CSCI 3500: Studio 3

Pointers

One tricky part of writing good C code is using pointers correctly. Understanding how pointers work, how to use them to index arrays, and how to reference and dereference data correctly is vital.

In this studio, you will:

1. Use pointers to index into an array
2. Write a simple makefile to make compilation easy

Please complete the required exercises below, as well as any optional enrichment exercises that you wish to complete.

As you work through these exercises, please record your answers in a text file. When finished, submit your work by sending your text file and source code to dferry@slu.edu with the phrase Pointers in the subject line.

Make sure that the name of each person who worked on these exercises is listed in the first answer, and make sure you number each of your responses so it is easy to match your responses with each exercise.

Required Exercises

1. As the answer to the first exercise, list the names of the people who worked together on this studio.

2. Download this file as a starting point. Open up the file and look at linString and winString. These variables demonstrate two different ways that you can declare character strings in C. Convince yourself of this by printing both strings with printf(). The correct format specifier to print a string looks like this:

   printf("%s\n", string_pointer);

3. Before we go any further, let's set up a Makefile in order to make compiling your program easy. This is a special file that specifies how to compile your project, so rather than having to invoke GCC every time you want to re-build your program, you can simply use the command make.

   Create a new file called "Makefile". Inside, on the first line, create a new target by typing "all:". On the next line, first press TAB, and then type your compilation instructions (gcc -o pointers pointers.c). Save and quit your file.

   Now type make at the Linux terminal. This command automatically looks for a Makefile in the current directory, and if it can find one, it will execute the instructions found under all:. The make program will print out the commands it executes so you can verify if it is working correctly. Makefiles can get much more complicated, but this simple method is suitable for small software projects.

   Leave the answer to this exercise blank, but attach your Makefile when you submit this studio. Note that a Makefile is mandatory for submitting Lab 1!

4. Now back to pointers. In C, a string is simply a consecutive sequence of characters in memory with an associated string pointer . (Notice that the definition of our strings both use char as their base type.)

   We can access a string by dereferencing the string pointer. A pointer points to data in memory, and dereferencing that pointer gives us the value of the data. You've already done dereferencing through the use of the square bracket index notation. The code linString[0] gives you the first character of linString, the code linString[1] gives you the second character, etc.

   Print out each character of linString using a loop with index notation.

5. The dereference operator in C is the asterisk (*) and is fundamental to using pointers. We already know that a pointer stores the memory address of data (i.e. it points to data). Just like indexing a pointer, the dereference operator obtains the value of the data that is pointed to.

   If the pointer winString is a pointer to a character, what character does it point to? In other words, what do you think is the value of the dereference operation on winString?

6. Check your answer to the last exercise by dereferencing winString and printing it out. The dereference operator is the asterisk when placed to the left of a pointer. You can print out a single character like so:

   printf("%c\n", *pointer_to_string);

   What was printed?

7. One useful way to use pointers is with pointer arithmetic. The index notation you just used is essentially pointer arithmetic, and in fact the C standard defines index notation in terms of pointer arithmetic.

   What character is stored in the byte after the first character of winString? Try printing the value of the next few bytes of winString using pointer arithmetic. To do so, add one, two, or three to the pointer before dereferencing. For example: *(pointer_to_string + 1).

8. Use a loop to print the entire contents of winString using pointer arithmetic.

9. You can also use pointers to assign values rather than simply read values. This is done by assigning a value to a dereferenced pointer (using either index notation or pointer arithmetic). For example, the following two statements turn the 'd' in "Windows!" into a 'u':

   winString[3] = 'u';

   *(winString + 3) = 'u';

   Write the code to change the string "Windows!" into the string "Linux! ". Then, print winString again. Copy and paste your program output as the answer to this exercise.

10. As has been said a few times already, pointers simply point to data that resides in memory. I've got one last experiment to convince you of this, if you don't believe me already.

    Open up your compiled executable file in a text editor. You will see a lot of gibberish but you'll also see a few recognizable things. Use the search function of your editor to search for the strings "Linux!" and "Windows!". Can you find them?

    Now, being slightly careful, use your text editor to change Windows! to Solaris! and save the file. What happens when you re-run the program?

    Note: This is not the advisable way to modify a program!

Optional Enrichment Exercises

1. No optional exercises