v1.0, May 2004
This document shows how to write a daemon in Linux using GCC. Knowledge of Linux and a familiarity with C are necessary to use this document. This HOWTO is Copyright by Devin Watson, under the terms of the BSD License.
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 userland 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.
First off, you'll need the following packages installed on your Linux machine to develop daemons, specifically:
GCC 3.2.2 or higher
Linux Development headers and libraries
If your system does not already have these installed (not likely, but check anyway), you'll need them to develop the examples in this HOWTO. To find out what version of GCC you have installed, use:
A daemon should do one thing, and do it well. That one thing may be as complex as managing hundreds of mailboxes on multiple domains, or as simple as writing a report and calling sendmail to mail it out to an admin.
In any case, you should have a good plan going in what the daemon should do. If it is going to interoperate with some other daemons that you may or may not be writing, this is something else to consider as well.
Daemons should never have direct communication with a user through a terminal. In fact, a daemon shouldn't communicate directly with a user at all. All communication should pass through some sort of interface (which you may or may not have to write), which can be as complex as a GTK+ GUI, or as simple as a signal set.
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:
A daemon is started either by the system itself or a user in a terminal or script. When it does start, the process is just like any other executable on the system. To make it truly autonomous, a child process must be created where the actual code is executed. This is known as forking, and it uses the fork() function:
Notice the error check right after the call to fork(). When writing a daemon, you will have to code as defensively as possible. In fact, a good percentage of the total code in a daemon consists of nothing but error checking.
The fork() function returns either the process id (PID) of the child process (not equal to zero), or -1 on failure. If the process cannot fork a child, then the daemon should terminate right here.
If the PID returned from fork() did succeed, the parent process must exit gracefully. This may seem strange to anyone who hasn't seen it, but by forking, the child process continues the execution from here on out in the code.
In order to write to any files (including logs) created by the daemon, the file mode mask (umask) must be changed to ensure that they can be written to or read from properly. This is similar to running umask from the command line, but we do it programmatically here. We can use the umask() function to accomplish this:
By setting the umask to 0, we will have full access to the files generated by the daemon. Even if you aren't planning on using any files, it is a good idea to set the umask here anyway, just in case you will be accessing files on the filesystem.
This part is optional, but it is recommended that you open a log file somewhere in the system for writing. This may be the only place you can look for debug information about your daemon.
From here, the child process must get a unique SID from the kernel in order to operate. Otherwise, the child process becomes an orphan in the system. The pid_t type, declared in the previous section, is also used to create a new SID for the child process:
Again, the setsid() function has the same return type as fork(). We can apply the same error-checking routine here to see if the function created the SID for the child process.
The current working directory should be changed to some place that is guaranteed to always be there. Since many Linux distributions do not completely follow the Linux Filesystem Hierarchy standard, the only directory that is guaranteed to be there is the root (/). We can do this using the chdir() function:
Once again, you can see the defensive coding taking place. The chdir() function returns -1 on failure, so be sure to check for that after changing to the root directory within the daemon.
One of the last steps in setting up a daemon is closing out the standard file descriptors (STDIN, STDOUT, STDERR). Since a daemon cannot use the terminal, these file descriptors are redundant and a potential security hazard.
The close() function can handle this for us:
It's a good idea to stick with the constants defined for the file descriptors, for the greatest portability between system versions.
At this point, you have basically told Linux that you're a daemon, so now it's time to write the actual daemon code. Initialization is the first step here. Since there can be a multitude of different functions that can be called here to set up your daemon's task, I won't go too deep into here.
The big point here is that, when initializing anything in a daemon, the same defensive coding guidelines apply here. Be as verbose as possible when writing either to the syslog or your own logs. Debugging a daemon can be quite difficult when there isn't enough information available as to the status of the daemon.
A daemon's main code is typically inside of an infinite loop. Technically, it isn't an infinite loop, but it is structured as one:
This typical loop is usually a while loop that has an infinite terminating condition, with a call to sleep in there to make it run at specified intervals.
Think of it like a heartbeat: when your heart beats, it performs a few tasks, then waits until the next beat takes place. Many daemons follow this same methodology.
Listed below is a complete sample daemon that shows all of the steps necessary for setup and execution. To run this, simply compile using gcc, and start execution from the command line. To terminate, use the kill command after finding its PID.
I've also put in the correct include statements for interfacing with the syslog, which is recommended at the very least for sending start/stop/pause/die log statements, in addition to using your own logs with the fopen()/fwrite()/fclose() function calls.
From here, you can use this skeleton to write your own daemons. Be sure to add in your own logging (or use the syslog facility), and code defensively, code defensively, code defensively!