vi first.c  //linux新建文件

 #include<stdio.h>

 int main() {
     printf("welcome to Linux ,this is the first C program!");
     ;
 }

 编译；gcc -o first first.c //linux编译文件
 执行: ./first //linux执行文件

 vi second.c

 #include<stdio.h>
 int main() {
     int x , y , sum ;
     x =  ;
     y =  ;
     sum = x + y ;
     printf("sum is %d",sum);
     ;
 }
 //编译执行命令与第一支C程序相同。

 #include<stdio.h>

 int main() {
     short int i;
     int j;
     long int k;
     int a,b,c;
     a = sizeof(i);
     b = sizeof(j);
     c = sizeof(k);    

     printf("a is %d\n",a);
     printf("b is %d\n",b);
     printf("c is %d\n",c);

     //return 0; don't write return is OK?
 }
 output :
 a
 b
 c 

 #include<stdio.h>

 int main(){
         float i ;
         double j;
         int a , b ;
         a = sizeof(i);
         b = sizeof(j);
         printf("a is %d \n b is %d \n",a , b);// bit number

         float c = 88888.88888;
         double d = 88888888888.88888888;
         printf("c is %f \n d is %f \n",c,d);//%f小数最多输出六位

 }

 output:
 a
 b
 c is 88888.890625 //i是单精度浮点数，有效位数为7，整数占据5位，小数占2位，第二位位四舍五入结果，后面均为无效数字
 d is 88888888888.888885//j双精度，有效16位，整数占11位，小数占5位，后面为无效数。

 //C语言字符用''单引号:eg : 'A'
 //转义字符：\n,换行，相当于enter
 //          \t,跳到下一个tab位置，相当于tab键
 //                    \b,退格，将当前位置移到前一列，相当于backspace
 //                    \\,反斜杠字符
 //                    \‘,单引号字符
 //                    \",双引号字符
 //                    \0,空字符，用在字符串中
 //                    \ddd,一到三位8进制代表的字符，如\101代表字符A
 //                    \xhh,1到2位十六进制代表的字符，如\x41代表字符A
 //字符变量定义：char c1 , c2 = 'A'; 占1字节,8bit,   '\n'是一个转义字符

 #include<stdio.h>

 int main(){
     int c1 ,c2 ;
     char c3;
     printf("c3 is %d \n",sizeof(c3));

     c1 = 'a' - 'A';
     c2 = 'b' - 'B';
     c3 = ;

     printf("c1 is %d and c2 is %d \n" , c1 ,c2);
     printf("c3 is %d and %c \n",c3,c3);
     ;
 }

 output:
 c3
 c1  and c2
 c3  and C