To clarify my question, let's start off with an example program:
#include <stdio.h>
#pragma pack(push,1)
struct cc {
unsigned int a : 3;
unsigned int b : 16;
unsigned int c : 1;
unsigned int d : 1;
unsigned int e : 1;
unsigned int f : 1;
unsigned int g : 1;
unsigned int h : 1;
unsigned int i : 6;
unsigned int j : 6;
unsigned int k : 4;
unsigned int l : 15;
};
#pragma pack(pop)
struct cc c;
int main(int argc, char **argv)
{ printf("%d\n",sizeof(c));
}
The output is "8", meaning that the 56 bits (7 bytes) I want to pack are being packed into 8 bytes, seemingly wasting a whole byte. Curious about how the compiler was laying these bits out in memory, I tried writing specific values to &c, e.g.:
int main(int argc, char **argv)
{
unsigned long long int* pint = &c;
*pint = 0xFFFFFFFF;
printf("c.a = %d", c.a);
...
printf("c.l = %d", c.l);
}
Predictably, on x86_64 using Visual Studio 2010, the following happens:
*pint = 0x00000000 000000FF :
c[0].a = 7
c[0].b = 1
c[0].c = 1
c[0].d = 1
c[0].e = 1
c[0].f = 1
c[0].g = 0
c[0].h = 0
c[0].i = 0
c[0].j = 0
c[0].k = 0
c[0].l = 0
*pint = 0x00000000 0000FF00 :
c[0].a = 0
c[0].b = 0
c[0].c = 0
c[0].d = 0
c[0].e = 0
c[0].f = 0
c[0].g = 1
c[0].h = 127
c[0].i = 0
c[0].j = 0
c[0].k = 0
c[0].l = 0
*pint = 0x00000000 00FF0000 :
c[0].a = 0
c[0].b = 0
c[0].c = 0
c[0].d = 0
c[0].e = 0
c[0].f = 0
c[0].g = 0
c[0].h = 32640
c[0].i = 0
c[0].j = 0
c[0].k = 0
c[0].l = 0
etc.
Forget portability for a moment and assume you care about one CPU, one compiler, and one runtime environment. Why can't VC++ pack this structure into 7 bytes? Is it a word-length thing? The MSDN docs on #pragma pack says "the alignment of a member will be on a boundary that is either a multiple of n [1 in my case] or a multiple of the size of the member, whichever is smaller." Can anyone give me some idea of why I get a sizeof 8 and not 7?
MSVC++ always allocates at least a unit of memory that corresponds to the type you used for your bit-field. You used unsigned int, meaning that a unsigned int is allocated initially, and another unsigned int is allocated when the first one is exhausted. There's no way to force MSVC++ to trim the unused portion of the second unsigned int.
Basically, MSVC++ interprets your unsigned int as a way to express the alignment requirements for the entire structure.
Use smaller types for your bit-fields (unsigned short and unsigned char) and regroup the bit-fields so that they fill the allocated unit entirely - that way you should be able to pack things as tightly as possible.
Bitfields are stored in the type that you define. Since you are using unsigned int, and it won't fit in a single unsigned int then the compiler must use a second integer and store the last 24 bits in that last integer.
Well you are using unsigned int which happens to be 32 Bit in this case. The next boundary (to fit in the bitfield) for unsigned int is 64 Bit => 8 Bytes.
pst is right. The members are aligned on 1-byte boundaries, (or smaller, since it's a bitfield). The overall structure has size 8, and is aligned on an 8-byte boundary. This complies with both the standard and the pack option. The docs never say there will be no padding at the end.
To give another interesting illustrates what's going on, consider the case where you want to pack a structure that crosses a type boundary. E.g.
struct state {
unsigned int cost : 24;
unsigned int back : 21;
unsigned int a : 1;
unsigned int b : 1;
unsigned int c : 1;
};
This structure can't be packed into 6 bytes using MSVC as far as I know. However, we can get the desired packing effect by breaking up the first two fields:
struct state_packed {
unsigned short cost_1 : 16;
unsigned char cost_2 : 8;
unsigned short back_1 : 16;
unsigned char back_2 : 5;
unsigned char a : 1;
unsigned char b : 1;
unsigned char c : 1;
};
This can indeed be packed into 6 bytes. However, accessing the original cost field is extremely awkward and ugly. One method is to cast a state_packed pointer to a specialized dummy struct:
struct state_cost {
unsigned int cost : 24;
unsigned int junk : 8;
};
state_packed sc;
state_packed *p_sc = ≻
sc.a = 1;
(*(struct state_cost *)p_sc).cost = 12345;
sc.b = 1;
If anyone knows a more elegant way of doing this, I would love to know!
来源：https://stackoverflow.com/questions/3919194/what-is-vc-doing-when-packing-bitfields