What is a Daemon?

A daemon (or service) is a background process that is designed to run autonomously,with little or not user intervention. The Apache web server http daemon (httpd) is one such example of a daemon. It waits in the background listening on specific ports, and serves up pages or processes scripts, based on the type of request.

Creating a daemon in Linux uses a specific set of rules in a given order. Knowing how they work will help you understand how daemons operate in user land Linux, but can operate with calls to the kernel also. In fact, a few daemons interface with kernel modules that work with hardware devices, such as external controller boards, printers,and PDAs. They are one of the fundamental building blocks in Linux that give it incredible flexibility and power.

Throughout this HOWTO, a very simple daemon will be built in C. As we go along, more code will be added, showing the proper order of execution required to get a daemon up and running.

Basic Daemon Structure

When a daemon starts up, it has to do some low-level housework to get itself ready for its real job. This involves a few steps:

  • Fork off the parent process
  • Change file mode mask (umask)
  • Open any logs for writing
  • Create a unique Session ID (SID)
  • Change the current working directory to a safe place
  • Close standard file descriptors
  • Enter actual daemon code