Qt Windows (MSDev not mingw) #pragma pack(push, 1) doesn't work

davecotter

i have some disk structures i need to read in that require structs to be tightly packed.
when i pack them with this:

#pragma pack(push, 1)

they still come out wrong.

example program:

typedef uint8_t		UInt8;

#pragma pack(push, 1)
typedef struct {
	UInt8	year		: 6;	//	[0..63] + 2000
	UInt8	month		: 4;	//	[1..12]
	UInt8	day		: 5;	//	[1..31]
	UInt8	hour		: 5;	//	[0..23]
	UInt8	minute		: 6;	//	[0..59]
	UInt8	second		: 6;	//	[0..59]
} ShortDateTime;
 #pragma pack(pop)

#define		kStructSize		(int)4

int main(int argc, char *argv[])
{
	int		actualI = (int)sizeof(ShortDateTime);
	
	if (actualI == kStructSize) {
		printf("it worked!");
	} else {
		printf("%s actual: %d, should be: %d\n", "ShortDateTime", actualI, kStructSize);
	}
}

output:

ShortDateTime actual: 6, should be: 4

so... what's missing?

davecotter

the answer to this case is to use uint32_t as the types not uint8_t

Kent-Dorfman

Two things come to mind.

pragmas are compiler specific. what works in the gnu compile may not work in the M$ compiler.

Also, the specification does not guarantee adjacent bits being stored in the same byte.

See This C++ reference

I would write code to manually stuff the bits when necessary, by using bit shift operators.

davecotter

It works on Mac with Xcode, and on Mac with QT. It works on windows with visual studio, and I am using VS2017 as the back end compiler for QT on windows, so why would it not work? We have all the information we need to get a definitive answer, it definitely should be working.

What is the proper way to pack bit fields? There is a way I just want to know what it is

Thanks

davecotter

the answer to this case is to use uint32_t as the types not uint8_t

jsulm

This post is deleted!

Kent-Dorfman

This post is deleted!