Lab 2: GDB Introduction and C Review

Note: You have an additional day for this lab as it requires a bit more work than we initially hoped for.

Goals for this lab

By the time you have completed this lab, you should:

  • completely understand how char arrays are used to represent c-strings
  • be comfortable writing your own c-string manipulation functions
  • be able to use gdb to discover where and why your program is crashing

The purpose of this lab is to get you back in the swing of things by completing small coding exercises in C involving c-strings and malloc.

To get full credit for this lab, you need to complete all of the functions specified.

Lab Pairing

For this lab you may work with a partner. If you do work with a partner (please do), you will need to use the same partner throughout the quarter. When working with a partner, I strongly encourage you follow the pair programming method of working together. If you are not familiar with pair programming please watch this video.

To facilitate your submissions as a pair you MUST pair up using the submission system. When you visit the project page from where you can view all your submission you should see a link to a page where you can join a group. Of the two people in the group, one of you must invite the other, and the other must confirm the invitation. Once you group up, any submissions that either you, or your partner have already made will be visible to each other in addition to all future submissions.

Lab Preparation

At this point, everyone should have a COE account. If you do not, you need to make that a priority and should partner up with someone who already has an account.

Before you begin, let’s prepare a cs24 directory in your home directory if you don’t already have one. In that directory we’ll create a subdirectory for the files for this lab. Note that these instructions assume you are using either a CSIL machine or one of the lab machines:

After logging in, create the directory (assuming you haven’t already done so):

mkdir -p cs24/lab2

Change the permissions of the cs24 directory such that only your account can access the files inside this directory. This step is incredibly important. If omitted, other students can peek at your work and you may be held partially responsible. It should also go without saying that you may not give other students access to your account. If you’ve done that in the past please change your password at this time.

chmod 700 cs24

Change into the lab2 directory and copy the files you will need for this lab:

cd cs24/lab2
cp ~bboe/public_html/cs24_f13/code/lab2/* .

Getting your feet wet with gdb

For the first part of the lab you will use gdb to figure out how to execute gdb_puzzle. The first thing you will want to do is try to execute gdb_puzzle by running:


You should notice that the program segfaults. That’s because the program expects to be executed with a specific command line argument to run successfully.

The first thing you will want to do is to read the awesome gdb tutorial that Dani has written. That should walk you through the gdb commands you will need to use to figure out the proper command line argument to use to execute gdb_puzzle.

Once you have discovered the command line arguments that should follow ./gdb_puzzle create a text file called part1.txt and place the extra arguments on a single-line in that file.

For instance if you discover that running ./gdb_puzzle hello world is the correct command to solve the challenge (it is not) then the contents of part1.txt should be hello world. You can verify that you have created this file correctly by running:

xargs ./gdb_puzzle < part1.txt

If you see the same correct output, then you have a valid part1.txt suitable for submission on the submission site.

It is important that you go through the process of using gdb, as anytime you have a segfault in your code this quarter, the first thing either myself, or the TAs will ask you is where in your code did the execution segfault, and how did it get there.

Building string functions

The second task for this lab is to complete three functions that operate on c-strings. The purpose is to practice two things: the first is to treat c-strings as nothing more than an array of characters ending with the null-terminating character, or '\0' (also 0 as a 1-byte number). That means c-strings are arbitrarily long and functions that operate on c-strings continue processing bytes (characters) until a '\0' is found. The second purpose is to reinforce the difference between pointers and arrays.


Take a look at cstring_fun.c. In this file, you must fill in the missing code in the following three functions. NOTE that you may not make use of any of the functions in string.h.

You are to complete three functions:

  • stringlength - counts the number of characters in a string, not counting the terminating character

  • countchars - counts the number of occurrences of a character in the string

  • safestringconcat - allocates new memory, copies in two strings concatenated, and returns the resulting pointer

The reason you are writing safestringconcat is a) to practice allocating memory and b) to emphasize that strcat in the strings library does NOT do this - instead, it adds the second string to the first one WITHOUT allocating more memory. That is dangerous if you are not aware of it.

Compile and test your implementation

I suggest you complete and test each function individually. That is, start with code that compiles (the template) and slowly add functionality and verify that the code continues to compile as you make changes. When you’ve completed a unit of work (i.e., one of the three functions) test your program to verify that function works correctly before moving on (this is also a good point to make a submission). You will not receive partial credit for units that don’t work.

To compile cstring_fun execute:

clang -g cstring_fun.c

Note that the -g argument to the clang compiler (this is consistent with gcc as well) means to build the executable (a.out in this case) with debugging information which allows you to more easily use gdb to discover what line any segfaults occur on.

If the compilation fails make the necessary changes and then repeat until your code compiles successfully.

To run the executable, execute:


Verify that your code produces the expected results. If not repeat this process until it does. If you code crashes, you should now know how to use gdb to locate the line it crashes on.


  • Remember that fgets retains the \n, so your stringlength will be one more than what you think.

Submitting the assignment

Only one person in a group need submit the assignment but make sure that both you and your partner can view the submission on the submission site. If you cannot, you need to complete the making a group process. Please review the submission instructions as needed. Note that you may resubmit this assignment as many times as necessary up until the deadline.

Special Thanks

Special thanks to Diana Franklin for allowing me use of her CS24 material. Portions of this lab were adapted from one of her Spring 2013 CS24 labs.


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