When building a custom daemon, you must prevent multiple instances of your process from running at the same time. This is where PID files become crucial. A PID file is a simple text file that stores the process ID (PID) of your running daemon. The main purpose of a PID file is to let you—and your system—know which process is currently running as the daemon. If your daemon starts up and finds that a PID file already exists and is locked, it knows that another instance is active and should not continue.

Without this protection, multiple instances of the same daemon could start simultaneously. This could lead to race conditions, resource conflicts, data corruption, or unexpected behavior, especially if your daemon manages shared resources, listens on a network port, or writes to a common log or database. Ensuring that only one instance runs at a time is a fundamental part of robust daemon design.

Both the C and Python examples demonstrate how to enforce a single instance policy for your daemon using a PID file and file locking. When the daemon starts, it tries to open and lock the PID file. If the file cannot be locked because another process is holding the lock, the daemon knows another instance is running and exits immediately. If the lock is successful, the daemon writes its process ID to the file. The lock remains held for the lifetime of the process, preventing others from acquiring it. This approach ensures that only one daemon instance can run at a time, and the PID file always reflects the active process.