NULL
From cppreference.com
                    
                                        
                    
                    
                                                            
                    |   Defined in header  
<stddef.h>
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|   Defined in header  
<string.h>
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|   Defined in header  
<wchar.h>
  | 
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|   Defined in header  
<time.h>
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|   Defined in header  
<locale.h>
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|   Defined in header  
<stdio.h>
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|   #define NULL /*implementation-defined*/ 
 | 
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Expands into implementation-defined null-pointer constant.