Example of Linux Daemon

This repository contains simple example of daemon for Linux OS. This repository also contains examples of starting scripts.

When you want to create super simple daemon, then it is very easy. You can write something like this in C and call it daemon.c:

/* Compile this with gcc -o daemon daemon.c */

#include <unistd.h>

int main(void)
{
    while(1) {
        /* TODO: do something usefull here ;-) */
        sleep(1);
    }
}

and write some super simple systemd service file called simple-daemon.service:

[Unit]
Description=Super simple daemon

[Service]
Type=simple
ExecStart=/usr/bin/daemon

[Install]
WantedBy=multi-user.target

and then you can run it as UNIX daemon, but such daemon do not have some nice features like reloadin configure files, loging, etc. This repository and sources can help you to understand how UNIX daemons works.

Requirements

To build example of the daemon you have to have following tools

CMake
GCC/CLang

Build

To build example of daemon you have to type following commands:

git clone https://github.com/jirihnidek/daemon.git
cd daemon
mkdir build
cd build
cmake -DCMAKE_INSTALL_PREFIX=/usr ../
make
sudo make install

Usage

You can test running daemon from command line:

./bin/daemon

But running the app in this way is not running running daemon. Let have a look at command line parameters and arguments

Usage: ./bin/daemon [OPTIONS]

 Options:
  -h --help                 Print this help
  -c --conf_file filename   Read configuration from the file
  -t --test_conf filename   Test configuration file
  -l --log_file  filename   Write logs to the file
  -d --daemon               Daemonize this application
  -p --pid_file  filename   PID file used by daemonized app

When you will run ./bin/daemon with parameter --daemon or -d, then it will become real UNIX daemon. But this is not the way, how UNIX daemons are started nowdays. Some init scripts or service files are used for this purpose.

When you use Linux distribution using systemd, then you can try start daemon using

systemctl start simple-daemon
systemctl status simple-daemon
systemctl reload simple-daemon
systemctl stop simple-daemon

Note: The unit files simple-daemon.service and forking-daemon.service are copied to the directory /usr/lib/systemd/system during installation using make install command.

When you use RedHat 4/5/6 or CentOS, then you can try to use init script:

cp daemon.init /etc/rc.d/init.d/daemond

Then it should be possible to control daemon using:

service daemon start
service daemon status
service daemon reload
service daemon stop