c++ - 对齐内存分配器 : memory corruption (game engine architecture[Jason Gregory])

Question

我刚刚在读Game Engine Architecture by Jason Gregory S.212，当我偶然发现他分配对齐内存的代码时。我认为他正在产生内存损坏并使用以下代码自行尝试:

void* myalloc( unsigned size )
{
    // this will not return a valid memory address but its ok since
    // it will never be dereferenced! its just to check what if a real malloc would
    // return this obviously UNALIGNED memory address
    return reinterpret_cast<void*>( 0x00001 );
}
void* allocatedAligned( unsigned size_bytes, unsigned alignment )
{
    unsigned expandedSize_bytes = size_bytes + alignment;
    unsigned rawAddress = (unsigned)myalloc( expandedSize_bytes );

    unsigned mask = alignment - 1;
    unsigned misalignment = rawAddress & mask;
    unsigned adjustment = alignment - misalignment;

    unsigned alignedAddress = rawAddress + adjustment;

    unsigned* ptrAdjustment = reinterpret_cast<unsigned*>( alignedAddress - 4 );
    //*ptrAdjustment = adjustment; //Corruption here
    //return reinterpret_cast<unsigned*>( alignedAddress ); this is what jasons function normally returns
    return ptrAdjustment;
}

int main()
{
    void* ptr = allocatedAligned( 4, 4 );
    std::cout << ptr << std::endl; // this shouldn't print an address lower than 0x0001
    return 1;
}

对齐工作正常，但我不得不反对 *ptrAdjustment = adjustment 行，因为恕我直言，它破坏了内存。它在从 myalloc() 收到的地址之前写入内存，还是我错了？main() 不应打印任何小于 myalloc() 返回的地址的地址？!

期待您的回答，谢谢!

注意:这个例子是关于:内存应该写到哪里，而不是关于:myalloc() returns wrong memory....

(我对代码的改动:

• 使用无符号代替 U32
• 使用 myalloc() 而不是 allocateUnaligned()
• C++ 转换而不是 C 风格)

Answer 1

代码已损坏。如果 malloc 会返回已经正确对齐的地址，比如 0 而你请求 8 字节对齐的地址，代码将返回地址 4，这显然是错误的。应删除以下行:

unsigned* ptrAdjustment = reinterpret_cast<unsigned*>( alignedAddress - 4 );

代码应该只返回 alignedAddress:

return reinterpret_cast<unsigned*>( alignedAddress );

(也应该是 void* 而不是 unsigned* 并且他应该使用 size_t 而不是 unsigned )。如果您希望代码返回原始的 malloc() 地址以防它已经正确对齐，您可以简单地将上面的行更改为:

return reinterpret_cast<void*>(misalignment?alignedAddress:rawAddress);

也很迂腐，函数中应该有断言来验证对齐是 2 的幂，例如assert((alignment&(alignment-1))==0);