我在此程序中得到了意外的输出

Question

I just want to replace specific character from file. for example, I want to replace character 'l' with character 'p'. is it correct way ?

int main() {
  FILE * ptr;
  ptr = fopen("D:\f4.txt", "r+");

  if (ptr == NULL) {
    printf("file cant be opened");
    exit(0);
  }

  char ch = fgetc(ptr);
  while (ch != EOF) {
    if (ch == 'l') {
      fseek(ptr, -1, 1);
      fputc('p', ptr);
    }

    ch = fgetc(ptr);
  }

  fclose(ptr);
}

suppose content in my file is "hello everyone" so output should be like "heppo everyone" but it writes in file "hepepepepepepepepepepepepepepep" continuesly. please help me to find why this happen.

Answer 1

Please note this from the man page for fopen() .

When the "r+" , "w+" , or "a+" access type is specified, both reading and writing are enabled (the file is said to be open for "update"). However, when you switch from reading to writing, the input operation must encounter an EOF marker. If there is no EOF, you must use an intervening call to a file positioning function. The file positioning functions are fsetpos , fseek , and rewind . When you switch from writing to reading, you must use an intervening call to either fflush or to a file positioning function. (my italics)

So after you wrote 'p' to the file, it is not enough to carry on reading as though nothing has happened, you must fseek to the original position, obtained by ftell , or fflush the file.

Also don't use magic numbers: in fseek you should use SEEK_CUR not 1 .

Finally, function fgetc returns an int type, not char . This allows EOF to be distinguished from the data byte 0xFF .