The psignal() function locates the descriptive message string for the given signal number sig and writes it to the standard error.
If the argument s is non-NULL it is written to the standard error file descriptor prior to the message string, immediately followed by a
colon and a space. If the signal number is not recognized (sigaction(2)), the string ``Unknown signal'' is produced.
The psiginfo() function produces the same output as the psignal() function, only it uses the signal number information from the si argument.
The message strings can be accessed directly using the external array sys_siglist, indexed by recognized signal numbers. The external array
sys_signame is used similarly and contains short, upper-case abbreviations for signals which are useful for recognizing signal names in user
input. The defined variable NSIG contains a count of the strings in sys_siglist and sys_signame.