Custom implementation offsetof

What is? offsetof？

offsetof, Programming language , This macro is used to find the offset of a member in the structure .

The header file ：stddef.h

Macro form ：

 size_t offsetof( structName, memberName )

explain ：
1. The first parameter is the name of the structure , The second parameter is the name of the structure member .

2. The macro returns the structure structName Member of the memberName The offset . The offset is size_t Type of .

The code implementation is as follows ：

typedef struct S
{
	char a;
	char c;
	int b;
}S;
#define OFFSETOF(type,name) ((int)&(((type*)0)->name))
int main()
{
	printf("%d\n", OFFSETOF(S, a));
	printf("%d\n", OFFSETOF(S, c));
	printf("%d\n", OFFSETOF(S, b));
	return 0;
}