C language programming | explanation and Simulation of offsetof macro

offsetof Macro explanation

First, let's take a look at cpluscplus about offsetof The definition given by macro

macro Represents macro

There are two parameters

• type It stands for type
• member Members representing

The function of this macro is to calculate the offset of the structure member relative to the first address .

The methods used are as follows , Code instance ：

struct S
{
    
	char c1;
	int i;
	char c2;
};

#include <stdio.h>
#include <stddef.h>
int main()
{
    
	struct S s = {
    0};
	printf("%d\n", offsetof(struct S, c1));
	printf("%d\n", offsetof(struct S, i));
	printf("%d\n", offsetof(struct S, c2));

	return 0;
}

Print the results ：

Explain ：

Simulation Implementation offsetof macro

In the explanation , We can see that , The offset is the number of the address minus the first address , Then let's assume the address is 0, We just need to take out the address , This address is the offset .

Code instance ：

#include <stddef.h>
#include <stdio.h>
struct S
{
    
	char c1;
	int i;
	char c2;
};

#define OFFSETOF(type, m_name) (size_t)&(((type*)0)->m_name)

int main()
{
    
	struct S s = {
    0};
	printf("%d\n", OFFSETOF(struct S, c1));
	printf("%d\n", OFFSETOF(struct S, i));
	printf("%d\n", OFFSETOF(struct S, c2));

	return 0;
}

Explain ：

First we set the address to 0, Then there is ：

#define OFFSETOF(type,m_name) ((type*)0)

Here, the address is set to 0, The type is type* Type of , Notice the type It's just a replacement , Does not represent the actual type

After taking out the address , Let's find the members of the structure , Then there is ：

#define OFFSETOF(type,m_name) (((type*)0)->m_name)

After finding the members of the structure here , Let's take out the address of this member , Then there is ：

#define OFFSETOF(type,m_name) &(((type*)0)->m_name)

After taking out the address , Let's cast the type .

#define OFFSETOF(type,m_name) (size_t)(&(((type*)0)->m_name))

Print the results ：