问题描述
为了澄清我的问题，让我们从一个示例程序开始：
#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\\\
，sizeof（c））;
}
输出为8，意味着56位（7字节）我想打包被打包成8字节，看来浪费一个整个字节。对于编译器如何将这些位存储在内存中，我尝试为& c 编写特定的值，例如：
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）;
}
可以预见，在x86_64上使用Visual Studio 2010，会发生以下情况：
* 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] 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
p>
暂时忘记可移植性，假设你关心一个CPU，一个编译器和一个运行时环境。为什么VC ++不能把这个结构打包成7个字节？它是一个字长的东西吗？ （ #pragma） pack 说一个成员的对齐将在一个边界上，该边界是n的倍数（在我的例子中为1）或成员大小的倍数，以较小者为准。任何人都可以给我一些想法为什么我得到一个sizeof 8而不是7？
MSVC ++总是分配至少一个单位内存对应于您用于位字段的类型。你使用 unsigned int ，意味着最初分配一个 unsigned int ，另一个 unsigned int 在第一个用尽时分配。没有办法强制MSVC ++修剪第二个 unsigned int 的未使用部分。
基本上，MSVC ++将 unsigned int 解释为表示对齐要求
为位字段使用较小的类型（无符号short 和 unsigned char ）并重新组合位字段，以便它们完全填充分配的单元 - 这样，您应该能够尽可能紧密地包装。
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.
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