c - 访问和存储结构元素

Question

我是编码新手，在一个事件中遇到了这个片段，这个代码到底是做什么的？提前致谢 。

 #include <stdio.h>
 #define OFFSETOF(TYPE,ELEMENT) ((size_t)&(((TYPE *)50)->ELEMENT))
 typedef struct PodTag
 {
 char i;
 int  d;
 int c;
 } PodType;
 int main()
 {
  printf("%d\n", OFFSETOF(PodType,c) );
  printf("%d\n", OFFSETOF(PodType,d) );
  printf("%d\n", OFFSETOF(PodType,i) );
  printf("%d \n %d",(&((PodType*)0)->d),sizeof(((PodType*)0)->i));
  getchar();
  return 0;
 }

Answer 1

这段代码演示了结构是如何存储在 C 中的内存中的。

以这种方式编写代码会更好：

#include <stdio.h>

#define OFFSETOF(TYPE,ELEMENT) ((size_t)&(((TYPE *)100)->ELEMENT))

typedef struct PodTag {
    char c;
    int  i;
    int  j;
} PodType;

int main()
{
    printf("The size of a char is %d\n", sizeof(((PodType*)0)->c));
    printf("The size of an int is %d\n", sizeof(((PodType*)0)->i));
    printf("The size of a PodType is %d\n", sizeof(PodType));
    printf("The size of a pointer to a PodType is %d\n\n", sizeof(PodType*));

    printf("If there was a PodType at location 100, then:\n");
    printf("    the memory location of char c would be: %d\n",
        OFFSETOF(PodType,c));
    printf("    the memory location of int  i would be: %d\n",
        OFFSETOF(PodType,i));
    printf("    the memory location of int  j would be: %d\n",
        OFFSETOF(PodType,j));

    return 0;
}

这段代码的输出是：

The size of a char is 1
The size of an int is 4
The size of a PodType is 12
The size of a pointer to a PodType is 8

If there was a PodType at location 100, then:
    the memory location of char c would be: 100
    the memory location of int  i would be: 104
    the memory location of int  j would be: 108