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How to fix C error 'NULL undeclared'

Problem:

You have C code like

char* ptr = NULL;

but when you try to compile it you see an error message like

main.c: In function ‘main’:
main.c:3:17: error: ‘NULL’ undeclared (first use in this function)
     void* ptr = NULL;
                 ^~~~
main.c:3:17: note: each undeclared identifier is reported only once for each function it appears in

Solution

Add

#include <stddef.h>

at the top of the source file where the error occured.

The reason for this error is that NULL is not defined in C itself but only in stddef.h.