C语言条件编译详解
预处理程序提供了条件编译的功能。可以按不同的条件去编译不同的程序部分，因而产生不同的目标代码文件。这对于程序的移植和调试是很有用的。条件编译有三种形式，下面分别介绍。
第一种形式
第一种形式的格式为：
#ifdef 标识符
程序段1
#else
程序段2
#endif
它的功能是，如果标识符已被 #define命令定义过则对程序段1进行编译；否则对程序段2进行编译。如果没有程序段2（它为空），本格式中的#else可以没有，即可以写为：
#ifdef 标识符
程序段
#endif
【例9-12】
- #include <stdio.h>
- #define NUM ok
- int main(void){
- struct stu{
- int num;
- char *name;
- char sex;
- float score;
- } *ps;
- ps=(struct stu*)malloc(sizeof(struct stu));
- ps->num=102;
- ps->name="Zhang ping";
- ps->sex=‘M‘;
- ps->score=62.5;
- #ifdef NUM
- printf("Number=%d\nScore=%f\n",ps->num,ps->score);
- #else
- printf("Name=%s\nSex=%c\n",ps->name,ps->sex);
- #endif
- free(ps);
- return 0;
- }
由于在程序的第14行插入了条件编译预处理命令，因此要根据NUM是否被定义过来决定编译那一个printf语句。而在程序的第一行已对NUM作过宏定义，因此应对第一个printf语句作编译故运行结果是输出了学号和成绩。
在程序的第1行宏定义中，定义NUM表示字符串OK，其实也可以为任何字符串，甚至不给出任何字符串，写为：
- #define NUM
也具有同样的意义。只有取消程序的第1行才会去编译第二个printf语句。读者可上机试作。
第二种形式
第二种形式的格式为：
#ifndef 标识符
程序段1
#else
程序段2
#endif
与第一种形式的区别是将“ifdef”改为“ifndef”。它的功能是，如果标识符未被#define命令定义过则对程序段1进行编译，否则对程序段2进行编译。这与第一种形式的功能正相反。
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第三种形式
第三种形式的格式为：
#if 常量表达式
程序段1
#else
程序段2
#endif
它的功能是，如常量表达式的值为真（非0），则对程序段1 进行编译，否则对程序段2进行编译。因此可以使程序在不同条件下，完成不同的功能。
【例9-13】
- #include <stdio.h>
- #define R 1
- int main(void){
- float c,r,s;
- printf ("input a number: ");
- scanf("%f",&c);
- #if R
- r=3.14159*c*c;
- printf("area of round is: %f\n",r);
- #else
- s=c*c;
- printf("area of square is: %f\n",s);
- #endif
- return 0;
- }
本例中采用了第三种形式的条件编译。在程序第1行宏定义中，定义R为1，因此在条件编译时，常量表达式的值为真，故计算并输出圆面积。
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