具有char，double，int和t的结构的大小 [英] Size of structure with a char, a double, an int and a t

问题描述

当我只运行代码片段

  int * t; 
 std :: cout<< sizeof（char）< std :: endl; 
 std :: cout<< sizeof（double）<< std :: endl; 
 std :: cout<< sizeof（int）< std :: endl; 
 std :: cout<< sizeof（t）< std :: endl; 
  

它给我这样的结果：

  1 
 8 
 4 
 4 
  

b $ b

总计：17。

但是当我测试包含这些数据类型的sizeof结构时，它给我24，我很困惑。

  
 
 
 
 
 $ b < #include< iostream> 
 #include< stdio.h> 
 struct struct_type {
 int i; 
 char ch; 
 int * p; 
 double d; 
} s; 
 
 int main（）{
 int * t; 
 // std :: cout<< sizeof（char）<< std :: endl; 
 // std :: cout<< sizeof（double）<< std :: endl; 
 // std :: cout<< sizeof（int）<< std :: endl; 
 // std :: cout<< sizeof（t）<< std :: end1; 
 
 printf（s_type is％d byes long，sizeof（struct struct_type））; 
 
 return 0; 
} 
  

：EDIT

我已经更新了这样的代码

  #include< iostream> 
 #include< stdio.h> 
 struct struct_type {
 double d_attribute; 
 int i__attribute __（int（packed））; 
 int * p__attribute_（int（packed））;; 
 char ch; 
} s; 
 
 int main（）{
 int * t; 
 // std :: cout<< sizeof（char）<< std :: endl; 
 // std :: cout<< sizeof（double）<< std :: endl; 
 // std :: cout<< sizeof（int）<< std :: endl; 
 // std :: cout<< sizeof（t）<< std :: endl; 
 
 printf（s_type is％d bytes long，sizeof（s））; 
 
 return 0; 
} 
  

现在它显示我16个字节。

http://en.wikipedia.org/wiki/Data_structure_alignment\">调整 alignments 正确。例如，为了提高效率，默认情况下，指针位于4字节边界上，即其地址必须是4的倍数。如果结构体只包含一个char和一个指针

  struct {
 char a; 
 void * b; 
}; 
  

那么 b 不能使用加法器＃1 - 它必须放置在＃4。

  0 1 2 3 4 5 6 7 
 + --- + -   -   -   -   -   -  + --------------- + 
 | a | （未使用）| b | 
 + --- +  -   -   -   -   -   -  + --------------- + 
  

在这种情况下，由于 int * 的对齐，额外的7个字节来自3个字节， double 。

  0 1 2 3 4 5 6 7 8 9 abcdef 
 + --------------- + --- +  -   -   -   -   -   -  + --------------- +  -   -  -   -   -   -   -   -  + 
 | i | ch | | p | | 
 + --------------- + --- +  -   -   -   -   -   -  + --------------- +  -   -  -   -   -   -   -   -  + 
 10 11 12 13 14 15 16 17 
 + -------------------------- ----- + 
 | d | 
 + ------------------------------- + 
  

When I run only the code fragment

int *t;
std::cout << sizeof(char)   << std::endl;
std::cout << sizeof(double) << std::endl;
std::cout << sizeof(int)    << std::endl;
std::cout << sizeof(t)      << std::endl;

it gives me a result like this:

1
8
4
4

Total: 17.

But when I test sizeof struct which contains these data types it gives me 24, and I am confused. What are the additional 7 bytes?

This is the code

#include <iostream>
#include <stdio.h>
struct struct_type{
    int i;
    char ch;
    int *p;
    double d;
} s;

int main(){
    int *t;
    //std::cout << sizeof(char)   <<std::endl;
    //std::cout << sizeof(double) <<std::endl;
    //std::cout << sizeof(int)    <<std::endl;
    //std::cout << sizeof(t)      <<std::endl;

    printf("s_type is %d byes long",sizeof(struct struct_type));

    return 0;
}

:EDIT

I have updated my code like this

#include <iostream>
#include <stdio.h>
struct struct_type{
    double d_attribute;
    int i__attribute__(int(packed));
    int * p__attribute_(int(packed));;
    char  ch;
} s;

int main(){
    int *t;
    //std::cout<<sizeof(char)<<std::endl;
    //std::cout<<sizeof(double)<<std::endl;
    //std::cout<<sizeof(int)<<std::endl;
    //std::cout<<sizeof(t)<<std::endl;

    printf("s_type is %d bytes long",sizeof(s));

    return 0;
}

and now it shows me 16 bytes. Is it good, or have I lost some important bytes?

解决方案

There is some unused bytes between some members to keep the alignments correct. For example, a pointer by default reside on 4-byte boundaries for efficiency, i.e. its address must be a multiple of 4. If the struct contains only a char and a pointer

struct {
  char a;
  void* b;
};

then b cannot use the adderss #1 — it must be placed at #4.

  0   1   2   3   4   5   6   7
+---+- - - - - -+---------------+
| a | (unused)  | b             |
+---+- - - - - -+---------------+

In your case, the extra 7 bytes comes from 3 bytes due to alignment of int*, and 4 bytes due to alignment of double.

  0   1   2   3   4   5   6   7   8   9   a   b   c   d   e   f
+---------------+---+- - - - - -+---------------+- - - - - - - -+
| i             |ch |           | p             |               |
+---------------+---+- - - - - -+---------------+- - - - - - - -+
 10  11  12  13  14  15  16  17
+-------------------------------+
| d                             |
+-------------------------------+

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