如何用GCC编译器强制执行结构位顺序？ [英] How to enforce the struct bit order with the GCC compiler?

问题描述

我想知道是否有一个GCC C编译器指令允许我确定打包结构的位顺序？一些喜欢的：

  #pragma bit_order left 
  

这种需求的基本原理是我有以下结构：

pre $ struct {
union {
unsigned char BYTE;
struct {
unsigned char B0：1;
unsigned char B1：1;
unsigned char B2：1;
unsigned char B3：1;
unsigned char B4：4;
} BIT;
}项目;
} myStruct;


有了这个结构，我希望编译器能够这样打包它：

 位次序：| 7 6 5 4 3 2 1 0 | 
标签：| B0 B1 B2 B3 B4 B5 B6 B7 | 
  

而不是GCC如何做：

 位次序：| 7 6 5 4 3 2 1 0 | 
标签：| B7 B6 B5 B4 B3 B2 B1 B0 | 
  

我正在处理具有巨大头文件的MCU，这些头文件具有根据规定的硬件计算位偏移的结构地址。我希望GCC C编译器中有一个编译器指令，在我尝试翻转制造商提供的文件中的所有字段之前，对我进行位顺序交换。

解决方案

你使用哪个版本的GCC和哪个平台？有一个编译指示可以做到这一点，但它无法在x86上以GCC 4开始。

  #pragma reverse_bitfields on 
  

更多详情：

http://groups.google.com/group/ gnu.gcc.help/browse_thread/thread/747918655affa5c0?pli=1

如果你不介意重建GCC，所有相关的构建设置都在这里（搜索 bitfield ）：

http://gcc.gnu.org/onlinedocs/gccint/Storage-Layout.html

一些关于位域不好的细节：

C / C ++：强制位域顺序和对齐方式

I was wondering if there is a GCC C Compiler directive that allows me to determine the bit order for packing of a structure? Something to the likes of:

#pragma bit_order left

The rationale for such a need is that I have the following structure:

struct {
       union {
             unsigned char BYTE;
             struct {
                 unsigned char B0: 1;
                 unsigned char B1: 1;
                 unsigned char B2: 1;
                 unsigned char B3: 1;
                 unsigned char B4: 4;
             }BIT;
       }ITEM;
} myStruct;

With this structure, I would like the compiler to pack it this way:

Bit order: | 7  6  5  4  3  2  1  0 |
Label:     |B0 B1 B2 B3 B4 B5 B6 B7 |

Rather than how GCC does it:

Bit order: | 7  6  5  4  3  2  1  0 |
Label:     |B7 B6 B5 B4 B3 B2 B1 B0 |

I am dealing with MCUs that have huge header files that have structures that compute bit offsets according to stipulated hardware addresses. I am hoping that there is a compiler directive in GCC C Compiler that does the bit order swap for me before I attempt the flip all the fields in the manufacturer supplied file.

解决方案

Which version of GCC are you using and which platform? A pragma exists that may do the trick, but it doesn't work on x86 starting with GCC 4.

#pragma reverse_bitfields on

More details at:

http://groups.google.com/group/gnu.gcc.help/browse_thread/thread/747918655affa5c0?pli=1

If you don't mind rebuilding GCC, all the relevant build settings are here (search for bitfield):

http://gcc.gnu.org/onlinedocs/gccint/Storage-Layout.html

Some details about bitfields being bad:

C/C++: Force Bit Field Order and Alignment

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