Deadline: End of lab session Tuesday, July 9th
Pull the lab files from the lab starter repository with
git pull staff master
Here’s one to help you in your interviews. In
ll_cycle.c, complete the
ll_has_cycle() to implement the following algorithm for checking if
a singly-linked list has a cycle.
ll_has_cycle(), the program you get when you compile
ll_cycle.cwill tell you that
ll_has_cycle()agrees with what the program expected it to output.
ACTION ITEM: Implement
ll_has_cycle() and execute the following commands
to make sure that the provided tests pass.
$ make ll_cycle $ ./ll_cycle
Hint: There are two common ways that students usually write this function. They differ in how they choose to encode the stopping criteria. If you do it one way, you’ll have to account for a special case in the beginning. If you do it another way, you’ll have some extra NULL checks, which is OK. The previous 2 sentences are meant to urge you to not stress over cleanliness. If they don’t help you, just ignore them. The point of this exercise is to make sure you know how to use pointers.
Here’s a Wikipedia article on the algorithm and why it works. Don’t worry about it if you don’t completely understand it. We won’t test you on this.
As a closing note, the story of the tortoise and the hare is always relevant, especially in CS61C. Writing your C programs slowly and steadily, using debugging programs like CGDB, is what will win you the race.
This exercise uses
vector.c, where we
provide you with a framework for implementing a variable-length array. This
exercise is designed to help familiarize you with C structs and memory
management in C.
ACTION ITEM: Explain why
bad and fill in the functions
vector.c so that our test code
vector-test.c runs without any memory management errors.
Comments in the code describe how the functions should work. Look at the
functions we’ve filled in to see how the data structures should be used. For
consistency, it is assumed that all entries in the vector are 0 unless set by
the user. Keep this in mind as
malloc() does not zero out the memory it
For explaining why the two bad functions are incorrect, keep in mind that one
of these functions will actually run correctly (assuming correctly modified
vector_set, etc.) but there may be other problems. Hint:
think about memory usage.
ACTION ITEM: Test your implementation of
vector_set() for both correctness and memory
management (details below).
# 1) to check correctness: $ make vector-test $ ./vector-test # 2) to check memory management using Valgrind: $ make vector-memcheck
does is run the following valgrind command on our executable. For a review,
read through the valgrind exercise from Lab 1. Explain to yourself what each of
the flags mean.
$ valgrind --tool=memcheck --leak-check=full --track-origins=yes [OS SPECIFIC ARGS] ./<executable>
The last line in the valgrind output is the line that will indicate at a glance if things have gone wrong. Here’s a sample output from a buggy program:
==47132== ERROR SUMMARY: 1200039 errors from 24 contexts (suppressed: 18 from 18)
If your program has errors, you can scroll up in the command line output to view details for each one. For our purposes, you can safely ignore all output that refers to suppressed errors. In a leak-free program, your output will look like this:
==44144== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 18 from 18)
Again, any number of suppressed errors is fine; they do not affect us.
Feel free to also use CGDB or add
printf statements to
vector-test.c to debug your code.
So far, we have been dealing with C program files (.c file extension), and have
been using the
gcc compiler to execute these higher-level language programs.
Now, we are learning about the RISC-V assembly language, which is a lower-level
language much closer to machine code. For context,
gcc takes the C code we
write, first compiles this down to assembly code (gcc uses a more complex
assembly language than RISC-V), and then assembles this down to machine
In this lab, we will deal with several RISC-V assembly program files, each of which have a .s file extension. To run these, we will need to use a RISC-V simulator. The simulator we will use was developed by Keyhan Vakil (now a CS161 TA) and improved by Stephan Kaminsky (a former CS61C TAs). The simulator is called Venus and can be found online here.
mainfunction must be put first.
ecallwith argument value 10. This signals for the program to exit. The ecall instructions are analogous to System Calls and allow us to do things such as print to the console or request chunks of memory from the heap.
For the following exercises, please save your completed code in a file on your local machine. This is crucial for the checkoff portion to work.
lab2_ex3.sinto the editor.
Record your answers to the following questions in a text file. Some of the questions will require you to run the RISC-V code using Venus’ simulator tab.
.textdirectives mean (i.e. what do you use them for)? Hint: think about the 4 sections of memory.
nstored in memory? Hint: Look at the contents of the registers.
At this point, make sure that you are comfortable with the following. Note that these will not be part of the lab checkoff, but are meant to benchmark how comfortable you are with the material in the exercise.
Open the files
lab2_ex4_assembly.s The assembly code
provided (.s file) is a translation of the given C program into RISC-V. Paste
the contents of
into the editor.
Find/explain the following components of the assembly file and put your answers in a text file.
After you’ve answered explained the above components, edit
lab2_ex4_assembly.s so that it
dest satisfies the following conditions.
dest[i] = 2 * source[i]for even
dest[i] = 1for odd
Hint: This can be done by adding one line of code and modifying another (in other words, you only need to make 2 changes). Look at the initial values of dest; how does this help you implement this modification?
Verify that your changes work for the given
dest arrays by
running your code in a new Venus tab and check that the output looks like:
3 1 4 1 5 9 6 1 8 1 10 1
Make sure you are comfortable with the following.
also_bad_vector_new()are bad. Also, show your TA/AI the output of running the program as well as the output of
make lab2_ex4_assemblyto run the assembly code through Venus locally and show the output to your TA/AI.
This exercise isn’t for a checkoff. Instead, we included it because we think it will be helpful for students working on Project 1. If you have time now, try doing this exercise.
When you used Valgrind before, it may not have looked very useful or it may have
been somewhat hard to understand what was going on. For this exercise, we’re
going to look at some more practical and interpretable examples of code where
we can use Valgrind. We have provided 4 programs for you to run Valgrind on:
You can run a memory check on a program by running the following command, shown
$ make a-memcheck
gcc -ggdb -Wall -std=c99 -o program_a program_a.c; \ valgrind --tool=memcheck --leak-check=full --track-origins=yes ./program_a; \ rm program_a ==29708== Memcheck, a memory error detector ==29708== Copyright (C) 2002-2015, and GNU GPL'd, by Julian Seward et al. ==29708== Using Valgrind-3.11.0 and LibVEX; rerun with -h for copyright info ==29708== Command: ./program_a ==29708== *pi is 3 ==29708== ==29708== HEAP SUMMARY: ==29708== in use at exit: 4 bytes in 1 blocks ==29708== total heap usage: 3 allocs, 2 frees, 1,032 bytes allocated ==29708== ==29708== 4 bytes in 1 blocks are definitely lost in loss record 1 of 1 ==29708== at 0x4C2DB8F: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so) ==29708== by 0x400611: main (program_a.c:14) ==29708== ==29708== LEAK SUMMARY: ==29708== definitely lost: 4 bytes in 1 blocks ==29708== indirectly lost: 0 bytes in 0 blocks ==29708== possibly lost: 0 bytes in 0 blocks ==29708== still reachable: 0 bytes in 0 blocks ==29708== suppressed: 0 bytes in 0 blocks ==29708== ==29708== For counts of detected and suppressed errors, rerun with: -v ==29708== ERROR SUMMARY: 1 errors from 1 contexts (suppressed: 0 from 0)</pre>
Alternatively, you can compile each C file yourself, and then run the Valgrind
command with whatever specific flags you would like (for example the flag
// compile program_a $ gcc -o program_a program_a.c // now run a memory check using Valgrind $ valgrind --tool=memcheck --leak-check=full --track-origins=yes [OS SPECIFIC ARGS] ./program_a
Read each C file as well as run
make X-memcheck (replace
d) on every program and try to determine what is happening that is
causing memory problems in each program (e.g. memory out of bounds,
segmentation fault, etc.).
program_a: memory leak (modified and lost pointer to malloc'd memory) program_b: memory out of bounds (outstep bound of array) program_c: illegal use of memory (trying to use freed memory) program_d: segmentation fault (accessing invalid memory)