如何口罩适用于一个const void *的地址？ [英] How to apply a mask to a const void* address?

问题描述

我在一个嵌入式目标工作，要定义内存池。

I work on an embedded target and want to define memory pools.

内存地址重新psented为 $ P $无效* 。然而，在特定情况下，这些地址被缓存，我想取消缓存他们直接获得真正的硬件地址。

Memory addresses are represented as void*. However in a particular case, those addresses are cached and I want to uncache them to get directly the "real" hardware address.

我想定义 memory_area 的起始地址（这仅仅是一个标记）：

I want to define the address of the beginning of memory_area (which is just a marker):

#define UNCACHE_MASK 0xABCDEF12UL // Value of the mask to apply
extern uint32_t memory_area; // Global, defined somewhere else
const void * virtual_address = &memory_area; // OK
const void * real_address = 
    (void*)(virtual_address | UNCACHE_MASK); // guilty line

不幸的是， GCC 不会让我这样做：

error: invalid operands to binary | (have 'const void *' and 'long unsigned int')

在绝望中我尝试：

const void * real_address = 
    (void*)(((uint32_t)virtual_address) | UNCACHE_MASK); // guilty line

在徒劳的：

error: initializer element is not constant

我真的想保持常量安全：这是可以实现的？

I really want to keep the const for safety : is it achievable ?

• 我使用 GCC V4.9（与 -std = gnu99 和大量的 -Wxxx Linux上的标志）。


• 摘录是从 .H 文件，变量是全局。

• I am using gcc v4.9 (with -std=gnu99 and a lot of -Wxxx flags) on Linux.
• The excerpt is from a .h file, variables are "global".

推荐答案

您可以将这个使用按位或前$ P $函数内pssion。你不能在全局范围内做初始化为已示出，因为确实的值是不知道的编译器（也许在C ++中，但不是在C）。如果添加石膏和移动分配的功能范围内，它会工作。

You can assign this using your bitwise-OR expression inside a function. You cannot do the initialization at global scope as you have shown, because indeed the value is not known to the compiler (maybe in C++, but not in C). If you add the casts and move the assignment inside a function scope, it will work.

当你这样说：

const void * real_address

这意味着real_address是指向未知类型的常量内存。如果你希望指针也是不可修改（除了它所指向的），你可以这样做：

It means "real_address is a pointer to constant memory of unknown type." If you want the pointer to also not be modifiable (in addition to what it points to), you can do this:

const void * const real_address

但你不会因为你需要做的是能够初始化函数内。我想你可以抛弃函数内部的常量性，但是这可能甚至不合法的温度。

But then you won't be able to initialize it inside a function as you need to do. I suppose you could cast away the constness inside the function, but this may not even be legal in C.

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