SIG_DFL, SIG_IGN
From cppreference.com
                    
                                        
                    
                    
                                                            
                    |   Defined in header  
<signal.h>
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|   #define SIG_DFL /*implementation defined*/ 
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|   #define SIG_IGN /*implementation defined*/ 
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The SIG_DFL and SIG_IGN macros expand into integral expressions that are not equal to an address of any function. The macros define signal handling strategies for signal() function.
| Constant | Explanation | 
  SIG_DFL
 | 
default signal handling | 
  SIF_IGN
 | 
signal is ignored |