Betriebssysteme/hw2/simu1/QUESTIONS.md

Questions 14-Memory-API

Overview

In this task, you will gain some familiarity with memory allocation. First, you’ll write some buggy programs (fun!). Then, you’ll use some tools to help you find the bugs you inserted. Then, you will realize how awesome these tools are and use them in the future, thus making yourself more happy and productive.

The first tool you’ll use is gdb, the debugger. There is a lot to learn about this debugger; here we’ll only scratch the surface. Here you can find a quick reference gdb. Also ddd is installed on lab workstations.

The second tool you’ll use is valgrind. This tool helps find memory leaks and other insidious memory problems in your program.

Please answer the questions, by giving the result and an explanation, why you got the result. Write your answers in markdown syntax in the new file ANSWERS.md. Also checkin your C-Files and one Makefile for all or your C-Files, so that all binaries are build. Do NOT checkin the binaries!

The installed gcc wrapper on you workstations does some optimization to your code examples, which will not show some of the bugs with an “Segmentation fault”. We have already disabled this behavior by setting:

export hardeningDisable=all

in your labshell bsys environment. So, you don’t have to do any further steps to produce unoptimized code with gcc, if you want to.

If you struggle with your own systems about some strange gcc optimization behavior, maybe it helps to check for gcc-wrapper variables like these.

Questions

1. First, write a simple program called null.c that creates a pointer to an integer, sets it to NULL, and then tries to dereference it. Compile this into an executable called null. What happens when you run this program?

2. Next, compile this program with symbol information included (with the -g flag). Doing so let’s put more information into the executable, enabling the debugger to access more useful information about variable names and the like. Run the program under the debugger by typing gdb null and then, once gdb is running, typing run. What does gdb show you?

3. Finally, use the valgrind tool on this program. We’ll use the memcheck tool that is a part of valgrind to analyze what happens. Run this by typing in the following: valgrind --leak-check=yes ./null. What happens when you run this? Can you interpret the output from the tool?

4. Write a simple program that allocates memory using malloc() but forgets to free it before exiting. What happens when this program runs? Can you use gdb to find any problems with it? How about valgrind (again with the --leak-check=yes flag)?

5. Write a program that creates an array of integers of size 100 using malloc(); then, set data[100] to zero. What happens when you run this program? What happens when you run this program using valgrind? Is the program correct?

6. Create a program that allocates an array of integers (as above),frees them, and then tries to print the value of one of the elements of the array. Does the program run? What happens when you use valgrind on it?

7. Now pass a funny value to free (e.g., a pointer in the middle of the array you allocated above). What happens? Do you need tools to find this type of problem?