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asandikci 7c9d75c5b0
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go-daemon

A Linux daemon runs continuously written in golang. Refer to ahenk-docs/dev for resources

This repo will be include two separate daemons. One of them is written with C language for understanding native daemon processes c-daemon . Other one is written in go for implement same deamon concepts to go language and using with native performance go-daemon


Go-Daemon

Dependencies git, go

Debian/Pardus

sudo apt install git

Install latest version of Go ! // TODO // 1. get tar.gz from original website // 2. extract tar.gz // 3. move go to /usr/local/go // 4. add to path // 5. check version

Build / Install with:

wget -qO- https://git.aliberksandikci.com.tr/Liderahenk/go-daemon/raw/branch/main/go-daemon/install.sh | bash

C-Daemon

Dependencies git, go, gcc

Debian/Pardus

sudo apt install git golang-go gcc

Build / Install with:

wget -qO- https://git.aliberksandikci.com.tr/Liderahenk/go-daemon/raw/branch/main/c-daemon/tmp-install.sh | bash


Notes

Summary - MakeUseOf

Usually daemons start on system startup and run continuously until the system shuts down. They do NOT send messages to the console or screen in any way.

It is not mandatory but daemon processes are usually named to end with the letter d

  • Initial operations, such as reading configuration files or obtaining necessary system resources, must be performed before the process becomes a daemon

  • A background running process is created with init as its parent process. For this purpose, a sub-process is forked from the init process first, and then the upper process is terminated with exit

  • A new session should open by calling the setsid function, and the process should be disconnected from the terminal

  • All open file descriptors inherited from the parent process are closed

  • Standard input, output, and error messages are redirected to /dev/null

  • The working directory of the process must change

The operating system groups processes into session and process groups. Each session consists of process groups.

Processes receive their inputs and send their outputs to controlling terminal. A controlling terminal is associated with only one session at a time

A session and process groups in it have identification (ID) numbers; these identification numbers are the process identification numbers (PID) of the session and process group leaders. A child process shares the same group as its parent process


Creating a Daemon Process

c-daemon

To create a demon process, we need a background process whose parent process is init. In the code _daemon creates a child process and then kils the parent process. In this case, our new process will be a subprocess of init and will continue to run in background!

After compiling our both c files (21df9b77fa/c-daemon/daemon.c, 21df9b77fa/c-daemon/test.c) with command gcc -o test test.c daemon.c run code with ./test and do NOT enter any other key.

Examine the status before _daemon starts with ps -C test -o "pid ppid pgid sid tty stat command" command and you will see PID, Parent PID, STAT and other useful info.

STAT field, you see that your process is running but waiting for an off-schedule event to occur which will cause it to run in the foreground (we don't want this in daemons)

Abbreviation Meaning
S Waiting asleep for an event to happen, interruptible sleep (waiting for an event to complete)
T stopped, either by a job control signal or because it is being traced.
s is a session leader
l is multi-threaded (using CLONE_THREAD, like NPTL pthreads do)
+ is in the foreground process group
< high-priority (not nice to other users)

run man ps for more info.

You will see that our process is now a member of the foreground process group S+. and parent process is probably a shell (bash,zsh,fish etc.) See parent process with command ps -jp PPID (replace PPID with parent process id you get above).

Now return to the terminal where you are running your application (./test) and press Enter to invoke the _daemon function. Then look at the process information on the other terminal again.

First of all, you can say that the new subprocess is running in the background since you do not see the + character in the STAT field. You can now see that the parent process of your process is the systemd process (or other init in use)

I'm too lazy to continue this readme(about c-daemon). just continue in original website or just review last versions of c files