File Descriptors and Standard Streams

Linux daemons should not use the terminal for input or output. Every process starts with three file descriptors: stdin (0), stdout (1), and stderr (2). For interactive programs they point to the terminal, but for daemons this is unsafe and unreliable.

To avoid issues, a daemon must redirect these streams, typically sending stdin to /dev/null and stdout and stderr to /dev/null or a log file. This prevents accidental terminal access and enables proper logging and error handling.

If you do not redirect the standard streams in a daemon, several problems can occur. Output sent to stdout or stderr may go to a terminal that no longer exists, causing errors or lost messages. In some cases, if the parent process closes its terminal, your daemon could receive a SIGHUP (hangup signal), potentially terminating it. Leaving stdin open may allow the daemon to block, waiting for input that will never come. By redirecting stdin to /dev/null, you ensure the daemon does not block on input. Redirecting stdout and stderr to a log file or /dev/null avoids polluting the terminal or losing important error messages. The C and Python code samples above show how to perform these redirections, ensuring your daemon behaves predictably and logs output in a controlled manner.