• SL <shadowlord13 gmail.com> Apr 24 2005
• Bertel Brander <bertel post4.tele.dk> Apr 24 2005
• SL <shadowlord13 gmail.com> Apr 24 2005

#pragma pack(1)
struct pvlist {
     Uword polygon_id : 12;
     Uchar vtxflag_0  : 1;
     Uchar vtxflag_1  : 1;
     Uchar vtxflag_2  : 1;
     Uchar vtxflag_3  : 1;
};
#pragma pack()

The total number of bits there is 16, of course, so it should fit in two 
bytes. However, DMC is making it 3 bytes in size. (If I omit the pragma 
pack stuff, it makes it 4 bytes instead)

I tried changing 'Uword polygon_id : 12' to 'Uchar polygon_id : 12' to 
see if that worked, but it didn't. The compiler complained that "12 
exceeds maximum bit field width of 8 bits."

Rearranging the variables in the struct doesn't make it smaller either.

Oddly, if I omit the type from polygon_id's declaration entirely, the 
struct becomes 5 bytes long. I wouldn't even have expected that to 
compile, heh.

Is this a compiler bug?

The reason I'm asking is because there's some asm code which depends on 
this being 2 bytes in size. If this isn't a compiler bug, I'll "fix" the 
code of course, but it seems to me that DMC is wasting a byte. Or 
possibly I shouldn't be using pragma pack(1), if there's some way to 
tell it not to try to align anything in the struct on any boundaries at 
all (if that's the problem).

-SL

SL wrote:
 #pragma pack(1)
 struct pvlist {
     Uword polygon_id : 12;
     Uchar vtxflag_0  : 1;
     Uchar vtxflag_1  : 1;
     Uchar vtxflag_2  : 1;
     Uchar vtxflag_3  : 1;
 };
 #pragma pack()
 
 The total number of bits there is 16, of course, so it should fit in two 
 bytes. However, DMC is making it 3 bytes in size. (If I omit the pragma 
 pack stuff, it makes it 4 bytes instead)


This one print 2:

#include <iostream>
#pragma pack(1)
struct pvlist {
     unsigned short polygon_id : 12;
     unsigned short vtxflag_0  : 1;
     unsigned short vtxflag_1  : 1;
     unsigned short vtxflag_2  : 1;
     unsigned short vtxflag_3  : 1;
};
#pragma pack()

int main()
{
    std::cout << sizeof(pvlist)  << std::endl;
}

Bertel Brander wrote:
 This one print 2:
 


[An hour or so of testing later]

Ah ha - the problem is that DMC doesn't like how I mixed Uword (unsigned 
short) with Uchar (unsigned char) in the struct. If all of the variables 
in the struct are Uword it comes out to 2 bytes.

I'm kinda wishing DMC had printed a warning that mixing types there is bad.

Thanks for the help.