29-保存结构的程序示例

14.8.1　保存结构的程序示例

为了演示如何在程序中使用这些函数，我们把程序清单14.2修改为一个新的版本（即程序清单14.14），把书名保存在 book.dat 文件中。如果该文件已存在，程序将显示它当前的内容，然后允许在文件中添加内容（如果你使用的是早期的Borland编译器，请参阅程序清单14.2后面的“Borland C和浮点数”）。

程序清单14.14　 booksave.c 程序

/* booksave.c -- 在文件中保存结构中的内容 */
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define MAXTITL  40
#define MAXAUTL  40
#define MAXBKS   10             /* 最大书籍数量 */
char * s_gets(char * st, int n);
struct book {                   /* 建立 book 模板 */
     char title[MAXTITL];
     char author[MAXAUTL];
     float value;
};
int main(void)
{
     struct book library[MAXBKS]; /* 结构数组  */
     int count = 0;
     int index, filecount;
     FILE * pbooks;
     int size = sizeof(struct book);
     if ((pbooks = fopen("book.dat", "a+b")) == NULL)
     {
          fputs("Can't open book.dat file\n", stderr);
          exit(1);
     }
     rewind(pbooks);            /* 定位到文件开始 */
     while (count < MAXBKS &&  fread(&library[count], size,
          1, pbooks) == 1)
     {
          if (count == 0)
               puts("Current contents of book.dat:");
          printf("%s by %s: $%.2f\n", library[count].title,
               library[count].author, library[count].value);
          count++;
     }
     filecount = count;
     if (count == MAXBKS)
     {
          fputs("The book.dat file is full.", stderr);
          exit(2);
     }
     puts("Please add new book titles.");
     puts("Press [enter] at the start of a line to stop.");
     while (count < MAXBKS && s_gets(library[count].title, MAXTITL) != NULL
          && library[count].title[0] != '\0')
     {
          puts("Now enter the author.");
          s_gets(library[count].author, MAXAUTL);
          puts("Now enter the value.");
          scanf("%f", &library[count++].value);
          while (getchar() != '\n')
               continue;            /* 清理输入行  */
          if (count < MAXBKS)
               puts("Enter the next title.");
     }
     if (count > 0)
     {
          puts("Here is the list of your books:");
          for (index = 0; index < count; index++)
               printf("%s by %s: $%.2f\n", library[index].title,
               library[index].author, library[index].value);
          fwrite(&library[filecount], size, count - filecount,
               pbooks);
     }
     else
          puts("No books? Too bad.\n");
     puts("Bye.\n");
     fclose(pbooks);
     return 0;
}
char * s_gets(char * st, int n)
{
     char * ret_val;
     char * find;
     ret_val = fgets(st, n, stdin);
     if (ret_val)
     {
          find = strchr(st, '\n');    // 查找换行符
          if (find)                // 如果地址不是 NULL，
               *find = '\0';            // 在此处放置一个空字符
          else
               while (getchar() != '\n')
                    continue;        // 清理输入行
     }
     return ret_val;
}

我们先看几个运行示例，然后再讨论程序中的要点。

$ ./booksave
Please add new book titles.
Press [enter] at the start of a line to stop.
Metric Merriment
Now enter the author.
Polly Poetica
Now enter the value.
18.99
Enter the next title.
Deadly Farce
Now enter the author.
Dudley Forse
Now enter the value.
15.99
Enter the next title.
[enter]
Here is the list of your books:
Metric Merriment by Polly Poetica: $18.99
Deadly Farce by Dudley Forse: $15.99
Bye.
$ ./booksave
Current contents of book.dat:
Metric Merriment by Polly Poetica: $18.99
Deadly Farce by Dudley Forse: $15.99
Please add new book titles.
Press [enter] at the start of a line to stop.
The Third Jar
Now enter the author.
Nellie Nostrum
Now enter the value.
22.99
Enter the next title.
[enter]
Here is the list of your books:
Metric Merriment by Polly Poetica: $18.99
Deadly Farce by Dudley Forse: $15.99
The Third Jar by Nellie Nostrum: $22.99
Bye.
$

再次运行 booksave.c 程序把这3本书作为当前的文件记录打印出来。