难道位域类型影响结构alignement [英] Does the type of bitfield affect structure alignement

问题描述

我有以下结构：

 结构bf_struct1
{
    uint64_t中BF1：1;
    uint64_t中BF2：6;
    uint64_t中BF3：2;
    uint64_t中BF4：55;
}结构bf_struct2
{
    uint8_t有BF1：1;
    uint8_t有BF2：6;
    uint8_t有BF3：2;
    uint64_t中BF4：55;
}
 

请问结构成员对齐取决于类型位域的成员？

解决方案

从的马的嘴：

6.7.2.1结构和联合说明符的结果
...结果
5位字段应具有一个类型，是一个合格或不合格的版本 _Bool，符号int ，无符号整型或某些其它实现定义的类型。它是
实现定义原子类型是否允许。结果
...结果
11实现可以分配任何可寻址存储单元，大到足以容纳一个比特。
如果有足够的空间仍然存在，紧跟在另一个位字段位域
结构应包装到同一单元的相邻比特。如果没有足够的空间仍然存在，
不适合的位字段是否被放入下一个单元或重叠相邻单元是
实现定义。位域的分配单位（高阶到内的顺序
低阶或低阶到高阶）是实现定义的。的对准
可寻址存储单元是不确定的。

简短的回答：可以，根据不同的实现。

I have the following structures:

struct bf_struct1
{
    uint64_t bf1 : 1;
    uint64_t bf2 : 6;
    uint64_t bf3 : 2;
    uint64_t bf4 : 55;
}

struct bf_struct2
{
    uint8_t bf1 : 1;
    uint8_t bf2 : 6;
    uint8_t bf3 : 2;
    uint64_t bf4 : 55;
}

Does the structure member alignment depend on type of a bitfield members?

解决方案

From the horse's mouth:

6.7.2.1 Structure and union specifiers
...
5 A bit-field shall have a type that is a qualified or unqualified version of _Bool, signed int, unsigned int, or some other implementation-defined type. It is implementation-defined whether atomic types are permitted.
...
11 An implementation may allocate any addressable storage unit large enough to hold a bitfield. If enough space remains, a bit-field that immediately follows another bit-field in a structure shall be packed into adjacent bits of the same unit. If insufficient space remains, whether a bit-field that does not fit is put into the next unit or overlaps adjacent units is implementation-defined. The order of allocation of bit-fields within a unit (high-order to low-order or low-order to high-order) is implementation-defined. The alignment of the addressable storage unit is unspecified.

Short answer: it can, depending on the implementation.

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