Thanks a lot for your answer. I cannot understand anything from the phrase
"POSIX.1-2004 (also known as POSIX.1-2001 Technical Corrigendum 2) requires an implementation to guarantee that the following functions can be safely called inside a signal handler:
...
list of functions
...
"
POSIZ.1-2004 requires an implementation? What does that mean? After I implement POSIX.1-2004 then I can safely call only the following functions that almost none of them have to do with writing to files?

Also, is there any way to exactly simulate shutdown for my process only so as not to shutdown my system all the time for testing purposes?

EDIT: Also, how come and the signal handler works each time if I send the SIGTERM? What different does the system do? I thought it sends SIGTERM through kill and waits a small timeout for the processes to end.