Overview

• Preliminaries.
  • Admin.
  • Questions.
• Memory issues in C programs.
• Checking memory problems with valgrind.
• Sorting, revisited.
• Function pointers.

Preliminaries

Admin

• I still need judges for the CSC 207 students Monday at 10:00 a.m.
• If you are going to be taking classes on campus this fall and you have not registered, do so ASAP!
• Congrats to AK!
• In honor of the Work/Life Balance Town Hall, there is no homework for next next week.
• Next week may be our last week of this class. We plan to have the CS senior celebration on the last Thursday of the semester.

Questions

Memory issues in C programs

• Great aspect of C: Memory is a first-class value.
• Also a huge problem; many program errors have to deal with failure to use memory correctly.

• How do programmers use memory incorrectly?

  • Allocated less than actually gets used. Uses more than allocated.
  • Typically allocated with malloc.
  • Typically used with arrays - outside bounds of arrays.

  • Allocating space for a data type, and use the wrong amount of space.

    double *d = malloc(2);

  • Run out of memory

  • Attempt to write your own alternative to malloc and screw it up.

  • Problems with freeing memory

    • You don't do it - Memory leak

    • You do it twice

      int *ip = ...;
      int *jp = ip;
      ...
      free (ip);
      free (jp);
      

• Memory issues sometimes have indirect effects

Checking memory problems with valgrind

• Catches a lot of different errors related to bad use of malloc (or allocation on the stack)

Sorting, revisited

Here's a standard signature

int strings_sort (int n, char *strings[]);

But wait! How do we specify what we want to sort by?

int strings_sort (int n, char *strings[], THING-TO-COMPARE-ELEMENTS compare);

Function pointers

int strings_sort (int n, char strings[], int (compare)(char *, char *))

How do we pass in a function that meets this signature?

• Just type the name of the function

  strings_sort (10, names, strcmp);
  
  
  int compare_by_length (char *str1, char *str2)
  {
    if (strlen(str1) > strlen(str2))
      return 1;
    else if (strlen(str1) < strlen(str2))
      return -1;
    else
      return 0;
  }
  
  
  strings_sort (10, names, compare_by_length);
  

Making the type name a bit easier

    typedef int (*StringComparator)(char *, char *);

    int strings_sort (int n, char *strings[], StringComparator compare);

Within out implementation of strings_sort, how do we call compare?

Really precise version

    (*compare)("alpha", "beta")

Normal C programmer version

    compare("alpha", "beta")

Challenge! Write a program that calls strings_sort to sort the strings on the command line alphabetically

    int 
    main (int argc, char *argv[])
    {
      int i;
      strings_sort (argc-1, argv+1, strcmp);
      for (i = 0; i < argc-1; i++)
        printf ("%s ", argv[i]);
      printf ("\n");
    } // main