Daemon Example in C
13 days ago
- #systemd
- #daemon
- #C-programming
- Daemons are long-running background processes without a controlling terminal, commonly used for services like nginx, sshd, and cron.
- Creating a daemon in C can be done programmatically using methods like the 'double fork' technique or using systemd services.
- The 'double fork' method ensures the daemon cannot acquire a controlling terminal and runs as a true orphan process with the init process as its parent.
- Systemd provides multiple ways to create daemons, including Type=simple, Type=notify, and Type=forking, each with different management capabilities.
- The glibc `daemon()` function can create a BSD-style daemon but lacks the 'double fork' technique, making it less secure on systems with System V semantics.
- A manually created daemon in C involves steps like forking, calling `setsid()`, and forking again to ensure proper session and process group leadership.
- Daemons typically close standard input, output, and error files, change their working directory to root, and clear the file creation mode mask.
- Debugging daemons is challenging, so it's recommended to test functionality without running as a daemon first.
- The example provided includes C code for creating a daemon, handling flags for various options, and writing logs using syslog.
- Known issues include logging problems on OS X due to its use of `os_log` instead of traditional syslog.