#pragma pack笔记

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本文详细解释了C/C++中#pragma pack指令的作用与用法。该指令用于指导编译器如何对结构体成员进行内存对齐及填充，避免不必要的内存浪费，并确保与硬件设备交互时数据的正确对齐。

申明：内容来自stackoverflow，仅为方便自己以后忘记了再看时方便而转，https://www.google.com.hk/url?sa=t&rct=j&q=&esrc=s&source=web&cd=7&ved=0CEUQFjAG&url=%68%74%74%70%3a%2f%2f%73%74%61%63%6b%6f%76%65%72%66%6c%6f%77%2e%63%6f%6d%2f%71%75%65%73%74%69%6f%6e%73%2f%33%33%31%38%34%31%30%2f%70%72%61%67%6d%61%2d%70%61%63%6b%2d%65%66%66%65%63%74&ei=jxC_VNfQNYjh8AWHhIKQBg&usg=AFQjCNECxkwOc-hCiFvuimFS7HyDEadhCw&bvm=bv.83829542,d.dGc&cad=rjt

#pragma pack instructs the compiler to pack structure members with particular alignment. Most compilers, when you declare a struct, will insert padding between members to ensure that they are aligned to appropriate addresses in memory (usually a multiple of the type's size). This avoids the performance penalty (or outright error) on some architectures associated with accessing variables that are not aligned properly. For example, given 4-byte integers and the following struct:

struct Test
{
   char AAA;
   int BBB;
   char CCC;
};

The compiler could choose to lay the struct out in memory like this:

 Bytes: |  1  |  2  3  4  |  5  6  7  8  |  9  |  10 11 12  |
Member: | AAA |  padding  |  BBB         | CCC |  padding   |

and sizeof(Test) would be 12, even though it only contains 6 bytes of data. The most common use case for the #pragma (to my knowledge) is when working with hardware devices where you need to ensure that the compiler does not insert padding into the data and each member follows the previous one. With #pragma pack(1), the struct above would be laid out like this:

 Bytes: |  1  |  2  3  4  5  |  6  |
Member: | AAA |     BBB      | CCC |

And sizeof(Test) would be 6.

With #pragma pack(2), the struct above would be laid out like this:

 Bytes: |  1  |    2    |  3  4  5  6  |  7  |    8    |
Member: | AAA | padding |     BBB      | CCC | padding |

And sizeof(Test) would be 8.