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!