C ++编译器正在更改我的结构的对齐方式.我该如何预防呢? [英] C++ The compiler is changing the alignment of my structures. How can I prevent this?

问题描述

我正在编写一些代码来读取位图文件.

I am writing some code to read bitmap files.

这是我用来读取位图标题的结构.另请参阅:

Here is the struct I am using to read the bitmap header. See also:

https://msdn.microsoft.com/zh-CN/library/windows/desktop/dd183374(v=vs.85).aspx

struct BITMAPFILEHEADER
{
        WORD  bfType; // 2
        DWORD bfSize; // 6
        WORD  bfReserved1; // 8
        WORD  bfReserved2; // 10
        DWORD bfOffBits; // 14
}; // should add to 14 bytes

如果在我的主要功能中输入以下代码:

If I put the following code in my main function:

std::cout << "BITMAPFILEHEADER: " << sizeof(BITMAPFILEHEADER) << std::endl;

程序打印:

BITMAPFILEHEADER: 16

似乎是在4字节边界上重新对齐结构中的数据，大概是为了提高效率.当然，这使我无法读取位图...即使是Microsoft和其他公司，也可以通过指定方式来做到这一点...

It appears to be re-aligning the data in the struct on 4-byte boundaries, presumably for efficiency. Of course this renders me unable to read a bitmap... Even though microsoft, and others, specifiy this is the way to do it...

如何防止结构重新对齐?

How can I prevent the structure re-alignment?

推荐答案

我发现的解决方案适用于Linux下的gcc编译器:

The solution I found which works on gcc compilers, under linux:

struct BITMAPFILEHEADER
{
        WORD  bfType;
        DWORD bfSize;
        WORD  bfReserved1;
        WORD  bfReserved2;
        DWORD bfOffBits;
} __attribute__((packed));

也许有更好的，更交叉的编译器/平台方式，但是我不知道它是什么.

There is probably a better, more cross compiler/platform way of doing things, but I don't know what it is.

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