Chapter 6

6.9 Bit-fields

When storage space is at a premium, it may be necessary to pack several objects into a single machine word; one common use is a set of single-bit flags in applications like compiler symbol tables. Externally-imposed data formats, such as interfaces to hardware devices, also often require the ability to get at pieces of a word.

在存储空间很宝贵的情况下，有可能需要将多个对象保存在一个机器字中。一种常用的方法是，使用类似于编译器符号表的单个二进制位标志集合。外部强加的数据格式（如硬件设备接口）也经常需要从字的部分值中读取数据。

Imagine a fragment of a compiler that manipulates a symbol table. Each identifier in a program has certain information associated with it, for example, whether or not it is a keyword, whether or not it is external and/or static, and so on. The most compact way to encode such information is a set of one-bit flags in a single char or int.

考虑编译器中符号表操作的有关细节。程序中的每个标识符都有与之相关的特定信息，例如，它是否为关键字，它是否是外部的且（或）是静态的，等等。对这些信息进行编码的最简洁的方法就是使用一个char或int对象中的位标志集合。

The usual way this is done is to define a set of ``masks'' corresponding to the relevant bit positions, as in

通常采用的方法是，定义一个与相关位的位置对应的“屏蔽码”集合，如：

#define KEYWORD 01

#define EXTRENAL 02

#define STATIC 04

or

enum { KEYWORD = 01, EXTERNAL = 02, STATIC = 04 };

The numbers must be powers of two. Then accessing the bits becomes a matter of ``bit-fiddling'' with the shifting, masking, and complementing operators that were described in Chapter 2.

这些数字必须是2 的幂。这样，访问这些位就变成了用第2 章中描述的移位运算、屏蔽运算及补码运算进行简单的位操作。

Certain idioms appear frequently:

下列语句在程序中经常出现

flags |= EXTERNAL | STATIC;

turns on the EXTERNAL and STATIC bits in flags, while

该语句将flags中的EXTERNAL和STATIC位置为1，而下列语句：

flags &= ~(EXTERNAL | STATIC);

turns them off, and

则将它们置为0。并且，当这两位都为0 时，下列表达式：

if ((flags & (EXTERNAL | STATIC)) == 0) ...

is true if both bits are off.

Although these idioms are readily mastered, as an alternative C offers the capability of defining and accessing fields within a word directly rather than by bitwise logical operators. A bit-field, or field for short, is a set of adjacent bits within a single implementation-defined storage unit that we will call a ``word.'' For example, the symbol table #defines above could be replaced by the definition of three fields:

尽管这些方法很容易掌握，但是，C语言仍然提供了另一种可替代的方法，即直接定义和访问一个字中的位字段的能力，而不需要通过按位逻辑运算符。位字段（bit-field），或简称字段，是“字”中相邻位的集合。“字”（word）是单个的存储单元，它同具体的实现有关。例如，上述符号表的多个#define语句可用下列3 个字段的定义来代替：

struct {

unsigned int is_keyword : 1;

unsigned int is_extern : 1;

unsigned int is_static : 1;

} flags;

This defines a variable table called flags that contains three 1-bit fields. The number following the colon represents the field width in bits. The fields are declared unsigned int to ensure that they are unsigned quantities.

这里定义了一个变量flags，它包含3 个一位的字段。冒号后的数字表示字段的宽度（用二进制位数表示）。字段被声明为unsigned int类型，以保证它们是无符号量。

Individual fields are referenced in the same way as other structure members: flags.is_keyword, flags.is_extern, etc. Fields behave like small integers, and may participate in arithmetic expressions just like other integers. Thus the previous examples may be written more naturally as

单个字段的引用方式与其它结构成员相同，例如：flags.is_keyword、flags.is_extern 等等。字段的作用与小整数相似。同其它整数一样，字段可出现在算术表达式中。因此，上面的例子可用更自然的方式表达为：

flags.is_extern = flags.is_static = 1;

to turn the bits on;

该语句将is_extern和is_static位置为1。下列语句：

flags.is_extern = flags.is_static = 0;

to turn them off; and

将is_extern和is_static位置为0。下列语句：

if (flags.is_extern == 0 && flags.is_static == 0)

...

to test them.

Almost everything about fields is implementation-dependent. Whether a field may overlap a word boundary is implementation-defined. Fields need not be names; unnamed fields (a colon and width only) are used for padding. The special width 0 may be used to force alignment at the next word boundary.

字段的所有属性几乎都同具体的实现有关。字段是否能覆盖字边界由具体的实现定义。字段可以不命名，无名字段（只有一个冒号和宽度）起填充作用。特殊宽度0 可以用来强制在下一个字边界上对齐。

Fields are assigned left to right on some machines and right to left on others. This means that although fields are useful for maintaining internally-defined data structures, the question of which end comes first has to be carefully considered when picking apart externally-defined data; programs that depend on such things are not portable. Fields may be declared only as ints; for portability, specify signed or unsigned explicitly. They are not arrays and they do not have addresses, so the & operator cannot be applied on them.

某些机器上字段的分配是从字的左端至右端进行的，而某些机器上则相反。这意味着，尽管字段对维护内部定义的数据结构很有用，但在选择外部定义数据的情况下，必须仔细考虑哪端优先的问题。依赖于这些因素的程序是不可移植的。字段也可以仅仅声明为int，为了方便移植，需要显式声明该int类型是signed还是unsigned类型。字段不是数组，并且没有地址，因此对它们不能使用&运算符。

上一章Chapter 6 - Structures（六）

at a premium 非常珍贵; 非常需要; 渴求