License

Learning Goals

  • Practice working with files and file pointers
  • More work with strings and arrays
  • Debugging with debug50
  • Use valgrind to check for memory leaks

Background

Suppose you work for a company that develops AI-enhanced license plate recognition technology. As you develop your technology, you may want to use actual license plate numbers as test data. You have a text file with some plate numbers you want to analyze, so you first try reading from that text file and printing out the license plates, to test your file-reading logic. But when you run your program, only the last number prints 8 times! What went wrong?

Demo

Getting Started

  1. Log into cs50.dev using your GitHub account.
  2. Click inside the terminal window and execute cd.
  3. Execute wget https://cdn.cs50.net/2022/fall/labs/4/license.zip followed by Enter in order to download a zip called license.zip in your codespace. Take care not to overlook the space between wget and the following URL, or any other character for that matter!
  4. Now execute unzip license.zip to create a folder called license.
  5. You no longer need the ZIP file, so you can execute rm license.zip and respond with “y” followed by Enter at the prompt.

Implementation Details

If you open the plates.txt file, you’ll notice that there are actually 8 different license plate numbers. There are a few errors in the distribution code that are causing this to happen. Let’s start by walking through the code.

We first check for command-line arguments, since argv[1] should be the text file that contains the license plates. We then create a character array of length 7, since the plate numbers are 6 chars long and we need to save space for the NUL terminator. Now we create an array of char *’s (character pointers, otherwise known as strings!) to store the 8 plate numbers. We create a file pointer to our external text file, create a variable to hold the index of each array element, and begin reading the file and saving the plate numbers to the array. Finally, to test that we did this correctly, we print out the values in the array.

However, when we compile and execute this code, something is clearly wrong. You’ll need to make the appropriate corrections!

Debugging

Take a close look at what values are actually being added to the array. You can experiment with debug50 to find the error in the distribution code.

  • Hints
    • What exactly does the variable buffer hold that’s being assigned to each element of the plates array?
    • Note that the plates array as an array of char *’s or string pointers.
    • So when we assign the char array buffer to a char *, what actually happens?

Is there a better way to copy license plate numbers to the license array?

  • More Hints
    • Would strcpy come in useful here?
    • If you copy data to a pointer, where will that data go?
    • Is malloc required here?

Checking for Memory Leaks

Once your program seems to be working properly, run valgrind to make sure there are no memory leaks.

valgrind ./license plates.txt

You may end up with something like this:

==24478== HEAP SUMMARY:
==24478==     in use at exit: 528 bytes in 9 blocks
==24478==   total heap usage: 10 allocs, 1 frees, 4,624 bytes allocated
==24478== 
==24478== 56 bytes in 8 blocks are definitely lost in loss record 1 of 2
==24478==    at 0x4848899: malloc (in /usr/libexec/valgrind/vgpreload_memcheck-amd64-linux.so)
==24478==    by 0x109207: main (license.c:30)
==24478== 
==24478== 472 bytes in 1 blocks are still reachable in loss record 2 of 2
==24478==    at 0x4848899: malloc (in /usr/libexec/valgrind/vgpreload_memcheck-amd64-linux.so)
==24478==    by 0x4A086CD: __fopen_internal (iofopen.c:65)
==24478==    by 0x4A086CD: fopen@@GLIBC_2.2.5 (iofopen.c:86)
==24478==    by 0x1091CD: main (license.c:20)
==24478== 
==24478== LEAK SUMMARY:
==24478==    definitely lost: 56 bytes in 8 blocks
==24478==    indirectly lost: 0 bytes in 0 blocks
==24478==      possibly lost: 0 bytes in 0 blocks
==24478==    still reachable: 472 bytes in 1 blocks
==24478==         suppressed: 0 bytes in 0 blocks
==24478== 
==24478== For lists of detected and suppressed errors, rerun with: -s
==24478== ERROR SUMMARY: 1 errors from 1 contexts (suppressed: 0 from 0)

There seem to be two kinds of memory errors here that need to be corrected!

  • Hint
    • Your line numbers may be different, but note here that there seems to be a memory problem in main on line 30 (something to do with malloc?), and line 20 (a problem related to fopen?). What might the distribution code be missing?

How to Test Your Code

Your program should behave per the examples below.

license/ $ ./license plates.txt
11ZT00
1KAD21
78ZZ01
99ZZ11
72ZZ21
98ZZ31
44ZW41
34ZZ51

You can check your code using check50, a program that CS50 will use to test your code when you submit, by typing in the following at the $ prompt. But be sure to test it yourself as well!

check50 cs50/labs/2023/x/license

To evaluate that the style of your code, type in the following at the $ prompt.

style50 license.c

How to Submit

No need to submit! This is a practice problem.