67 lines
No EOL
4.5 KiB
Markdown
67 lines
No EOL
4.5 KiB
Markdown
# go-daemon
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A Linux daemon runs continuously written in golang. Refer to [ahenk-docs/dev](https://git.aliberksandikci.com.tr/Liderahenk/ahenk-docs/src/branch/main/dev/resources.md#daemon-linux) for resources
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This repo will be include two separate daemons. One of them is written with C language for understanding native daemon processes [c-daemon](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/)
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<br>
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---
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<br>
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### Summary - [MakeUseOf](https://www.makeuseof.com/create-daemons-on-linux/)
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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.
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It is not mandatory but daemon processes are usually named to end with the letter **d**
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- Initial operations, such as reading configuration files or obtaining necessary system resources, must be performed before the process becomes a daemon
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- 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
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- A new session should open by calling the setsid function, and the process should be disconnected from the terminal
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- All open file descriptors inherited from the parent process are closed
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- Standard input, output, and error messages are redirected to `/dev/null`
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- The working directory of the process must change
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The operating system groups processes into session and process groups. Each session consists of process groups.
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Processes receive their inputs and send their outputs to **controlling terminal**. A controlling terminal is associated with only **one** session at a time
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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
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<br>
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#### Creating a Daemon Process
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<sub>c-daemon</sub>
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To create a demon process, we need a background process whose parent process is init. In the code [_daemon](https://git.aliberksandikci.com.tr/Liderahenk/go-daemon/src/commit/ca61ee4ac26ae3855d41581baba2b44dbabc0cd3/c-daemon/daemon.c#L10) 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!
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After compiling our both c files (https://git.aliberksandikci.com.tr/Liderahenk/go-daemon/src/commit/21df9b77fac68aeae40b09df7f9f5e0135f31e0d/c-daemon/daemon.c, https://git.aliberksandikci.com.tr/Liderahenk/go-daemon/src/commit/21df9b77fac68aeae40b09df7f9f5e0135f31e0d/c-daemon/test.c) with command `gcc -o test test.c daemon.c` run code with `./test` and do NOT enter any other key.
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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.
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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)
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| Abbreviation | Meaning |
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| ------------ | ------- |
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| `S` | Waiting asleep for an event to happen, `interruptible sleep (waiting for an event to complete)` |
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| `T` | `stopped, either by a job control signal or because it is being traced.` |
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| `s` | `is a session leader` |
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| `l` | `is multi-threaded (using CLONE_THREAD, like NPTL pthreads do)` |
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| `+` | `is in the foreground process group`|
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| `<` | `high-priority (not nice to other users)` |
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run `man ps` for more info.
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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).
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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.
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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)
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I'm too lazy to continue this readme(about c-daemon). just continue in [original website](https://www.makeuseof.com/create-daemons-on-linux/) or just review last versions of c files |