宏与分号

转自：宏与分号

细节决定成败！

如果留心的话，可以看到 linux内核代码经常有这么奇怪的宏定义：

引用
#define wait_event(wq, condition)                                  
do {                         
        if (condition)  
                break;
        __wait_event(wq, condition);
} while (0)

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上面的宏中，while(0) 后是没有分号的，而且这个宏的 do 里执行一次，那为什么要这么定义呢？

这是因为，在程序里使用宏时，我们按照会在语句的后面加个分号 ; ，如：
wait_event(wq,conditon);
展开后，成为：

引用
do {                         
        if (condition)  
                break;
        __wait_event(wq, condition);
} while (0);

View Code

由此可见，分号自己加上了。假如在宏定义中已在 while(0) 后添加了分号。那么对于在程序中，单独的调用这个宏，不管在宏的后面是否添加分号，那都是可以的；如果是添加了分号，那就是执行了一个空语句。

但是对于 if  ... else ... 结构，情况就不一样了，像下面的程序：

引用
#include <stdio.h>

int condition = 0;

#define wait_condition         /
do {                /
    if (condition == 10)    /
        break;        /
    condition++;        /
    sleep(1);        /
} while (0);            /

int main()
{
    int k = 1;

    if (k == 1)
        wait_condition;       /*添加了分号编译会报错*/
    else
        printf ("hello world/n");

    printf ("%d/n", condition);

    return (0);
}

View Code

编译报错

引用
[beyes@localhost programming]$ gcc -g temp.c -o temp.exe
temp.c: 在函数‘main’中:
temp.c:19: 错误：‘else’没有对应的‘if’

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如果在程序中将 wait_condition;后的分号去掉，那就可以编译通过了，但这看上去就不符合 C 语言的表达式习惯。所以，在宏定义里的 while(0) 后不需要添加分号。
然而，不管宏里加不加分号，但在程序里如下使用，也不会出现问题：

引用
{wait_condition; }

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将宏语句用大括号括起来，这样整个宏将被当成一个整体看待，所以也是正确的。

那为什么会定义 do ... while(0)  这样的形式呢？
原因是，这样做会让程序的看起来更加简洁清晰(在程序中使用大括号把一个单独的宏括起来多少总觉得有些碍眼)。

---

分割线上是原文。跟人感觉分析的不够透彻，为此写下个人感想。

上代码，每段代码分为  源码 + 编译情况 + 预处理后的.i文件。所有代码均在Linux环境下实验

正确代码

#include <stdio.h>
#include <stdlib.h>
int condition = 0;

#define wait_condition  \
do {\
    if (condition == 10)\
        break;\
    condition++;\
    sleep(1);\
} while (0)\

int main()
{
    int k = 1;
    wait_condition; 
    if (k == 1)
        wait_condition;       /*添加了分号编译会报错*/
    else
        printf ("hello world/n");
    
    printf ("%d/n", condition);
    return (0);
}

编译OK

# 1 "/usr/include/bits/stdlib-float.h" 1 3 4
# 952 "/usr/include/stdlib.h" 2 3 4
# 964 "/usr/include/stdlib.h" 3 4

# 3 "hello.c" 2
int condition = 0;
# 13 "hello.c"
int main()
{
    int k = 1;
    do { if (condition == 10) break; condition++; sleep(1);} while (0);
    if (k == 1)
        do { if (condition == 10) break; condition++; sleep(1);} while (0);
    else
        printf ("hello world/n");

    printf ("%d/n", condition);
    return (0);
}

View Code

以下均为错误代码

#include <stdio.h>
#include <stdlib.h>
int condition = 0;

#define wait_condition  \
do {\
    if (condition == 10)\
        break;\
    condition++;\
    sleep(1);\
} while (0);\

int main()
{
    int k = 1;
    wait_condition; 
    if (k == 1)
        wait_condition;       /*添加了分号编译会报错*/
    else
        printf ("hello world/n");
    
    printf ("%d/n", condition);
    return (0);
}


编译报错
hello.c: In function ‘main’:
hello.c:19:5: error: ‘else’ without a previous ‘if’
     else
     ^

# 1 "/usr/include/bits/stdlib-float.h" 1 3 4
# 952 "/usr/include/stdlib.h" 2 3 4
# 964 "/usr/include/stdlib.h" 3 4

# 3 "hello.c" 2
int condition = 0;
# 13 "hello.c"
int main()
{
    int k = 1;
    do { if (condition == 10) break; condition++; sleep(1);} while (0);;
    if (k == 1)
        do { if (condition == 10) break; condition++; sleep(1);} while (0);;
    else
        printf ("hello world/n");

    printf ("%d/n", condition);
    return (0);
}

View Code

#include <stdio.h>
#include <stdlib.h>
int condition = 0;

#define wait_condition  \
    if (condition == 10)\
        break;\
    condition++;\
    sleep(1);\

int main()
{
    int k = 1;
    wait_condition; 
    if (k == 1)
        wait_condition;       /*添加了分号编译会报错*/
    else
        printf ("hello world/n");
    
    printf ("%d/n", condition);
    return (0);
}


编译报错
hello.c: In function ‘main’:
hello.c:7:9: error: break statement not within loop or switch
         break;\
         ^
hello.c:14:5: note: in expansion of macro ‘wait_condition’
     wait_condition; 
     ^
hello.c:7:9: error: break statement not within loop or switch
         break;\
         ^
hello.c:16:9: note: in expansion of macro ‘wait_condition’
         wait_condition;       /*添加了分号编译会报错*/
         ^
hello.c:17:5: error: ‘else’ without a previous ‘if’
     els
     
# 1 "/usr/include/bits/stdlib-float.h" 1 3 4
# 952 "/usr/include/stdlib.h" 2 3 4
# 964 "/usr/include/stdlib.h" 3 4

# 3 "hello.c" 2
int condition = 0;

int main()
{
    int k = 1;
    if (condition == 10) break; condition++; sleep(1);;
    if (k == 1)
        if (condition == 10) break; condition++; sleep(1);;
    else
        printf ("hello world/n");

    printf ("%d/n", condition);
    return (0);
}

View Code

#include <stdio.h>
#include <stdlib.h>
int condition = 0;

#define wait_condition  \
    if (condition == 10)\
        break;\
    condition++;\
    sleep(1)\

int main()
{
    int k = 1;
    wait_condition; 
    if (k == 1)
        wait_condition;       /*添加了分号编译会报错*/
    else
        printf ("hello world/n");
    
    printf ("%d/n", condition);
    return (0);
}


编译报错
hello.c: In function ‘main’:
hello.c:7:9: error: break statement not within loop or switch
         break;\
         ^
hello.c:14:5: note: in expansion of macro ‘wait_condition’
     wait_condition; 
     ^
hello.c:7:9: error: break statement not within loop or switch
         break;\
         ^
hello.c:16:9: note: in expansion of macro ‘wait_condition’
         wait_condition;       /*添加了分号编译会报错*/
         ^
hello.c:17:5: error: ‘else’ without a previous ‘if’
     else
     ^

     # 1 "/usr/include/bits/stdlib-float.h" 1 3 4
# 952 "/usr/include/stdlib.h" 2 3 4
# 964 "/usr/include/stdlib.h" 3 4

# 3 "hello.c" 2
int condition = 0;

int main()
{
    int k = 1;
    if (condition == 10) break; condition++; sleep(1);
    if (k == 1)
        if (condition == 10) break; condition++; sleep(1);
    else
        printf ("hello world/n");

    printf ("%d/n", condition);
    return (0);
}

View Code

#include <stdio.h>
#include <stdlib.h>
int condition = 0;

#define wait_condition  \
    if (condition == 10)\
        return;\
    condition++;\
    sleep(1)\

int main()
{
    int k = 1;
    wait_condition; 
    if (k == 1)
        wait_condition;       /*添加了分号编译会报错*/
    else
        printf ("hello world/n");
    
    printf ("%d/n", condition);
    return (0);
}


编译报错
hello.c: In function ‘main’:
hello.c:17:5: error: ‘else’ without a previous ‘if’
     else

     
# 1 "/usr/include/bits/stdlib-float.h" 1 3 4
# 952 "/usr/include/stdlib.h" 2 3 4
# 964 "/usr/include/stdlib.h" 3 4

# 3 "hello.c" 2
int condition = 0;

int main()
{
    int k = 1;
    if (condition == 10) return; condition++; sleep(1);
    if (k == 1)
        if (condition == 10) return; condition++; sleep(1);
    else
        printf ("hello world/n");

    printf ("%d/n", condition);
    return (0);
}

View Code

#include <stdio.h>
#include <stdlib.h>
int condition = 0;

#define wait_condition  \
    if (condition == 10)\
        return;\
    condition++;\
    sleep(1);\

int main()
{
    int k = 1;
    wait_condition; 
    if (k == 1)
        wait_condition;       /*添加了分号编译会报错*/
    else
        printf ("hello world/n");
    
    printf ("%d/n", condition);
    return (0);
}


编译报错
llo.c:17:5: error: ‘else’ without a previous ‘if’
     else
     ^

# 1 "/usr/include/bits/stdlib-float.h" 1 3 4
# 952 "/usr/include/stdlib.h" 2 3 4
# 964 "/usr/include/stdlib.h" 3 4

# 3 "hello.c" 2
int condition = 0;

int main()
{
    int k = 1;
    if (condition == 10) return; condition++; sleep(1);;
    if (k == 1)
        if (condition == 10) return; condition++; sleep(1);;
    else
        printf ("hello world/n");

    printf ("%d/n", condition);
    return (0);
}

View Code

我给出的结论：

如果带参宏，宏体有判断。请一定要加上do while(0),而且不能再while（0）后面加；

 这种情况报找不到if是因为多出来的那个；造成的

单独；表示一句话，他隔开了else

 这种情况的找不到else不是由于；造成的

 从这句话开始就隔断了else。

C认为什么是语句？

①int a=10;  这种有指令内容，有分号，是一条语句。

② ；  这种也是一句话，一条语句。只不过事空语句

说白了，C判断是不是语句实际上是在查 ；