valgrind note

2022-06-14

Using Valgrind to find segfaults

Edit the Makefile to include the -g flag in CFLAGS to provide debugging information to Valgrind.

Compile

linked_list.c

and

test_linked_list.c

by executing

make linked_list

By default, memcheck is the tool that is run when you invoke Valgrind. The documentation on Valgrind’s memcheck is very useful, as it provides examples of the most common error messages, what they mean, and some optional arguments you can use to help debug them.

Box 1. This shows us the command that we are running through Valgrind.

Box 2. This is a print statement from our program.

Box 3. We are reading 8 bytes from an invalid memory address on linked_list.c line 62.

Box 4. Our program received a segfault by accessing invalid memory on linked_list.c line 62

Box 5. There were no memory leaks at the time that the program exited.

Box 6. We encountered 1 error.

Remember from Lab01 that (*head)->next produced a segfault because we forgot to check if *head==NULL. Fix this error, run your code through Valgrind again, and you should see that there are no errors reported by valgrind.

Using Valgrind to detect memory leaks

Let’s cause a memory leak in test_linked_list.c. Comment out the line that calls free_list.
Run make linked_list to compile your code.
Run valgrind
```
valgrind ./linked_list
```
We can see that our program is still producing the correct result based on the printed messages “Congrats…”; however, we are now experiencing memory leaks. Valgrind tells us to “Rerun with –leak-check=full to see details of leaked memory”, so let’s do that
```
valgrind --leak-check=full ./linked_list
```

Box 1. Summary of heap usage. There were 80 bytes allocated in 5 different blocks in the heap at the time of exit.

Box 2. Stack trace showing where the unfreed blocks were allocated.

Direct blocks are those which are root nodes (blocks of memory that the programmer has direct access to, ex stack/global pointer to the heap).
Indirect blocks are those which are not root nodes (ex a pointer inside of a struct).

Box 3. Summary of leak. You can find more info about memory leaks in the Valgrind docs

You can use the stack trace to see where the unfreed blocks were allocated. Hopefully this example will help you understand Valgrind messages when you are completing your projects