Overview

• Preliminaries.
  • Admin.
  • Upcoming Work.
  • Extra Credit.
  • Questions.
• Sorting, revisited.
  • Function pointers.
• Testing C programs.
  • Testing with macros.
  • Testing through the command line.
  • Testing with assert.
• Memory issues in C programs.
  • Checking memory problems with valgrind.

Preliminaries

Admin

• Homework: Find or write a program with interesting memory errors and show how valgrind helps you identify them.
• Cool CS talk today at 4:30 p.m.
• Alumni talk tonight about social media.

Questions

Sorting, revisited

In Scheme

    ;;; Procedure:
    ;;;   sort
    ;;; Parameters:
    ;;;   lst, a list (or perhaps vec, a vector)
    ;;;   order, a two-parameter predicate that represents
    ;;;     a total order.  ("Does a come before b?")

In Java, we also think about providing a Comparator (or using Comparables)

What about C? What's your signature for a procedure that sorts an array of strings?

• Question: Do we sort "in place" or do we return a sorted array?

  /**
   * Sort strings.  Return 1 upon success and 0 upon failure.
   */
  int
  sort (char *strings[], int len)
  {
  } // sort
  

Function pointers

• Can we make this take a comparator?

  /**
   * Sort strings.  Return 1 upon success and 0 upon failure.
   */
  int
  sort (char *strings[], int len, int (*compare)(char *x, char *y))
  {
  } // sort
  

• This says "compare is a function that takes two strings as input and returns an integer."

• The parentheses are important!

• Many novice C programmers find this hard to read. Sometimes typedefs are useful.

  /**
   * Functions that return negative if x comes before y, 0 if
   * x "equals" y, and pos if x is greater than y.
   */
  typedef int (*Comparator)(char *x, char *y);
  
  
  int
  sort (char *strings[], int len, Comparator compare)
  {
  } // sort
  

• How do we call sort?

  int 
  compare_by_length (char *x, char *y)
  {
    return strlen (x) - strlen (y);
  } // compare_by_length
  
  
  sort (strings, adfas, compare_by_length);
  

Memory issues in C programs

• Everyone gets segfaults.
• How do you find/fix them?
  • Run gdb, look at a stack trace
  • Unfortunately, not all memory errors happen where the program crashes.
    • Free earlier and use later. (The error may be in the free.)
    • Using freed memory, and only sometimes that fails.

Checking memory problems with valgrind

• Approximate demos.

Testing C programs

Testing with macros

Testing through the command line

Testing with assert