibility to reattach, it’s useful to perform a technique known as double fork.

The steps are:

1. the parent forks to create the child and exits;
2. the child becomes a new session leader by calling setsid();
3. the child calls fork() again and exits.

Creating a new session during step 2 detaches child from parent’s TTY, but now as a session leader, the process could acquire TTY (accidentally or intentionally), which is unwanted for daemons. To prevent that, second fork (a grandchild) is created, which is not a session leader and thus cannot acquire a terminal.

So strictly speaking I forked twice and not double forked (maybe I will one day), but also strictly speaking I am not a UNIX environment.