Linux Network Programming, Part 2
Daemon processes are servers which run in the background servicing various clients. You should be aware of the following few issues when creating daemon processes. During development, it is always advisable to run the server in the foreground in order to use printf or write for debugging. Also, if the server happens to go berserk, it can be killed by sending the interrupt character (typically ctrl-c). While being deployed for use, the server should be coded to act like a daemon. Daemon programs in Unix typically end in the letter d, e.g., httpd for the HTTP daemon (web server).
It is always nice to have a daemon automatically put itself in the background when run. This is quite easy to achieve using the fork() call. A well-behaved daemon will close all the open file descriptors it inherited from its parent after the fork. This is especially important if the files are terminal devices, as they must be closed to allow the terminal state to reset when the user who starts the daemon logs out. Use the getrlimit() call to determine the maximum number of open file descriptors and to close them.
The process must then change its process group. Process groups are used in distributing signals—those processes with the same group as the terminal are in the foreground and are allowed to read from the terminal. Those in a different group are considered in the background (and will be blocked if they attempt to read).
Closing the controlling terminal and changing the session group prevents the daemon process from receiving implicit (i.e., not sent by the user with the kill command) signals from the previous group leader (commonly a shell).
Processes are organized within process groups and process groups within sessions. With the setsid() system call, a new session (and thus, a new process group) is then created with the process as the new session leader.
Once the daemon is without a controlling terminal, it must not re-acquire one. Controlling terminals are automatically acquired when a process group leader opens a terminal device. The easiest way to prevent this is to fork again after calling setsid(). The daemon runs in this second child. Since the parent (the session and process group leader) will terminate, the second child will obtain a new process group of zero (since it becomes a child of init). Therefore, it cannot acquire a new controlling terminal, since it is not a process leader. Many standard library routines may assume the three standard I/O descriptors are open. As a result, servers commonly open all three descriptors, connected to a harmless I/O device such as /dev/null.
Daemons are typically started at boot-up and remain running throughout the uptime life of the system. If a daemon was started from a mounted file system, it would be impossible to unmount the file system until the daemon was killed. As a result, it is a sensible daemon programming practice to perform a chdir() to / (or perhaps to a file system which holds files relevant to the operation of the daemon). Daemons inherit umask information from the process which created them. To prevent problems with file creation later within the daemon, it is commonly set to zero using umask(). Listing 1 illustrates these points with some sample code.
For systems that do not support sessions (e.g., some systems other than Linux and Solaris), you can achieve the same result as setsid() using the code from Listing 2.
When the forked children of the main server code exit, their memory is deallocated but their entries in the process table are not removed. In other words, the processes are dead, i.e., they do not consume system resources, but they have not been reaped yet. The reason they stay around in this zombie-like form is to allow the parent to gather statistics from the child if necessary (such as CPU usage, etc). Obviously, a daemon does not want to fill the process table with zombie processes.
When a child dies, it sends its parent a SIGCHLD signal. The default handler of this signal causes the child to turn into a zombie, unless it is explicitly reaped by its parent, as in Listing 3. Alternatively, as shown in Listing 4, you can ignore the signal and allow the zombie to die.
It is also quite common for daemons to ignore most other signals or to re-read any configuration files and restart after being sent SIGHUP. Many daemons save their PID (process identification) to a log file, typically called /var/run/foobar.pid (where foobar is the name of the daemon), which can aid in stopping the daemon.
When the system is being shut down (or changing from multi-user to single-user), the SIGTERM signal is sent to notify all processes. The init process then waits a specific amount of time (20 seconds for SVR4, 5 seconds for BSD, 5 seconds for Linux init, 3 seconds or a passed command-line argument Linux shutdown). If the process is still alive, a SIGKILL signal which cannot be ignored is sent to it. Therefore, daemon processes should catch the SIGTERM signal to ensure they shut down gracefully.
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