Why extra 'ÿ' added when writing into a .txt file in C?

Question

I searched about this problem everywhere, but none of the suggested solutions worked for me.

char currentChar;
FILE *fp_read = fopen("../input.txt", "r");
FILE *fp_write = fopen("../textArranged.txt", "w");
while (!feof(fp_read)){
      currentChar = fgetc(fp_read);
      ...
}

I tried to change the while condition (using getc()), but it didn't work.

Answer 1

feof() seems to return 0 after reading the last byte of the file. It returns 1 after fgetc() already made the attempt to read one more byte after the end of the file.

When fgetc() makes the attempt to read data after the end of the file, fgetc() returns -1.

If you perform fputc(x, ...) and x is not in the range 0...255, fputc() will actually write the byte (x & 0xFF).

On nearly all modern computers, (-1 & 0xFF) is 0xFF which equals the character 'ÿ'.

So the following happens:

• ...
• Your program reads the last byte of the file using fgetc()
• It writes that character using fputc()
• Although there are no more bytes left in the file, feof() returns 0 because you did not make the attempt to read bytes after the end of the file, yet.
• Your program calls fgetc() and because there are no more bytes left, fgetc() returns -1.
• Your program calls fputc(-1, ...) which writes the character 'ÿ'.
• feof() returns 1 because fgetc() already tried to read bytes after the end of the file.