CODEWITHSUNDEEP
c++bitmapc++14file-format
user2138149
user2138149

Reputation: 17296

C++ The compiler is changing the alignment of my structures. How can I prevent this?

I am writing some code to read bitmap files.

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

https://msdn.microsoft.com/en-us/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

If I put the following code in my main function:

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

the program prints:

BITMAPFILEHEADER: 16

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...

How can I prevent the structure re-alignment?

Upvotes: 3

Views: 2261

Answers (2)

Mark
Mark

Reputation: 16

To avoid this, you can obviously designate the compiling granularity. Just use this switch: 

##pragma pack(1)

This tells the compiler to align to 1-byte boundaries (do nothing)

To resume normal padding (from before the previous #pragma pack):

#pragma pack(pop)

Upvotes: 1

user2138149
user2138149

Reputation: 17296

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));

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

Upvotes: 5

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