结构体中的冒号代表什么
在windows API中定义的_DCB结构体中的冒号如
DWORD fDtrControl :2;
和它后面的数值2是什么意思？

原定义如下：


typedef struct _DCB {
DWORD DCBlength;
DWORD BaudRate;
DWORD fBinary :1;
DWORD fParity :1;
DWORD fOutxCtsFlow :1;
DWORD fOutxDsrFlow :1;
DWORD fDtrControl :2;
DWORD fDsrSensitivity :1;
DWORD fTXContinueOnXoff :1;
DWORD fOutX :1;
DWORD fInX :1;
DWORD fErrorChar :1;
DWORD fNull :1;
DWORD fRtsControl :2;
DWORD fAbortOnError :1;
DWORD fDummy2 :17;
WORD wReserved;
WORD XonLim;
WORD XoffLim;
BYTE ByteSize;
BYTE Parity;
BYTE StopBits;
char XonChar;
char XoffChar;
char ErrorChar;
char EofChar;
char EvtChar;
WORD wReserved1;
} DCB, *LPDCB;


[解决办法]
C Bit Fields

Syntax:
struct-declarator:
declarator

type-specifier declarator opt : constant-expression

The constant-expression specifies the width of the field in bits. The type-specifier for the declarator must be unsigned int, signed int, or int, and the constant-expression must be a nonnegative integer value.and the constant-expression must be a nonnegative integer value. If the value is zero, the declaration has no declarator. Arrays of bit fields, pointers to bit fields, and functions returning bit fields are not allowed. The optional declarator names the bit field. Bit fields can only be declared as part of a structure. The address-of operator (&) cannot be applied to bit-field components.

Bit fields are allocated within an integer from least-significant to most-significant bit. In the following code

Copy Code
struct mybitfields
{
unsigned short a : 4;
unsigned short b : 5;
unsigned short c : 7;
} test;

int main( void );
{
test.a = 2;
test.b = 31;
test.c = 0;
}


the bits would be arranged as follows:

Copy Code
00000001 11110010
cccccccb bbbbaaaa




[解决办法]
常量表式的度
[解决办法]
你BAIDU一下位域
[解决办法]
位域 或者 位段

属于C中的一种基本语法
[解决办法]
这是占位的用这种方法可以使占的位数最少，例如
<PRE> struct date_struct {
BYTE day : 5, //
month : 4, //
year : 14; //
} date;则
|0 0 0 0 0 0 0 0|0 0 0 0 0 0 0 0|0 0 0 0 0 0 0 0|
| | | |
+------ year ---------------+ month +-- day --+

这样的话就可以控制该结构所占的字节数，只占用了23位，不到三个字节
操作的话可以这样
date.day = 12;

dateptr = &date;
dateptr-> year = 1852;