[SOLVED] Struct size issue

vmelkon · 01-24-2024, 11:11 AM

Hello,
I have my own BMP file reader. It is written in C++.
I am compiling it for Linux, using Qt Creator so underneath it, I think it uses gcc.

So, I think it is only natural to have structure that englobe a set of data.
A lot of file formats use some kind of structure.

Code:

struct BITMAPFILEHEADER
{
	ushort	bfType;
	uint	bfSize;
	ushort	bfReserved1;
	ushort	bfReserved2;
	uint	bfOffBits;
};

We tend to write the structure to file via something like

Code:

BITMAPFILEHEADER BitmapFileHeader;
....
....
fwrite(&BitmapFileHeader, 1, sizeof(BITMAPFILEHEADER), hfile);

BITMAPFILEHEADER should be 14 bytes but gcc has turned that into 16 bytes.
In other words, the BMP file created is not valid.

There is this:
https://en.cppreference.com/w/cpp/language/alignas

but I think that just effects start address. It helps with CPU optimization to have data start at certain addresses.

Is there something I can put in the C++ code to make it NOT ADD additional bytes to my struct?

It is possible to do it with compiler flags but I would prefer to do it in the C++ code.

That sort of code is very common for writing/reading a file, for sending some data over a network.
If the compiler does whatever it wants, then it can break your code.

NevemTeve · 01-24-2024, 12:35 PM

Code:

#pragma pack(push,2)
struct BITMAPFILEHEADER
{
	ushort	bfType;
	uint	bfSize;
	ushort	bfReserved1;
	ushort	bfReserved2;
	uint	bfOffBits;
};
#pragma pack(pop)

sundialsvcs · 01-28-2024, 12:09 PM

“Slack bytes” within structs are common with many modern compilers, because they allow faster code to be generated for modern pipelined CPUs. But “packing” is available when you need to control exactly how the bytes are laid out. Different compilers (and, versions thereof) do it in different ways, and may use different pragmas (if they do).