4.30.1 Introduction to PID files

A lock PID file is a textual file in ASCII encoding created at process start time and deleted at process exit time; it contains a string representing the PID number of the current process followed by a newline character.

The lock PID file is especially useful whenever a server is executed as daemon:

• It allows the execution of a single instance of the daemon, causing instances launched later to recognise themselves as such and so exit.
• It allows for quick retrieval of the PID number, which can be used to send interprocess signals to the daemon; for example: to shut it down, to restart it, to update its configuration.

The lock PID file must be created after a server daemonisation process has completed: this way we correctly store the PID of the daemon process, rather than the PID of the starting process.

The lock PID file must be created after a process has changed its user and group IDs to drop special privileges: this way the file is created with appropriate owner and group.

All the operations performed by this library are logged; logging operations related to the lock PID file rely on a function handed as argument.