How does a program save data in memory to the disk before shutdown?

How does a program save data in memory to the disk before shutdown? -- Linux general technology-Linux programming and kernel information. The following is a detailed description. Assume that you write a background service program, which runs automatically after startup and always runs in the background.
If other users use other commands (such as the root user input the reboot command) or other programs to shut down the computer, how can our programs start to save their data to the disk? Can the system shut down after we save the data?

First, let's do an experiment. Check your/etc/inittab to see this:

CODE: # The default runlevel. Id: 2: initdefault: