C语言常用方法技巧
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))
