Class 28: Files, Contined

Held: Friday, 7 March 2003

Summary: Today we continue our investigation into files in C.

Related Pages:

• EBoard.
• Lab on files.

Due

• Homework 5.

Notes:

• I am on my way to Chicago. I'll be back Sunday. Mr. Gum and Mr. Brantley will sub.
• Send me email if you have questions on the exam.
• Amazingly enough, you have no reading for Monday (although you might want to start reading ahead in Gries).
• Mr. Walker says that he doesn't recognize the readLine procedure that you're going to critique today. It may be that he wrote it when he first started learning C. It may be that he reused it from someone else. He suggests that you use it as an example of why to be careful about legacy code.

Overview:

• What's wrong with readLine?
• What's wrong with the display procedure?
• Lab.

A Flawed readLine

• Here's a flawed version of readLine.

  void readLine(FILE *cityFile, char line[MAX]) {
  /* This procedures reads a line from the given file */
    int i = 0;
    if (fscanf(cityFile, "%c", &line[i]) == EOF) {
      line[0] = EOF;
      return;
    }
  
    while (line[i] != '\n') {
      i++;
      fscanf(cityFile, "%c", &line[i]);
    }
    line[i] = 0;
  }
  

• What's wrong with it? Many things. Here are some of my initial lists.
• Four significant problems:
  • line[i] = getc(cityFile) is a more standard way of reading a character (and is more efficient).
  • EOF is not a character, and shouldn't be assigned to a character variable (line[0]).
  • No attempt is made to prevent buffer overflow.
  • Very strange things can happen if the EOF character appears in the middle of the line. A few minor problems:
  • (char *) 0 is the end-of-string character, not 0.
  • In the documentation, "This procedures" should be "This procedure".
  • By tradition, I/O functions return some sensible value (e.g., 1 for success and 0 for failure or #of characters read for success and EOF for failure).

Lab

• Do the lab.
• Be prepared to reflect.