同时在一个流中使用fread和fwrite导致问题

Question

该程序想要从文件读取。该文件中的内容是字符串“Hello，world”。然后判断字符串的每个字符，以查看字符是否大于或等于常量字符'e'，如果字符符合条件，则按字母顺序将字符更改为其前一个字符（例如，'b'更改到'一'，'E'改为'D'）。最后，将更改的文件内容输出到屏幕上。

问题是，fwrite和fread是如何工作的？为什么我不能摆脱变量pos2来简化表达式。如果有人能帮忙，非常感谢！

#include <stdio.h> 

int main() 

{ 
    FILE *fp; 
    char s[20]; 
    char t[20]; 
    char transfer; 
    int i; 
    int pos; // storing the position of the file before reading from the file 
    int pos1; // check the position of the file 
    int pos2; // storing the position of the file after reading from the file 
#pragma region create a file named "Hello", write "Hello, world" into the file, close it 
    if ((fp = fopen("Hello", "wb"))== NULL) 
    { 
     printf("can't open file\n"); 
     exit(0); 
    } 
    strcpy(s, "Hello, world"); 
    fwrite(s, sizeof(char)*20, 1, fp); 
    fseek(fp, 0, SEEK_SET); 
    fclose(fp); 
#pragma endregion create a file named "Hello", write "Hello, world" into the file, close it 
#pragma region read from the file named "Hello", deal with its current, write the change into the file. 
    if ((fp = fopen("Hello", "rb+")) == NULL) 
    { 
      printf("can't open file\n"); 
      exit(1); 
    } 
    i = 0; 
    while(i < 20) 
    { 
      // 提问，该处为何不能利用fwrite的自动定位免去注释掉的语句行（即使用了pos2的语句行）。 
      // Here is the problem. since the fread and fwrite function can move the position of the 
      // file, I think I can get rid off the following commented two sentences with the 
      // variable pos2 in it 
      pos = ftell(fp);  // storing the position before reading from file 
      fread(&transfer, sizeof(char), 1, fp); // the position of the file moved to the next char 
      // pos2 = ftell(fp); // storing the position after reading from file 
      pos1 = ftell(fp); 
      if (transfer >= 'e') // if the character greater or equal to 'e' minus 1. 
      { 
       transfer -= 1; 
      } 
      fseek(fp, pos, SEEK_SET); // back to the position where the character is read to change the char 
      fwrite(&transfer, sizeof(char), 1, fp);// the position of the file moved to the next char 
      // fseek(fp, pos2, SEEK_SET); // 
      pos1 = ftell(fp); 
      i++; 
    } 
    fseek(fp, 0, SEEK_SET); 
    fclose(fp); 
#pragma endregion read from the file named "Hello", deal with its current, write the change into the file. 
#pragma region read from the file named "Hello", output the changed string 
    if((fp = fopen("Hello", "rb")) == NULL) 
    { 
      printf("Can't open file\n"); 
      exit(2); 
    } 
    fread(t, sizeof(char)*20, 1, fp); 
    printf("The output is: %s \n", t); 
// the right output is (the two sentences above with pos2 in it is commented) ： 
// The output is: Hdkkn,vnqkd 
// the wrong output is (the two sentences above with pos2 in it isn't commented): 
// The output is: Hddddddddddddddddddd烫烫烫烫Hello, world 
    fseek(fp, 0, SEEK_SET); 
    fclose(fp); 
#pragma endregion read from the file named "Hello", output the changed string 
    system("pause"); 
} 

Answer 1

我并没有真正明白为什么你想要评论这两条线。因为无论你评论他们还是评论他们，什么都不会改变。你已经摆脱了你的代码中的pos2（这是你问的）。

所以，如果你使用下面的代码为while循环

pos = ftell(fp);  // storing the position before reading from file 
fread(&transfer, sizeof(char), 1, fp); 
pos1 = ftell(fp); 
if (transfer >= 'e') // if the character greater or equal to 'e' minus 1. 
{ 
    transfer -= 1; 
} 
fseek(fp, pos, SEEK_SET); 
fwrite(&transfer, sizeof(char), 1, fp); 
i++; 

然后你拿“的输出是：Hdkkn，vnqkd”，这是预期的结果。

你也可以从文件中的每一行读取数组并对其进行操作，然后将其写回到文件中。通过这种方式，它可以是更通用的，你不必使用像“20”这样的幻数。

编辑：

我用gcc 4.5.2在我的Linux平台。我不想在其他平台上发表评论，但正如我之前提到的，您可以将该行放入缓冲区，然后在操作后将其写回。你可以尝试用你的while循环替换下面的代码：

char line[20] = {0}; 

fread(line, sizeof(char), 20, fp); 

for(i = 0; i < strlen(line); i++) 
{ 
    if(line[i] >= 'e') 
     line[i] -= 1; 
} 

fseek(fp, 0, SEEK_SET); 
fwrite(line, sizeof(char), strlen(line), fp); 

通过这种方式，你可以摆脱许多变量。这是你的选择。