为什么'sizeof'给出错误的测量？ [英] Why does 'sizeof' give wrong measurement?

问题描述

可能重复：

struct sizeof结果不期望

我有这个C ++结构：

I have this C++ struct:

struct bmp_header {

  //bitmap file header (14 bytes)
  char Sign1,Sign2; //2
  unsigned int File_Size; //4
  unsigned int Reserved_Dword; //4
  unsigned int Data_Offset; //4

  //bitmap info header (16 bytes)
  unsigned int Dib_Info_Size; //4
  unsigned int Image_Width; //4
  unsigned int Image_Height; //4

  unsigned short Planes; //2
  unsigned short Bits; //2  
};

它应该是30个字节，但是'sizeof（bmp_header）'给了我32。错误？

It is supposed to be 30 bytes, but 'sizeof(bmp_header)' gives me value 32. What's wrong?

推荐答案

原因是因为填充。如果你把 char 放在结构的末尾， sizeof 可能会给你30个字节。整数通常存储在为4的倍数的存储器地址上。因此，由于字符占用2个字节，所以它与第一个unsigned int之间有两个未使用的字节。与 int 不同， char 通常不填充。

The reason is because of padding. If you put the chars at the end of the struct, sizeof will probably give you 30 bytes. Integers are generally stored on memory addresses that are multiples of 4. Therefore, since the chars take up 2 bytes, there are two unused bytes between it and the first unsigned int. char, unlike int, is not usually padded.

一般来说，如果空间是一个很大的问题，请务必将 structs 的元素从大到最小排序。

In general, if space is a big concern, always order elements of structs from largest in size to smallest.

注意，padding不总是（或通常） sizeof（element）。这是巧合， int 在4个字节对齐， char 在1字节。

Note that padding is NOT always (or usually) the sizeof(element). It is a coincidence that int is aligned on 4 bytes and char is aligned on 1 byte.

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