Lab Time!

Using pointers

Where can a pointer point to?

Heap

Stack

Global

Text

…Anywhere!

What is a function pointer?

A pointer that stores the address of a function’s code

With this, we can pass a function as an argument

This allows us to reuse code

E.g., sorting an array of arbitrary objects could use a function pointer as the comparison function

Pointer Arithmetic

Part 1

You will be debugging several functions that all use pointers incorrectly

Part 2

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We’ve given you a set of functions as tools

Your job is to use these tools in the right order to print “Illinois”

Hints about the MP

What is this NULL-terminated array?

Maybe, we don’t want to pass around the size of the array everywhere

We know that NULL is not a valid string (but this doesn’t work for every type)

char *ptr[] = ...;// vs
int arr[] = ...;
arr[len] = NULL; // NULL = 0, meaning we have a valid element

Useful Functions

• strdup: return a string copy
• strcpy,strncpy: copy a string to another string
• toupper, tolower: uppercases/lowercases an input character

• ispunct,isspace,isalpha: decide whether a character is punctuation/alphabetical/whitespace

• Not so useful: strtok

Debugging

Valgrind

Valgrind is a framework for building program analysis tools. The most popular Valgrind tool is memcheck, which detects memory leaks. You will use Valgrind very often in CS 341, and the autograder will run Valgrind against your code to check for memory leaks.

Usage

Given a program myprog arg1 arg2:

valgrind --leak-check=yes myprog arg1 arg2

Leak Types

1. Memory block: A block of allocated, not-freed memory
2. Definitely lost: A memory block wasn’t freed, and no pointers point to it.
3. Still reachable: A memory block wasn’t freed, but there are pointers to it still left.
4. Indirectly lost: A memory block wasn’t freed that contained pointers to other memory blocks.
5. Possibly lost: A memory block wasn’t freed, and the pointer to it still exists but was moved (e.g. array)

Questions?