问题描述
大家好，
以下是示例代码段：
....
typedef PACKED struct
{
union
{
PACKED struct
{
char red ：1;
char green：1;
char蓝色：1;
} color_bit;
char颜色;
} color_u;
} color_t;
color_t颜色;
....
我的问题是：color.color_u.color中的位是什么？反映
" color.color_u.color_bit.red"，" color.color_u.color_bit.green"和
" color.color_u.color_bit.blue"？这个定义得好吗？
换句话说，如果我设置color.color_u.color_bit.red为0，
" color.color_u.color_bit.green" as 0和color.color_u.color_bit.blue
为1，什么是color.color_u.color？它是0x04吗？
关于位字段的几乎所有内容都是实现定义的。它完全取决于你的编译器如何将位置放在它用于存储的类型中./ b
。检查编译器的文档。
-
Jack Klein
主页：
comp的常见问题解答.lang.c
comp.lang.c ++
alt.comp.lang.learn.c-c ++
不一定。事实上，并不能保证
`color.color_u.color''将具有有效值：工会
经常（ab）用于这种类型的惩罚，但
语言并不保证任何合理的将会发生如果你尝试的话会发生b $ b。
如果你想在一个
`char''中将颜色表示为三位，你最好使用按位运算符>
pre- #define（或者enum-erated）面具：
typedef unsigned char color_t;
#define COLOR_RED 1
#define COLOR_GREEN 2
#define COLOR_BLUE 4
#define COLOR_YELLOW（COLOR_GREEN | COLOR_RED）
...
- -
Eric Sosman
盖子
什么是包装？它不是C构造 - 看起来好像它可能是一个#defined宏扩展到（一个
会猜测）某种特定于实现的扩展对C来说比如`__attribute __（（packed））''。如果是这样，你需要查阅你的实施文件，找出发生了什么，因为它不是C.
好吧，它*可能是*，如果'PACKED''扩展为像'volatile'或'const''或者什么都没有的东西。在接下来的内容中，我会假设情况就是这样。
不是用C语言编写的。编译器如何选择在结构中排列位字段有很大的自由。
不同的编译器会以不同的方式排列。
不一定。事实上，并不能保证
`color.color_u.color''具有有效值：联合
经常（ab）用于这种类型的惩罚，但
如果你想在一个字符中用三个字符表示颜色''，你最好使用带有
预定义（或枚举）掩码的按位运算符：
typedef unsigned char color_t;
#define COLOR_RED 1
#define COLOR_GREEN 2
#define COLOR_BLUE 4
#define COLOR_YELLOW（COLOR_GREEN | COLOR_RED）
...
-
Eric Sosman
lid
Hi all,
Here is a sample code segment:
....
typedef PACKED struct
{
union
{
PACKED struct
{
char red:1;
char green:1;
char blue:1;
} color_bit;
char color;
} color_u;
} color_t;
color_t color;
....
My question is: which bit in "color.color_u.color" reflect
"color.color_u.color_bit.red", "color.color_u.color_bit.green" and
"color.color_u.color_bit.blue"? Is this well defined?
In other words, if I set "color.color_u.color_bit.red" as 0,
"color.color_u.color_bit.green" as 0 and "color.color_u.color_bit.blue"
as 1, What will be "color.color_u.color"? is it 0x04?
Almost everything about bit-fields is implementation defined. It is
entirely up to your compiler as to how it places the bits in the type
it uses for storage. Check your compiler''s documentation.
--
Jack Klein
Home: http://JK-Technology.Com
FAQs for
comp.lang.c http://www.eskimo.com/~scs/C-faq/top.html
comp.lang.c++ http://www.parashift.com/c++-faq-lite/
alt.comp.lang.learn.c-c++
http://www.contrib.andrew.cmu.edu/~a...FAQ-acllc.html
Not necessarily. In fact, it''s not guaranteed that
`color.color_u.color'' will have a valid value at all: unions
are frequently (ab)used for this sort of type punning, but
the language doesn''t promise that anything reasonable will
happen if you try it.
If you want to represent a color as three bits in a
`char'', you''re better off using bitwise operators with
pre-#defined (or enum-erated) masks:
typedef unsigned char color_t;
#define COLOR_RED 1
#define COLOR_GREEN 2
#define COLOR_BLUE 4
#define COLOR_YELLOW (COLOR_GREEN | COLOR_RED)
...
--
Eric Sosman
es*****@acm-dot-org.invalid
What is `PACKED''? It isn''t a C construct -- it looks
like it''s probably a #defined macro that expands to (one
would guess) some kind of implementation-specific extension
to C like `__attribute__((packed))''. If so, you''ll need to
consult your implementation''s documentation to find out
what''s going on, because it isn''t C.
Well, it *could* be C, if `PACKED'' expands to something
like `volatile'' or `const'' or nothing at all. In what
follows, I''ll assume that''s the case.
Not by the C language. The compiler has great freedom
in how it chooses to arrange bit-fields within a struct.
Different compilers will arrange things differently.
Not necessarily. In fact, it''s not guaranteed that
`color.color_u.color'' will have a valid value at all: unions
are frequently (ab)used for this sort of type punning, but
the language doesn''t promise that anything reasonable will
happen if you try it.
If you want to represent a color as three bits in a
`char'', you''re better off using bitwise operators with
pre-#defined (or enum-erated) masks:
typedef unsigned char color_t;
#define COLOR_RED 1
#define COLOR_GREEN 2
#define COLOR_BLUE 4
#define COLOR_YELLOW (COLOR_GREEN | COLOR_RED)
...
--
Eric Sosman
es*****@acm-dot-org.invalid
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