C 中未命名的零长度位域是什么意思？

Question

我在C标准草案(n1570)中看到了如下例子：

$3.14 第 4 段：结构声明为：

struct 
{
        char a;
        int b:5, c:11, :0, d:8;
        struct 
        { 
            int ee:8; 
        } e;
}

那么，:0是什么意思？

我知道位域是什么，但是:0没有名字，我不明白。

:0 没有任何标识符的目的是什么？

Answer 1

正如您之前链接的文档所解释的那样：

A bit-field and an adjacent non-bit-field member are in separate memory locations. The same applies to two bit-fields, if one is declared inside a nested structure declaration and the other is not, or if the two are separated by a zero-length bit-field declaration, or if they are separated by a non-bit-field member declaration

这是一种告诉编译器 b 和 c can/will 位于同一内存位置而 d 必须与它们分开并且可以同时修改为b/c

Answer 2

首先，让我们看章节§6.7.2.1，结构和联合说明符，P11。它说，

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. [...]

但是，以防万一，我们明确想要两个连续的位域成员，"might be"打包到一个内存位置以驻留在单独的内存位置（即，可寻址存储单元），以上是强制它的方法。

下一段，P12，提到，

A bit-field declaration with no declarator, but only a colon and a width, indicates an unnamed bit-field.126) As a special case, a bit-field structure member with a width of 0 indicates that no further bit-field is to be packed into the unit in which the previous bit-field, if any, was placed.

按照你的例子，这确保 :0 周围的两个位域成员将驻留在单独的内存位置（而不是在单个可寻址存储单元内，即使仍有足够的内存来打包它们合为一）。这与在两个位域之间有一个非位域成员具有类似的效果，以强制分隔内存位置。

引用 C11，章节 §3.14，NOTE 2（强调我的）

A bit-field and an adjacent non-bit-field member are in separate memory locations. The same applies to two bit-fields, if one is declared inside a nested structure declaration and the other is not, or if the two are separated by a zero-length bit-field declaration, or if they are separated by a non-bit-field member declaration.

此外，关于用法（"why it is needed"部分）

[...] The bit-fields b and c cannot be concurrently modified, but b and a, for example, can be.

附录：

关于并发部分，来自NOTE 1

Two threads of execution can update and access separate memory locations without interfering with each other.

并且，从第 §5.1.2.4/P1 章开始，

Under a hosted implementation, a program can have more than one thread of execution (or thread) running concurrently. [...]

因此，根据标准，这在理论上是可行的选择。

Answer 3

这是一种确保位文件不合并到单个内存位置的方法。

例如，假设您有一个 8 位字符，但您希望确保两个 3 位字段位于不同的位置（因此可以同时修改）。为此，您可以使用：

struct xyzzy {
    int first  : 3,
               : 0,
    int second : 3;
};

而且您不必担心手动填写 space，例如 junk : 5。

对于语言律师，C11 3.14 memory location /3 声明（我强调）：

A bit-field and an adjacent non-bit-field member are in separate memory locations. The same applies to two bit-fields, if one is declared inside a nested structure declaration and the other is not, or if the two are separated by a zero-length bit-field declaration, or if they are separated by a non-bit-field member declaration.

C 中未命名的零长度位域是什么意思？

What does an unnamed zero length bit-field mean in C?

c

structure

bit-fields