C語言條件編譯詳解

標籤：

預先處理程式提供了條件編譯的功能。可以按不同的條件去編譯不同的程式部分，因而產生不同的目標代碼檔案。這對於程式的移植和調試是很有用的。條件編譯有三種形式，下面分別介紹。

第一種形式

第一種形式的格式為：
    #ifdef  標識符
        程式段1
    #else
        程式段2
    #endif

它的功能是，如果標識符已被 #define命令定義過則對程式段1進行編譯；否則對程式段2進行編譯。如果沒有程式段2（它為空白），本格式中的#else可以沒有，即可以寫為：
    #ifdef  標識符
        程式段
    #endif

【例9-12】

1. #include <stdio.h>
2. #define NUM ok
3. int main(void){
4. struct stu{
5. int num;
6. char *name;
7. char sex;
8. float score;
9. } *ps;
10. ps=(struct stu*)malloc(sizeof(struct stu));
11. ps->num=102;
12. ps->name="Zhang ping";
13. ps->sex=‘M‘;
14. ps->score=62.5;
15. #ifdef NUM
16. printf("Number=%d\nScore=%f\n",ps->num,ps->score);
17. #else
18. printf("Name=%s\nSex=%c\n",ps->name,ps->sex);
19. #endif
20. free(ps);
21. return 0;
22. }

由於在程式的第14行插入了條件編譯預先處理命令，因此要根據NUM是否被定義過來決定編譯那一個printf語句。而在程式的第一行已對NUM作過宏定義，因此應對第一個printf語句作編譯故運行結果是輸出了學號和成績。

在程式的第1行宏定義中，定義NUM表示字串OK，其實也可以為任何字串，甚至不給出任何字串，寫為：

1. #define NUM

也具有同樣的意義。只有取消程式的第1行才會去編譯第二個printf語句。讀者可上機試作。

第二種形式

第二種形式的格式為：
    #ifndef 標識符
        程式段1 
    #else 
        程式段2 
    #endif
與第一種形式的區別是將“ifdef”改為“ifndef”。它的功能是，如果標識符未被#define命令定義過則對程式段1進行編譯，否則對程式段2進行編譯。這與第一種形式的功能正相反。

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第三種形式

第三種形式的格式為：
    #if 常量運算式
        程式段1
    #else 
        程式段2
    #endif
它的功能是，如常量運算式的值為真（非0），則對程式段1 進行編譯，否則對程式段2進行編譯。因此可以使程式在不同條件下，完成不同的功能。

【例9-13】

1. #include <stdio.h>
2. #define R 1
3. int main(void){
4. float c,r,s;
5. printf ("input a number: ");
6. scanf("%f",&c);
7. #if R
8. r=3.14159*c*c;
9. printf("area of round is: %f\n",r);
10. #else
11. s=c*c;
12. printf("area of square is: %f\n",s);
13. #endif
14. return 0;
15. }

本例中採用了第三種形式的條件編譯。在程式第1行宏定義中，定義R為1，因此在條件編譯時間，常量運算式的值為真，故計算並輸出圓面積。

C語言條件編譯詳解