C语法简单测试

1、未初始化的枚举变量

 /* uninitialized-enum.c */
 #include <stdio.h>

 enum color{white = , black, blue};

 int main(void)
 {
     enum color bird;

     printf("bird = %d\n", bird);

     return ;
 }

输出结果：

$ ./uninitialized-enum
bird = 2130567168
$

2、十六进制数与switch语句测试

 /* test.cpp */
 #include <iostream>

 using namespace std;

 enum color{green, yellow, red, blue, black};

 int main(void)
 {
     int i = 0x3e - ;

     cout << "i = " << i << endl;

     color bird = blue;
     switch(bird){
         case green:
         break;
         case yellow:
         break;
         cout << "Hello, world!" << endl;
         case red:
         break;
         cout << "Hello, world!" << endl;
         case blue:
         cout << "Yes, the color of this bird is blue!" << endl;
         break;
         default:
         cout << "No, byebye!" << endl;
         break;
     }

     return ;
 }

输出结果：

$ ./test
i = 61
Yes, the color of this bird is blue!
$

3、sizeof关键字和处理器的字节序

 #include <stdio.h>

 int main(void)
 {
     char str[] = { 0x01, 0x00, 0x00, 0x00, 0x04, 0x01, 0x06, 0x00, 0x01, 0x02, 0x03, 0x04, 0x10, 0x10 };
     unsigned ret, lid;

     ret = sizeof str;
     printf("ret = %d\n", ret);
     lid = *(unsigned *)(str+);
     printf("lid = %d\n", lid);

     return ;
 }

备注：Intel Core处理器的字节序是小端序。

输出结果：

$ ./test
ret = 14
lid = 393476 (0x00, 0x06, 0x01, 0x04)
$

4、C语言中的整数

 #include <stdio.h>

 int main(void)
 {
     int i = , value;

     if((i << ) == 0x0138)
         printf("YES!\n");
     else
         printf("NO!\n");

     value = i + 0x0138;
     printf("value = %d\n", value);

     return ;
 }

输出结果：

$ ./test
YES!
value = 390
$

从输出结果可以看出，十进制数据与十六进制数据可以直接进行运算，而不需要经过转换。

另外，C语言中的整数有多种形式，二进制数、八进制数、十进制数、十六进制数、字符（'A'）都是整数，可以使用任何一种形式，或者在任何形式的整数之间进行整数运算。

5、循环结构、switch结构中的break语句

 #include <stdio.h>

 int main(void)
 {
     enum color {yellow, green, red, black};
     enum color bird = red;

     while(){

         switch(bird){
         case yellow:
             printf("the bird is yellow!\n");
             break;
         case green:
             printf("the bird is green!\n");
             break;
         case red:
             printf("the bird is red!\n");
             break;
         default:
             printf("the bird is black!\n");
             break;
         }

         printf("use a break to get out of while!\n");
         break;
     }

     printf("we are now at outside of while loop!\n");

     return ;
 }

输出结果：

$ ./test
the bird is red!
use a break to get out of while!
we are now at outside of while loop!
$

从输出结果可以看出：switch中的break只对switch结构有用，对更外层的while循环无用；while循环中break语句可以跳出while循环。

因此，我们有如下结论：

1、switch结构并非循环结构，遇到break则不再执行该结构中剩下的语句；
2、break语句可以跳出当前的循环结构（一层）。

6、结构体的存储空间

 #include <stdio.h>

 typedef struct {
     unsigned            request;
     unsigned            len;
     unsigned char       data[];
 }rms_message_t;

 int main(void)
 {
     printf("sizeof(unsigned) = %d\n", sizeof(unsigned));
     printf("sizeof(unsigned char) = %d\n", sizeof(unsigned char));
     printf("sizeof(rms_message_t) = %d\n", sizeof(rms_message_t));

     return ;
 }

输出结果：

$ ./test
sizeof(unsigned) = 4
sizeof(unsigned char) = 1
sizeof(rms_message_t) = 8
$