The strsignal() function shall return a pointer to a string describing the signal number sig. The string can only be used until the next call to strsignal().
If sig is a valid signal number, strsignal() shall return a pointer to the appropriate description string. If sig is not a valid signal number, strsignal() shall return a pointer to an error string. The contents of either type of string are unspecified.
Although the function is not declared as returning a pointer to a constant character string, applications shall not modify the returned string.