NULL

From cppreference.com
< c | types
Defined in header <stddef.h>
Defined in header <string.h>
Defined in header <wchar.h>
Defined in header <time.h>
Defined in header <locale.h>
Defined in header <stdio.h>
#define NULL /*implementation-defined*/

Expands into implementation-defined null-pointer constant.

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