c++ - 对齐的内存分配器: 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，我自己用以下代码进行了尝试:

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，但我必须反对行*ptrAdjustment = adjustment，因为恕我直言，它会损坏内存。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？!

期待您的答复，谢谢!

注意:这个例子是关于:内存将被写入哪里，而不是关于:myalloc() 返回错误的内存....

(我对代码的更改:

• 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 返回已经正确对齐的地址(例如 0)并且您请求 8 字节对齐的地址，则代码将返回地址 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 )。如果您希望代码在已正确对齐的情况下返回原始 malloc() 地址，您只需将上面的行更改为:

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

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