Question

问题

我的同学和我无法计算内存并找到最后一个问题的起始地址（见下文）。我们不确定如何计算联合中的字节来查找用户的起始地址[20] .userinfo.donor.amount [1]。
根据我们的教授的说法，答案是8009.这是正确的吗？

Assume that a char = 1 byte, int = 4 bytes, double = 8 bytes, and pointers are 4 bytes.

struct address {
   char street[100];
   char city[20];
   char state[2];
   char zip[10];
};
struct date {
   int month;
   int day;
   int year;
};
struct user {
   char login[20];
   char fullname[100];
   char password[30];
   struct address physical_address;
   struct date birthday;
   int user_type;
   union {
      struct {
         double salary;
     char *clearance;
      } admin;
      struct {
         date donationdate[2];
         double amount[2];
      } donor;
      struct {
         double wage;
     date datehired;
      } worker;
   } userinfo;
};

struct users[200];

If the users begins at a memory address 1000, what are the starting addresses (in bytes) of each of the following:
a) users[10]
b) users[15].physical_address.street
c) users[20].birthday.year
d) users[20].userinfo.donor.amount[1]

Solutions:
a) 4380
b) 6220
c) 8050
d) 8009

Answer 1

通过下面的测试程序确认@paddy和@jxh评论 8090 是正确的答案。

struct user *users = (struct user *) 1000;
// used __attribute__((packed)) on each structure
int main() {
  printf("size %zu %zu %zu %zu\n", sizeof(char), sizeof(int), sizeof(double), sizeof(void *));
  printf("%zu\n", (size_t)&users[10]                         );
  printf("%zu\n", (size_t)&users[15].physical_address.street );
  printf("%zu\n", (size_t)&users[20].birthday.year           );
  printf("%zu\n", (size_t)&users[20].userinfo.donor.amount[1]);
  return 0;
}
//size 1 4 8 4
//4380
//6220
//8050
//8090

使用C中的联合/结构来转发内存中的起始地址

1 个答案: