一起来看看C语言的预处理注意点

#ifndef     __DEMO_H_
#define    __DEMO_H_


#define PAI 3.14


#endif // __DEMO_H_

#include <stdio.h>
#include "demo.h"

/* 在 #include 与 main 函数中间区域使用#undef来取消以定义的宏，然后重新定义宏 */

#undef PAI
#define PAI 33.14


int main()
{
    printf("PAI = %G\n", PAI);
}

/*
输出结果
PAI = 33.14
*/

#ifdef DEBUG

/* 你的调试代码 */

#endif // DEBUG

#include <stdio.h>


#define PAI  \
                3.1415926

int main()
{
    printf("%.8f\n", PAI);
}

#include <stdio.h>


#define PAI 3.1415926
//字符串常量化
#define CONVERT_TO_STR1(arg) "字符串常量化：" #arg
//字符串常量化
#define CONVERT_TO_STR2(arg) CONVERT_TO_STR1(arg)

int main()
{
    printf("%s\n", CONVERT_TO_STR1(ABC));
    printf("%s\n", CONVERT_TO_STR1(PAI));
    printf("%s\n", CONVERT_TO_STR2(PAI));
}

/*
输出结果
字符串常量化：ABC
字符串常量化：PAI
字符串常量化：3.1415926
*/

#include <stdio.h>


#define PRINTF1(arg) printf("var" #arg "  = %d\n" , var##arg)

int main()
{
    int var1 = 20;
    PRINTF1(1);
}

#define MAX(a,b) ((a)>(b)?(a):(b))

#include <stdio.h>


#define MIN(A,B) ((A)<(B)?(A):(B))

int main()
{
    char *p="ghi";
    char a;
    a=MIN(*p++,'f');//不要使用自增/自减运算符
    printf("%c\n",a);
}

#define SWAP(a, b, size)    \
    unsigned int __size = (size);    \
    char *__a = (a), *__b = (b);    \
    do    \
    {    \
        char __tmp = *__a;    \
        *__a++ = *__b;    \
        *__b++ = __tmp;    \
    } while (--__size > 0);