Prototypes for these functions are only available if _BSD_SOURCE is defined before the inclusion of any system header file.
It is not possible to block SIGKILL or SIGSTOP - this restriction is silently imposed by the system.
4.4BSD. These function calls appeared in BSD 4.3 and are deprecated. Use the POSIX signal facilities for new programs.
kill(2), sigprocmask(2), signal(7)