C++结构对齐问题

问题描述

我有一个预定义的结构(实际上有几个)，其中变量跨越 32 位字边界.在 Linux(和使用 GCC 的 Windows)中，我能够使用attribute((packed))"将结构打包到正确的大小.但是，我无法使用 VC++ 和 #pragma pack 使其以相同的方式工作.

I have a predefined struct (actually several) where variables span across 32-bit word boundary. In Linux (and Windows using GCC) I am able to get my structs to pack to the correct size using 'attribute((packed))'. However I cannot get it to work the same way using VC++ and #pragma pack.

使用 GCC 这会返回 6 个字节的正确大小:

Using GCC this returns a correct size of 6 bytes:

struct
{
    unsigned int   a                : 3;
    unsigned int   b                : 1;
    unsigned int   c                : 15;
    unsigned int   troubleMaker     : 16;
    unsigned short padding          : 13;
} __attribute__((packed)) s;

使用 VC++ 会返回错误的 8 字节大小

Using VC++ this returns an incorrect size of 8 bytes

#pragma pack(push)
#pragma pack(1)

struct
{
    unsigned int   a                : 3;
    unsigned int   b                : 1;
    unsigned int   c                : 15;
    unsigned int   troubleMaker     : 16;
    unsigned short padding          : 13;
} s;

#pragma pack(pop)

我可以通过手动跨边界拆分troubleMaker"来使事情发挥作用，但我不想这样做.有什么想法吗?

I can get things to work by splitting 'troubleMaker' across the boundary manually but I'd prefer not to. Any ideas?

推荐答案

疯狂的想法:首先只写一个符合 C99 或 C++03 的程序

我建议不要使用供应商特定的 C 语言扩展来匹配设备或网络位格式.即使您使用一系列每个供应商的语言扩展来排列字段，您仍然需要担心字节顺序，并且您仍然有一个需要额外指令才能访问的结构布局.

Crazy idea: just write a C99 or C++03 -conforming program in the first place

I would suggest not using vendor-specific C language extensions to match device or network bit formats. Even if you get the fields to line up using a series of one-per-vendor language extensions, you still have byte order to worry about, and you still have a struct layout that requires extra instructions to access.

通过使用标准化的 C API 字符串和内存复制函数以及 Posix hton 和 ntoh 函数，您可以编写符合 C99 的程序，该程序可以在任何架构或主机上运行，​​并以最大速度和缓存效率运行.

You can write a C99 conforming program that will work on any architecture or host and at maximum speed and cache efficiency by using the standardized C API string and memory copy functions and the Posix hton and ntoh functions.

一个好的做法是使用以下已发布标准的函数:

A good practice is to use the following functions for which there exist published standards:

C99: memcpy(), Posix: htonl(), htons(), ntohl(), ntohs()

更新:这里有一些代码应该在任何地方都一样.您可能需要从该项目中获取 如果微软仍然没有为 C99 实现它，或者只是对 int 大小做出通常的假设.

Update: here is some code that should work the same everywhere. You may need to get <stdint.h> from this project if Microsoft still hasn't implemented it for C99, or just make the usual assumptions about int sizes.

#include <stdlib.h>
#include <stdint.h>
#include <string.h>
#include <stdio.h>
#include <arpa/inet.h>

struct packed_with_bit_fields {  // ONLY FOR COMPARISON
    unsigned int   a        : 3;
    unsigned int   b        : 1;
    unsigned int   c        : 15;
    unsigned int   troubleMaker : 16;
    unsigned short padding  : 13;
} __attribute__((packed));       // USED ONLY TO COMPARE IMPLEMENTATIONS

struct unpacked { // THIS IS THE EXAMPLE STRUCT
    uint32_t a;
    uint32_t b;
    uint32_t c;
    uint32_t troubleMaker;
}; // NOTE NOT PACKED

struct unpacked su;
struct packed_with_bit_fields sp;
char *bits = "Lorem ipsum dolor";

int main(int ac, char **av) {
  uint32_t x;   // byte order issues ignored in both cases

  // This should work with any environment and compiler
  memcpy(&x, bits, 4);
  su.a = x & 7;
  su.b = x >> 3 & 1;
  su.c = x >> 4 & 0x7fff;
  memcpy(&x, bits + 2, 4);
  su.troubleMaker = x >> 3 & 0xffff;

  // This section works only with gcc
  memcpy(&sp, bits, 6);
  printf( sp.a == su.a
      &&  sp.b == su.b
      &&  sp.c == su.c
      &&  sp.troubleMaker == su.troubleMaker
      ? "conforming and gcc implementations match
" : "huh?
");
  return 0;
}

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