GCC C ++（ARM）和const指向struct字段 [英] GCC C++ (ARM) and const pointer to struct field

问题描述

假设有一个简单的测试代码

  typedef struct 
 {
 int first; 
 int second; 
 int third; 
} type_t; 
 
 #define ADDRESS 0x12345678 
 
 #define REGISTER（（type_t *）ADDRESS）
 
 const int data =（int）（& REGISTER- >第二个）* 2; 
 
 int main（void）
 {
 volatile int data_copy; 
 
 data_copy = data; 
 
 while（1）{}; 
} 
  

在CodeSourcery G ++（gcc 4.3.2）中编译用于裸机ARM 。它还有一个非常标准的链接描述文件。

当编译为C（作为main.c）时，对象数据按照预期进入Flash。当以C ++（作为main.cpp）编译时，该对象进入RAM，并添加了附加代码，只将该值从Flash复制到RAM（该值已经计算，只是复制！）。所以编译器可以计算地址，但不知何故不想只需
使用它。问题的根源是地址的乘法 - 没有* 2乘法，两个版本都按预期工作 - 数据放在Flash中。另外 - 当data声明为：

  const int data =（int）（REGISTER）* 2; 
  

也一切都很好。

用于C和C ++编译的文件是相同的，唯一的区别是调用编译器 - g ++ for main.cpp，gcc为main.c（与警告级别不同，c ++已禁用RTTI和异常）。

有没有任何简单而优雅的方式来克服这个C ++问题？我需要这样的操作来创建Cortex-M3的位带区域的位地址的const数组。这是一个错误，也许这是C ++编译器的一些奇怪的限制？

我知道我可以在C文件中创建数据对象，只是extern - 在C ++中包含它们，但是这不是很优雅[;

感谢您的帮助！

解决方案

正确的解决方案是使用 offsetof（）宏从stddef.h标题。

基本上这是：

  const int data =（int）（& REGISTER-> second）* 2; 
  

必须用

  #include< stddef.h> 
 const int data =（int）（REGISTER + offsetof（type_t，second））* 2; 
  

然后对象被放置在Flash中，用于C和C ++。

Let's say there is a simple test code

typedef struct
{
    int first;
    int second;
    int third;
} type_t;

#define ADDRESS 0x12345678

#define REGISTER ((type_t*)ADDRESS)

const int data = (int)(&REGISTER->second)*2;

int main(void)
{
    volatile int data_copy;

    data_copy = data;

    while(1) {};
}

Which is compiled in CodeSourcery G++ (gcc 4.3.2) for bare metal ARM. It also has a very standard linker script.

When compiled in C (as main.c) the object "data" goes into Flash, as expected. When compiled in C++ (as main.cpp) this object goes into RAM, and additional code is added which does nothing more than copy the value from Flash to RAM (the value is already calculated, just copy!). So the compiler can calculate the address, but somehow doesn't want to "just use it". The root of the problem is the multiplication of the address - without "*2" multiplication both versions work as expected - "data" is placed in Flash. Also - when "data" is declared as:

const int data = (int)(REGISTER)*2;

also everything is fine.

All files for C and C++ compilation are identical, the only difference is the call to compiler - g++ for main.cpp, gcc for main.c (with differences in the level of warnings, and c++ has RTTI and exceptions disabled).

Is there any easy and elegant way to overcome this "C++ problem"? I do require such operations for creating const arrays of addresses of bits in bitband region of Cortex-M3. Is this a bug, or maybe that is some strange limitation of the C++ compiler?

I know that I can create data objects in "C" files and just "extern"-include them in C++, but that's not very elegant [;

Thank you for all help!

解决方案

The right solution is using the offsetof() macro from the stddef.h header.

Basically this:

const int data = (int)(&REGISTER->second)*2;

has to be replaced with

#include <stddef.h>
const int data = (int)(REGISTER + offsetof(type_t,second))*2;

And then the object is placed in Flash both for C and C++.

这篇关于GCC C ++（ARM）和const指向struct字段的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！