uninterruptible process

Hi all, I'm looking for system call that can make my process uninterrupted until it completes successfully. My app receives a SIGTERM signal (when an shutdown is triggerd), then needs to write into a file with a simple fopen(). Actually it can't write in my file because I think it is automatically killed (SIGKILL) by systemd. Do you have any advice on this ? Thanks -- Thanks, Axel M.