C language structure

Preface

Structure alignment is divided into ： Structure member variables Alignment and Structural variable Alignment of

---

One 、 Structure member variable alignment

The alignment value of the structure member variable is
MemberAlign = Min(Zp, sizeof(MemberType))
among Zp Alignment values specified for compilation options .MemberType Is the type of the member , If the member variable is an array ,MemberType That's the type of the array .

Structure member variables Distance from the first address of the structure The migration MemberOffset Is the member's MemberAlign Integer multiple . namely MemberOffset % MemberAlign == 0

Two 、 Alignment of structural variables

The alignment value of the structure variable is
StructAlign = Min(Zp, Max(sizeof(MemberType1),...))

Structure size StructSize yes StructAlign Integer multiple . namely StructSize % StructAlign == 0

3、 ... and 、 Structure addressing mode

Structural members Address = The first address of the structure variable + The offset of the structure member variable MemberOffset

struct s;
s.member = *(MemberType*)((int)(&s) + MemberOffset);

struct* ps;
ps->member = *(MemberType*)((int)ps + MemberOffset);
//  If ps by NULL, You can calculate the offset of the member according to this formula , It's the following offsetof macro 

offsetof According to the passed structure and member variables , Calculate the offset of the structure member variable

#include <stddef.h>
#define offsetof(s,m) ((size_t)&(((s*)0)->m))

---