SIG_ERR
From cppreference.com
                    
                                        
                    
                    
                                                            
                    | Defined in header  <csignal> | ||
| #define SIG_ERR /* implementation defined */ | ||
A value of type void (*)(int). When returned by std::signal, indicates that an error has occurred.
See also
| sets a signal handler for particular signal (function) | |
| C documentation for SIG_ERR | |