C 语言常用方法技巧

C语言常用方法技巧

*:first-child {
margin-top: 0 !important;
}

body>*:last-child {
margin-bottom: 0 !important;
}

/* BLOCKS
=============================================================================*/

p, blockquote, ul, ol, dl, table, pre {
margin: 15px 0;
}

/* HEADERS
=============================================================================*/

h1, h2, h3, h4, h5, h6 {
margin: 20px 0 10px;
padding: 0;
font-weight: bold;
-webkit-font-smoothing: antialiased;
}

h1 tt, h1 code, h2 tt, h2 code, h3 tt, h3 code, h4 tt, h4 code, h5 tt, h5 code, h6 tt, h6 code {
font-size: inherit;
}

h1 {
font-size: 28px;
color: #000;
}

h2 {
font-size: 24px;
border-bottom: 1px solid #ccc;
color: #000;
}

h3 {
font-size: 18px;
}

h4 {
font-size: 16px;
}

h5 {
font-size: 14px;
}

h6 {
color: #777;
font-size: 14px;
}

body>h2:first-child, body>h1:first-child, body>h1:first-child+h2, body>h3:first-child, body>h4:first-child, body>h5:first-child, body>h6:first-child {
margin-top: 0;
padding-top: 0;
}

a:first-child h1, a:first-child h2, a:first-child h3, a:first-child h4, a:first-child h5, a:first-child h6 {
margin-top: 0;
padding-top: 0;
}

h1+p, h2+p, h3+p, h4+p, h5+p, h6+p {
margin-top: 10px;
}

/* LINKS
=============================================================================*/

a {
color: #4183C4;
text-decoration: none;
}

a:hover {
text-decoration: underline;
}

/* LISTS
=============================================================================*/

ul, ol {
padding-left: 30px;
}

ul li > :first-child,
ol li > :first-child,
ul li ul:first-of-type,
ol li ol:first-of-type,
ul li ol:first-of-type,
ol li ul:first-of-type {
margin-top: 0px;
}

ul ul, ul ol, ol ol, ol ul {
margin-bottom: 0;
}

dl {
padding: 0;
}

dl dt {
font-size: 14px;
font-weight: bold;
font-style: italic;
padding: 0;
margin: 15px 0 5px;
}

dl dt:first-child {
padding: 0;
}

dl dt>:first-child {
margin-top: 0px;
}

dl dt>:last-child {
margin-bottom: 0px;
}

dl dd {
margin: 0 0 15px;
padding: 0 15px;
}

dl dd>:first-child {
margin-top: 0px;
}

dl dd>:last-child {
margin-bottom: 0px;
}

/* CODE
=============================================================================*/

pre, code, tt {
font-size: 12px;
font-family: Consolas, "Liberation Mono", Courier, monospace;
}

code, tt {
margin: 0 0px;
padding: 0px 0px;
white-space: nowrap;
border: 1px solid #eaeaea;
background-color: #f8f8f8;
border-radius: 3px;
}

pre>code {
margin: 0;
padding: 0;
white-space: pre;
border: none;
background: transparent;
}

pre {
background-color: #f8f8f8;
border: 1px solid #ccc;
font-size: 13px;
line-height: 19px;
overflow: auto;
padding: 6px 10px;
border-radius: 3px;
}

pre code, pre tt {
background-color: transparent;
border: none;
}

kbd {
-moz-border-bottom-colors: none;
-moz-border-left-colors: none;
-moz-border-right-colors: none;
-moz-border-top-colors: none;
background-color: #DDDDDD;
background-image: linear-gradient(#F1F1F1, #DDDDDD);
background-repeat: repeat-x;
border-color: #DDDDDD #CCCCCC #CCCCCC #DDDDDD;
border-image: none;
border-radius: 2px 2px 2px 2px;
border-style: solid;
border-width: 1px;
font-family: "Helvetica Neue",Helvetica,Arial,sans-serif;
line-height: 10px;
padding: 1px 4px;
}

/* QUOTES
=============================================================================*/

blockquote {
border-left: 4px solid #DDD;
padding: 0 15px;
color: #777;
}

blockquote>:first-child {
margin-top: 0px;
}

blockquote>:last-child {
margin-bottom: 0px;
}

/* HORIZONTAL RULES
=============================================================================*/

hr {
clear: both;
margin: 15px 0;
height: 0px;
overflow: hidden;
border: none;
background: transparent;
border-bottom: 4px solid #ddd;
padding: 0;
}

/* TABLES
=============================================================================*/

table th {
font-weight: bold;
}

table th, table td {
border: 1px solid #ccc;
padding: 6px 13px;
}

table tr {
border-top: 1px solid #ccc;
background-color: #fff;
}

table tr:nth-child(2n) {
background-color: #f8f8f8;
}

/* IMAGES
=============================================================================*/

img {
max-width: 100%
}
-->

除法向上取整

#define DIV_ROUND_UP(n, d) (((n)+(d)-1) / (d))

大端小端选择

low-endian or high-endian

typedef union {
	short W;	/* Word access */
	struct {	/* Byte access */
#ifdef LOW_ENDIAN
		byte low, high;		/* in low-endian arch */
#else
		byte high, low;		/* in high-endian arch */
#endif
	} B;
} word;

求余数运算

a = a % 8;
=>
a = a & 7;

说明：位运算只需一个指令周期；取余通常需要调用子程序。

平方运算

a = pow(a, 2.0);
=>
a = a * a;

说明：内置乘法运算器的处理器中，乘法运算比求平方运算更快；即使没有内置乘法运算器，乘法运算的子程序也比平方运算子程序效率高。

移位实现乘除法运算

a = a * 4;
b = b / 4;
=>
a = a << 2;
b = b >> 2;

说明：通常乘以或者除以2n，可使用移位方法代替。

a = a * 9;
=>
a = (a << 3) + a;

while 循环和 do...while 循环

说明：do...while 循环编译后生成的代码长度短于 while 循环。

重定义类型，扩展移植性

typedef unsigned char       boolean; /* Boolean value type. */
typedef unsigned long int   uint32; /* Unsigned 32 bit value */
typedef unsigned short      uint16; /* Unsigned 16 bit value */
typedef unsigned char       uint8; /* Unsigned 8 bit value */
typedef signed long int     int32; /* Signed 32 bit value */
typedef signed short        int16; /* Signed 16 bit value */
typedef signed char         int8; /* Signed 8 bit value */

得到指定地址上的一个字节或字

typedef unsigned char   byte; /* Unsigned 8 bit value type */
typedef unsigned short  word; /* Unsigned 16 bit value type */

#define MEM_B(x)    (*((byte*)(x)))
#define MEM_W(x)    (*((word*)(x)))

求取极值

#define MAX(x, y)   ((x) > (y) ? (x) : (y))
#define MIN(x, y)   ((x) < (y) ? (x) : (y))

得到一个 field 在结构体(struct)中的偏移量

typedef unsigned long   dword; /* Unsigned 32 bit value type */

#define FPOS(type, field)\
    ( (dword)&((type*)0)->field )

得到一个结构体中 field 所占用的字节数

#define FSIZE(type, field)\
    ( sizeof(((type*)0)->field) )

按照 LSB 格式把一个 word(16 bit) 转换成两个字节

#define FLOPW(ray, val)\
    do {\
        (ray)[0] = ((val)>>8);\
        (ray)[1] = ((val)&0xFF);\
    }while(0);

得到一个变量的地址

typedef unsigned char   byte; /* Unsigned 8 bit value type */
typedef unsigned short  word; /* Unsigned 16 bit value type */  

#define B_PTR(var)  ((byte*)(void*)&(var))
#define W_PTR(var)  ((word*)(void*)&(var))

得到一个字节的低位和高位

typedef unsigned char   byte; /* Unsigned 8 bit value type */
typedef unsigned short  word; /* Unsigned 16 bit value type */

#define WORD_L(var) ((byte)(word)(var)&(0xFF))
#define WORD_H(var) ((byte)(word)(var)>>(8))

返回一个比 X 大的接近 8 的倍数

#define RND8(x) ((((x) + 7) >> 3) << 3)

防止溢出的方法

#define INC_SAT(val)\
    ((val) = ( ((val) + 1) > (val)) ? ((val) + 1):(val) )

返回数组元素的个数

#define ARR_SIZE(a)\
    ( (sizeof(a)) / (sizeof(a[0])) )

返回一个无符号数的后 n 位数

typedef unsigned long   dword; /* Unsigned 32 bit value type */ 

#define MOD_BY_POWER_OF_TWO(val, mod_by)\
    ((dword)(val)&(dword)(2<<(mod_by) - 1))

IO 空间映射在存储空间中的结构

typedef unsigned char   byte;   /* Unsigned 8 bit value type */
typedef unsigned short  word;   /* Unsigned 16 bit value type */
typedef unsigned long   dword;  /* Unsigned 32 bit value type */

#define outp(port)      (*((volatile byte *)(port)))
#define outpw(port)     (*((volatile word *)(port)))
#define outpdw(port)    (*((volatile dword *)(port)))

#define inp(port, val)      (*((volatile byte *)(port)))    = (byte)(val)
#define inpw(port, val)     (*((volatile word *)(port)))    = (word)(val)
#define inpdw(port, val)    (*((volatile dword *)(port)))   = (dword)(val)

宏中 "#" 和 "##" 的用法

一、使用 "#" 把宏参数变为一个字符串， 用 "##" 把两个宏参数贴合在一起。
    #define STR(val)    (#val)
    #define CONS(a, b)  (int)(a##e##b)
    ->
    STR(hello) ==> "hello"
    CONS(2, 3) ==> 2000 // 2e3

二、当宏参数是另一个宏的时候
    需要注意的是凡宏定义里有用 '#' 或 '##' 的地方宏参数是不会再展开.
    1, 非 '#' 和 '##' 的情况
        #define TOW         (2)
        #define MUL(a,b)    (a*b)

        printf("%d*%d=%d\n", TOW, TOW, MUL(TOW,TOW));
        ==>
        printf("%d*%d=%d\n", (2), (2), ((2)*(2)));

        MUL里的参数TOW会被展开为(2)。

    2, 当有 '#' 或 '##' 的时候
        #define A           (2)
        #define STR(s)      #s
        #define CONS(a,b)   int(a##e##b)

        printf("int max: %s\n", STR(INT_MAX));  // INT_MAX #include
        ==>
        printf("int max: %s\n", "INT_MAX"); 

        printf("%s\n", CONS(A, A));             // compile error
        ==>
        printf("%s\n", int(AeA));

        INT_MAX和A都不会再被展开, 然而解决这个问题的方法很简单，加多一层中间转换宏，
        加这层宏的用意是把所有宏的参数在这层里全部展开, 那么在转换宏里的那一个
        宏(_STR)就能得到正确的宏参数.

        #define A           (2)
        #define _STR(s)     (#s)
        #define STR(s)      _STR(s)     // 转换宏
        #define _CONS(a,b)  int(a##e##b)
        #define CONS(a,b)   _CONS(a,b)  // 转换宏

        printf("int max: %s\n", STR(INT_MAX)); // INT_MAX,int型的最大值，为一个变量 #include

        输出为: int max: 0x7fffffff

        STR(INT_MAX) --> _STR(0x7fffffff) 然后再转换成字符串;
        printf("%d\n", CONS(A, A));

        输出为：200

        CONS(A, A) --> _CONS((2), (2)) --> int((2)e(2))

三、'#' 和 '##' 的一些应用特例
    1、合并匿名变量名
        #define ___ANONYMOUS1(type, var, line)  type var##line
        #define __ANONYMOUS0(type, line)        ___ANONYMOUS1(type, _anonymous, line)
        #define ANONYMOUS(type)                 __ANONYMOUS0(type, __LINE__)

        例：ANONYMOUS(static int);
            即: static int _anonymous70; 70表示该行行号;
        第一层：ANONYMOUS(static int);
                --> __ANONYMOUS0(static int, __LINE__);
        第二层： --> ___ANONYMOUS1(static int, _anonymous, 70);
        第三层： --> static int _anonymous70;
            即每次只能解开当前层的宏，所以__LINE__在第二层才能被解开;

    2、填充结构
        #define FILL(a)     {a, #a}
        enum IDD {OPEN, CLOSE};
        typedef struct MSG{
            IDD id;
            const char * msg;
        }MSG;

        MSG _msg[] = {FILL(OPEN), FILL(CLOSE)};
        ==>
        MSG _msg[] = {{OPEN, "OPEN"},

        {CLOSE, "CLOSE"}};

    3、记录文件名
        #define _GET_FILE_NAME(f)   #f
        #define GET_FILE_NAME(f)    _GET_FILE_NAME(f)

        static char FILE_NAME[]     = GET_FILE_NAME(__FILE__);

    4、得到一个数值类型所对应的字符串缓冲大小
        #define _TYPE_BUF_SIZE(type)    sizeof #type
        #define TYPE_BUF_SIZE(type)     _TYPE_BUF_SIZE(type) 

        char buf[TYPE_BUF_SIZE(INT_MAX)];
        --> char buf[_TYPE_BUF_SIZE(0x7fffffff)];
        --> char buf[sizeof "0x7fffffff"];
        这里相当于： char buf[11];

关于平方根倒数速算法（雷神之锤3，牛B）