null pointer trick in C

Description

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The null pointer trick is a classic C programming technique that allows you to access a struct member’s offset or size without creating an actual instance of the struct.

Example

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Let’s say you have a struct like this:

struct Name {
  int member;
};

And you want to get the size of the member field without creating an instance

sizeof(((struct Name *)0)->member)

How it works:

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1. (struct Name *)0 casts the integer value 0 to a pointer to your struct
2. The ->member accesses a field of this hypothetical struct “located” at memory address 0
3. sizeof() evaluates the size of the member’s type at compile time

Alternative:

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In modern C, you can use the offsetof macro from stddef.h to achieve the same:

#include <stddef.h>
offsetof(struct Name, member)