最小化总的结构内存 [英] Minimize total struct memory

问题描述

我有一个struct:

struct st
{
    short a;
    int *b;
    char ch;
};

short是2个字节
int*在x64中为8个字节
char是1个字节

short is 2 bytes
int* is 8 bytes in x64
char is 1 bytes

以上所有内容都应该给我11个字节.但是如果执行sizeof(st)，我将获得24个字节.为什么struct使用更多的内存以及如何将内存减少到11个字节?

All the above together should give me 11 bytes. But if I do sizeof(st) I get 24 bytes. Why the struct uses more memory and how to reduce the memory to 11 bytes?

推荐答案

pragma pack通常是使用的，但是它不如您希望的那样可移植.这是上面的文档:

pragma pack is usually what is used, but its not as portable as you'd like. Here's the docs on it:

• Microsoft的产品包
• GCC的结构打包实用程序

• Microsoft's pack
• GCC's Structure-Packing Pragmas

均提供#pragma pack(n)，push和pop.

在没有包装的情况下，请尝试重新排序struct:

In the absence of the packing, try reordering the struct:

struct st
{
    int *b;
    short a;
    char ch;
};

如果打包，访问数据时必须格外小心.您可能必须memmov(或memcpy)，以确保在所有平台上的可移植性.如果不这样做，则可能会在Windows上遇到EXCEPTION_DATATYPE_MISALIGNMENT或在Linux上遇到SIGBUS错误.

You have to be careful about accessing data if its packed. You will probably have to memmov (or memcpy) it out to ensure portability across all platforms. If you don't, then you could encounter a EXCEPTION_DATATYPE_MISALIGNMENT on Windows or a SIGBUS error on Linux.

Microsoft在 IPF，x86和x64上的Windows数据对齐.

Microsoft has a good writeup on it at Windows Data Alignment on IPF, x86, and x64.

-Wstrict-aliasing和-Wcast-align将帮助您找到痛处.

-Wstrict-aliasing and -Wcast-align will help you find the sore spots.

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