定义结构中字段的冒号（:)语法 [英] Colon (:) syntax in defining fields in a struct

问题描述

大家好，


我最近遇到了以下代码段，它定义了一个C

结构：


typedef struct

{

unsigned char unused_bits：4;

unsigned char wchair_state：2;

} xyz;


数字4和2指的是什么？


如果我定义第二个结构如下：


typedef struct

{

unsigned char unused_bits;

unsigned char wchair_state;

} abc;

然后声明


void main（无效）

{

xyz _xyz;

abc _abc;

}


在内存分配方面，它是否有任何区别

分配给_xyz和_abc？


任何反馈都会非常感激。


谢谢

Raj

解决方案

"："表示比特分配。

系统将为struct _xyz分配1个字节，为struct _abc分配2个字节。

这是mem结构图：

_________________________________________________

| 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 |

---------------------------------------- ---------

|< - _xyz.u_bits - > |< - _xyz.w-> |


_________________

| 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | < ---- _abc.u_bits;

+ - + - + - + - + - + - + - + - +

| 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | < ---- _abc.w

-----------------

那就是，并且有效。

我真的希望你能得到它。


-

Shark Vanue

< blockquote> ra********@hotmail.com （Raj Kotaru）写道：

＃Hello all，

＃

＃我最近遇到了以下代码段，它定义了一个C $ / $
＃struct ：

＃

#typedef struct

＃{

＃unsigned char unused_bits：4;

＃unsigned char wchair_state：2;

＃} xyz;

＃

＃数字4和2是指什么？


unused_bits是四位宽，wchair_state是两位。字段可能会尽可能紧密地打包在一个字符大小的单元中。


在典型的CPU和C实现中，没有字段宽度，

结构将是两个字符宽，只有一个字符宽，上面是

。


＃

＃如果我定义第二个结构如下：

＃

＃typedef struct

＃{

＃unsigned char unused_bits;

＃unsigned char wchair_state;

＃} abc;

＃

＃

＃然后声明

＃

#void main（void）

＃{

＃xyz _xyz;

＃abc _abc;

＃}

＃

＃就...而言内存分配，

＃分配给_xyz和_abc之间有什么区别吗？


添加

printf（"％） d％d \ n"，sizeof（xyz），sizeof（abc））;

我希望看到它打印

1 2


- -

SM Ryan http://www.rawbw.com / ~wyrmwif /

成为烈士的一个缺点就是你必须死。

ra ******** @ hotmail.com （Raj Kotaru）写道：

I最近遇到了以下定义C />结构的代码段：

typedef struct
{unsigned char unused_bits：4;
unsigned char wchair_state ：2;
} xyz;

查找位字段在任何C教科书中。


-

Keith Thompson（The_Other_Keith） ks ***@mib.org < http://www.ghoti.net/~kst>

圣地亚哥超级计算机中心< *> < http://users.sdsc.edu/~kst>

我们必须做点什么。这是事情。因此，我们必须这样做。

Hello all,

I recently came across the following segment of code that defines a C
struct:

typedef struct
{
unsigned char unused_bits:4;
unsigned char wchair_state:2;
} xyz;

What do the numbers 4 and 2 refer to?

If I define a second struct as below:

typedef struct
{
unsigned char unused_bits;
unsigned char wchair_state;
} abc;
and then declare

void main(void)
{
xyz _xyz;
abc _abc;
}

In terms of memory allocation, is there any difference between that
allocated for _xyz and _abc?

Any feedback would be much appreciated.

Thanks
Raj

解决方案

":" means bit-allocation.
System will allocate 1 byte to struct _xyz and 2 byte to struct _abc.
Here is mem structure chart:
_________________________________________________
| 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 |
-------------------------------------------------
|<- _xyz.u_bits ->|<- _xyz.w->|

_________________
|1|2|3|4|5|6|7|8| <---- _abc.u_bits;
+-+-+-+-+-+-+-+-+
|1|2|3|4|5|6|7|8| <---- _abc.w
-----------------
That is,and works .
I really want you get it.

--
Shark Vanue

ra********@hotmail.com (Raj Kotaru) wrote:
# Hello all,
#
# I recently came across the following segment of code that defines a C
# struct:
#
# typedef struct
# {
# unsigned char unused_bits:4;
# unsigned char wchair_state:2;
# } xyz;
#
# What do the numbers 4 and 2 refer to?

unused_bits is four bits wide and wchair_state is two bits. The fields may
be packed as tightly as possible, in one char sized unit possibly.

On a typical CPU and C implementation, without the field widths, the
struct would be two characters wide, and only one character wide with
the above.

#
# If I define a second struct as below:
#
# typedef struct
# {
# unsigned char unused_bits;
# unsigned char wchair_state;
# } abc;
#
#
# and then declare
#
# void main(void)
# {
# xyz _xyz;
# abc _abc;
# }
#
# In terms of memory allocation, is there any difference between that
# allocated for _xyz and _abc?

Add
printf("%d %d\n",sizeof(xyz),sizeof(abc));
I would expect to see it print
1 2

--
SM Ryan http://www.rawbw.com/~wyrmwif/
One of the drawbacks of being a martyr is that you have to die.

ra********@hotmail.com (Raj Kotaru) writes:

I recently came across the following segment of code that defines a C
struct:

typedef struct
{
unsigned char unused_bits:4;
unsigned char wchair_state:2;
} xyz;

Look up "bit fields" in any C textbook.

--
Keith Thompson (The_Other_Keith) ks***@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <*> <http://users.sdsc.edu/~kst>
We must do something. This is something. Therefore, we must do this.

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