Reading binary file data into struct object?

[SirEnder125]

Hello. I do not know if it's the right way to do this, but I have this code:

#include <iostream>
#include <fstream>

typedef unsigned int u32;
typedef unsigned short u16;

struct MyStruct {
	u16 Size;
	u32 Data;
};

int main(int argc, char **argv) {
	std::ifstream ifs("data.bin", std::ios::binary);

	if (!ifs) {
		std::cout << "FATAL: File data.bin could not be opened.\n";
		return 1;
	}

	MyStruct ms;
	ifs.read(reinterpret_cast<char*>(&ms), 6);

	if (ifs.gcount() != 6) {
		std::cout << "Could not read 6 bytes." << std::endl;
		return 1;
	}

	std::cout << "Size: " << ms.Size << std::endl;
	std::cout << "Data: " << ms.Data << std::endl;

	return 0;
}

And here is data.bin (bytes):

06 00 FF FF FF FF

And what I would expect is this:

Size: 6
Data: 4294967295

But it gave me this:

Size: 6
Data: 65535

I don't know what's wrong, but I think it's reading bytes 4 & 5. Is the code the problem? Also, FWIW sizeof u32 returns 4 and sizeof u16 returns 2.

[helios]

The compiler is inserting two bytes of padding between 'size' and 'data'. You need to tell it not to do this. Which compiler are you using?

[SirEnder125]

I'm pretty sure that I am using MinGW G++, but I am in eclipse and forgot what I selected at the creation of this project. I generally use MinGW G++ though.

[helios]

In GCC you can add the attribute packed at the end of a struct to disable padding. E.g.

struct MyStruct {
    u16 Size;
    u32 Data;
} __attribute__((packed));

[SirEnder125]

Thanks! That works well.

[jlb]

How did you write the information to the file?

You really shouldn't be using that magic number in your read() call, use the actual sizeof the structure.

	MyStruct ms;
	ifs.read(reinterpret_cast<char*>(&ms), sizeof(ms));

Also you may want to consider using the standard fixed size destinations for the types instead of the typedef.