课前：一些样例 ¶

把讲义里的小程序和命令行跑一跑，思考一下留下的问题～

1. 重定向运算符

   echo_eof.c

   /* echo_eof.c -- repeats input to end of file */
   #include <stdio.h>
   int main(void)
   {
       int ch;
   
       while ((ch = getchar()) != EOF)
       putchar(ch);
   
        return 0;
   }
   

   echo_eof < echo_eof.c
   echo_eof < echo_eof.c > echo_eof.txt
   echo_eof < echo_eof.c | cat
   

2. 关于格式化字符串

   printf.c

   #include <stdio.h>
   
   int main()
   {
       // 类型
       printf("%5d\n", 1000);  // 默认右对齐，左边补空格
       printf("%-5d\n", 1000); // 左对齐，右边补空格
       printf("%+d %+d\n", 1000, -1000); // 输出正负号
       printf("% d % d\n", 1000, -1000); // 正号用空格替代，负号输出
       printf("%x %#x\n", 1000, 1000); // 输出 0x
       printf("%.0f %#.0f\n", 1000.0, 1000.0); // 当小数点后不输出值时依然输出小数点
       printf("%g %#g\n", 1000.0, 1000.0); // 保留小数点后后的 0
       printf("%05d\n", 1000); // 前面补 0
   
       // 宽度
       double a = 3.141592653589;
       int x = 5;
       printf("%.*f", x, a);
   
       // 精度
       printf("%.8d\n", 1000);           // 不足指定宽度补前导 0，效果等同于%06d
       printf("%.8f\n", 1000.123456789); // 超过精度，截断
       printf("%.8f\n", 1000.123456);    // 不足精度，补后置 0
       printf("%.8g\n", 1000.123456);    // 最大有效数字为 8 位
       printf("%.8s\n", "abcdefghij");   // 超过指定长度截断
   
       // 类型长度
       printf("%hhd\n", 'A');               // 输出有符号 char
       printf("%hhu\n", 'A' + 128);         // 输出无符号 char
       printf("%hd\n", 32767);              // 输出有符号短整型 short int
       printf("%hu\n", 65535);              // 输出无符号短整型 unsigned short int
       printf("%ld\n", 0x7fffffffffffffff); // 输出有符号长整型 long int
       printf("%lu\n", 0xffffffffffffffff); // 输出有符号长整型 unsigned long int
   }
   

3. 关于printf()

   wrong_cnv.c

   #include <stdio.h>
   #define num 336
   #define b 65618
   int main(void)
   {
       printf("num as short and unsigned short:  %hd %hu\n", num,num);
       printf("-num as short and unsigned short: %hd %hu\n", -mnum,-mnum);
       printf("num as int and char: %d %c\n", num, num);
       printf("b as int, short, and char: %d %hd %c\n",b, b, b);
   
       float n1 = 3.0;
       double n2 = 3.0;
       long n3 = 2000000000;
       long n4 = 1234567890;
       printf("%.1e %.1e %.1e %.1e\n", n1, n2, n3, n4);
       printf("%ld %ld\n", n3, n4);
       printf("%ld %ld %ld %ld\n", n1, n2, n3, n4);
       return 0;
   }
   

   该程序的输出结果为 (由于实现是未定义的，部分结果因系统和编译器而异)：

   num as short and unsigned short:  336 336
   -num as short and unsigned short: -336 65200
   num as int and char: 336 P
   b as int, short, and char: 65618 336 R
   3.0e+00 3.0e+00 2.0e+09 1.2e+09
   2000000000 1234567890
   0 1074266112 0 1074266112
   

   你能解释为什么使用 %ld 输出 long 类型的值时，也会出现错误吗？

4. 关于scanf()

   confusing_scanf.c

   #include <stdio.h>
   int main(void)
   {
       char c[100] = {0};
       scanf("%s\n",&c);
       printf("%s",c);
   }
   

   这个程序为什么这么奇怪？

5. 文件

   c_file.c

   #include<stdio.h>
   #include<string.h>
   
   int main(int argc, char *argv[]){
       const char *str = "ckc-agc programming lec3";
   
       FILE *fp1 = fopen("test1.txt", "wb+");
       FILE *fp2 = fopen("test2.txt", "w+");
       fprintf(fp1, "%s\n%s\n", str, argv[0]);
       fprintf(fp2, "%s\n%s\n", str, argv[0]);
       fwrite(str, sizeof(char), strlen(str), fp1);
   
       // fseek(fp1, 0, SEEK_SET);
       // fprintf(fp1, "%s\n", "SEEK_SET");
       fseek(fp1, 0, SEEK_END);
       fprintf(fp1, "%s\n", "SEEK_END");
   
       rewind(fp1);
       fprintf(fp1, "%s\n", "rewind");
   
       fclose(fp1);
       fclose(fp2);
   }
   

   file c_file.c
   file c_file.o
   file c_file
   objdump -h -s -d c_file.o
   objdump -h -s c_file