struct TriangleList
{
union
{
int Vertices[3];
struct
{
int Vertex1,Vertex2,Vertex3;
};
};
union
{
int TextureUV[3];
struct
{
int Uv1,Uv2,Uv3;
};
};
struct
{
union
{
double Normals[3];
struct
{
double Normal1, Normal2, Normal3;
};
};
} NormalCoords[3];
int MaterialNumber;
};
Linux sizeof different
April 20, 2005 02:46 AM
I have a structure:
864
April 20, 2005 03:00 AM
That's strange, especially since I count 64 bytes:
3*int = 3*4 = 12
3*int = 3*4 = 12
3*3*double = 3*3*4 = 36
int = 4
12+12+36+4 = 64
3*int = 3*4 = 12
3*int = 3*4 = 12
3*3*double = 3*3*4 = 36
int = 4
12+12+36+4 = 64
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187
April 20, 2005 03:05 AM
Quote: Original post by rick_appleton
That's strange, especially since I count 64 bytes:
3*int = 3*4 = 12
3*int = 3*4 = 12
3*3*double = 3*3*4 = 36
int = 4
12+12+36+4 = 64
double is 8 bytes large, so that should be :
3*int = 3*4 = 12
3*int = 3*4 = 12
3*3*double = 3*3*8 = 72
int = 4
total = 100
It probably has something todo with byte alignment. MSVC probably aligns on 8 byte boundaries hence the extra 4 bytes. (104 is dividable by 8, 100 isn't)
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5,545
April 20, 2005 03:12 AM
Weird, it's the int MaterialNumber; causing to add 8 bytes, but shouldn't actually? The struct is nicely on a 32 bit boundary.
Using pragma pack(1) will make the size be 100 on MSVC too, but i don't really see where the padding should be.
Using pragma pack(1) will make the size be 100 on MSVC too, but i don't really see where the padding should be.
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1,658
April 20, 2005 03:16 AM
I'd argue between the second union and the struct.
You can use the offsetof macro to get the offsets of each and every member.
You can use the offsetof macro to get the offsets of each and every member.
"Debugging is twice as hard as writing the code in the first place. Therefore, if you write the code as cleverly as possible, you are, by definition, not smart enough to debug it." — Brian W. Kernighan
978
April 20, 2005 03:33 AM
Maybe alignment is the reason.
If you create an array of TriangleList, then the double members still need 8 byte alignment. So the structure is padded to make its size a multiple of 8, which is 104 = 8 * 13. And GCC is incorrect in this case.
If you create an array of TriangleList, then the double members still need 8 byte alignment. So the structure is padded to make its size a multiple of 8, which is 104 = 8 * 13. And GCC is incorrect in this case.
April 20, 2005 03:49 AM
Thanks for the help. It is definitly an alignment issue. So I fixed it by putting in a float after int MaterialNumber and now it is 104 on both. So i guess MSVC was padding to the end of the struct. So now they both act exactly the same.
1,098
April 20, 2005 02:51 PM
Because you have an element that is 8-byte naturally aligned (the doubles) MSVC is padding the structure out to a multiple of 8 bytes. Presumably so that if you have an array of them all the elements of the array would be aligned properly instead of just every other one.
I'm a bit suprised gcc doesn't do that also. What does gcc give for sizeof(TriangleList[2]) and is the second element aligned properly?
I'm a bit suprised gcc doesn't do that also. What does gcc give for sizeof(TriangleList[2]) and is the second element aligned properly?
-Mike
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