1、未初始化的枚举变量
1 /* uninitialized-enum.c */ 2 #include <stdio.h> 3 4 enum color{white = 1, black, blue}; 5 6 int main(void) 7 { 8 enum color bird; 9 10 printf("bird = %d ", bird); 11 12 return 0; 13 }
输出结果：
$ ./uninitialized-enum bird = 2130567168 $
2、十六进制数与switch语句测试
1 /* test.cpp */ 2 #include <iostream> 3 4 using namespace std; 5 6 enum color{green, yellow, red, blue, black}; 7 8 int main(void) 9 { 10 int i = 0x3e - 1; 11 12 cout << "i = " << i << endl; 13 14 color bird = blue; 15 switch(bird){ 16 case green: 17 break; 18 case yellow: 19 break; 20 cout << "Hello, world!" << endl; 21 case red: 22 break; 23 cout << "Hello, world!" << endl; 24 case blue: 25 cout << "Yes, the color of this bird is blue!" << endl; 26 break; 27 default: 28 cout << "No, byebye!" << endl; 29 break; 30 } 31 32 return 0; 33 }
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
1 #include <stdio.h> 2 3 int main(void) 4 { 5 char str[] = { 0x01, 0x00, 0x00, 0x00, 0x04, 0x01, 0x06, 0x00, 0x01, 0x02, 0x03, 0x04, 0x10, 0x10 }; 6 unsigned ret, lid; 7 8 ret = sizeof str; 9 printf("ret = %d ", ret); 10 lid = *(unsigned *)(str+4); 11 printf("lid = %d ", lid); 12 13 return 0; 14 }
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
1 #include <stdio.h> 2 3 int main(void) 4 { 5 int i = 78, value; 6 7 if((i << 2) == 0x0138) 8 printf("YES! "); 9 else 10 printf("NO! "); 11 12 value = i + 0x0138; 13 printf("value = %d ", value); 14 15 return 0; 16 }
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
1 #include <stdio.h> 2 3 int main(void) 4 { 5 enum color {yellow, green, red, black}; 6 enum color bird = red; 7 8 while(1){ 9 10 switch(bird){ 11 case yellow: 12 printf("the bird is yellow! "); 13 break; 14 case green: 15 printf("the bird is green! "); 16 break; 17 case red: 18 printf("the bird is red! "); 19 break; 20 default: 21 printf("the bird is black! "); 22 break; 23 } 24 25 printf("use a break to get out of while! "); 26 break; 27 } 28 29 printf("we are now at outside of while loop! "); 30 31 return 0; 32 }
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
1 #include <stdio.h> 2 3 typedef struct { 4 unsigned request; 5 unsigned len; 6 unsigned char data[0]; 7 }rms_message_t; 8 9 int main(void) 10 { 11 printf("sizeof(unsigned) = %d ", sizeof(unsigned)); 12 printf("sizeof(unsigned char) = %d ", sizeof(unsigned char)); 13 printf("sizeof(rms_message_t) = %d ", sizeof(rms_message_t)); 14 15 return 0; 16 }
输出结果：
$ ./test sizeof(unsigned) = 4 sizeof(unsigned char) = 1 sizeof(rms_message_t) = 8 $