同時在一個流中使用fread和fwrite導致問題

Question

該程序想要從文件讀取。該文件中的內容是字符串「Hello，world」。然後判斷字符串的每個字符，以查看字符是否大於或等於常量字符'e'，如果字符符合條件，則按字母順序將字符更改爲其前一個字符（例如，'b'更改到'一'，'E'改爲'D'）。最後，將更改的文件內容輸出到屏幕上。

問題是，fwrite和fread是如何工作的？爲什麼我不能擺脫變量pos2來簡化表達式。如果有人能幫忙，非常感謝！

#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) 
    { 
      // 提問，該處爲何不能利用fwrite的自動定位免去註釋掉的語句行（即使用了pos2的語句行）。 
      // 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燙燙燙燙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

我並沒有真正明白爲什麼你想要評論這兩條線。因爲無論你評論他們還是評論他們，什麼都不會改變。你已經擺脫了你的代碼中的pos2（這是你問的）。

所以，如果你使用下面的代碼爲while循環

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++; 

然後你拿「的輸出是：Hdkkn，vnqkd」，這是預期的結果。

你也可以從文件中的每一行讀取數組並對其進行操作，然後將其寫回到文件中。通過這種方式，它可以是更通用的，你不必使用像「20」這樣的幻數。

編輯：

我用gcc 4.5.2在我的Linux平臺。我不想在其他平臺上發表評論，但正如我之前提到的，您可以將該行放入緩衝區，然後在操作後將其寫回。你可以嘗試用你的while循環替換下面的代碼：

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); 

通過這種方式，你可以擺脫許多變量。這是你的選擇。