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<!DOCTYPE html>
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<title>CS 144: Lab FAQs</title>
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<div class="page-header">
<h3>Lab FAQs</h3>
</div>
<h4>General</h4>
<ol>
<li><b>How will code style be graded?</b>
<p>Here are some guidelines:
<ul><li>Complex pieces of code should be documented in comments so that they become easier for the reader to understand—but don't overdo it with comments for trivial things.
<li>Common, general-purpose functionality should be decomposed into helper functions to avoid repeated code—don't overdo this either, e.g. by making it difficult to understand the flow of execution.
<li>Code should use consistent naming conventions and not contain commented-out code or vestiges of debugging.
<li>Please check the return values of all functions in which an error could occur, and handle these errors gracefully. Throwing an exception is one graceful way
to handle an unrecoverable error.
<li>Avoid memory errors and leaks, egregious performance issues (e.g. allocating large amounts of memory for no reason) or unnecessarily complex approaches (e.g. a complicated data structure that is only marginally more efficient than something much simpler, or something available in the C++ standard library).
<li>You can check your code for memory leaks, invalid accesses, and undefined behavior by compiling with <code>cmake .. -DCMAKE_BUILD_TYPE=RelASan</code> and then <code>make clean</code>, <code>make</code>, then <code>make check</code> or running any of the individual apps.
<li>Before submitting each assignment, please run <code>make format</code> (to standardize the formatting), and make sure to remove any dummy code or comments that were included with the starter code.
</ul>
Your style grade may also include components of functionality not covered by the tests.
<p>
<li><b>How much of my grade will come from style and to the quality of my writeup?</b></li>
<p>Grading breakdown for labs: 15% style, 10% writeup, 75% functionality.</p>
<li><b>How do I run an individual test?</b></li>
<p><code>ctest -R '^test_name$'</code></p>
<li><b>Can I add include lines in the header files?</b></li>
<p>Sure.</p>
<li><b>Why does 'make check' fail even though it's in the Sponge README?</b></li>
<p>Short Answer: This is a typo in our documentation. You should run <code>make check_lab0</code>, not <code>make check</code>.
<br>
Long Answer: The make target, <code>check</code>, runs the entire sponge test suite. The full test suite requires a shell script, <code>tun.sh</code>, which you won't get until a later lab. The make target <code>check_lab0</code> will just test lab0. You should be able to run it with the lab0 starter code (although it will return an error until you've completed the lab!).</p>
<li><b>What's the procedure if I want to use some of my late days?</b></li>
<p>To use a late day, please email the staff list ([email protected]) by the lab deadline, letting us know if you are using late days. With weekly labs, the CAs want to be able to get feedback to you quickly. As such, we will start grading as soon as the deadline passes, and want to avoid redundantly grading submissions that weren't intended to be final.</p>
<li><b>How do I use <code>git</code>?</b></li>
<p>Here are some links to <a href="https://try.github.io/">resources to learn Git</a>.
<p><b>Brief summary:</b> Git commits store a snapshot of the current state of your code. For instance, say you break a feature that you know used to work, but aren't sure which code caused it to break. If you made a commit after getting that feature working, you can checkout that commit to see what things looked like in the good old days when it worked. So, make a commit every time you get something working! To make a commit:<br>
First, git add the files you've changed - you must add before every commit. e.g. <code>git add ../libsponge/byte_stream.*</code><br>
Then, commit with a message describing the state of the code. e.g. <code>git commit -m "All tests pass except many_writes times out."</code><br>
You can see which files you modified by running <code>git status</code>. To really understand the state of a Git repository, the command-line <code>tig</code> tool, or the graphical <code>gitk</code> tool, can be very helpful.<br>
A handy shortcut to commit all modified files is <code>git commit -am "[message]"</code>.<br>
Don't <code>git push</code> - when you're ready to submit, you'll use git bundle instead to package your commits.
<p><b>Sanity check:</b> If you want to verify that you have submitted exactly the files you intended to, you may find the following useful.<br>
First, once you've committed, run <code>git status</code>. You should see a message saying <code>nothing to commit, working tree clean</code>. If you have uncommitted changes, you'll see this instead: <code>Changes not staged for commit:</code>.
<br>
Second, after you've copied your git bundle to rice, and logged into rice:
<ol>
<li>Run <code>$ git clone [bundle name] [unbundled name]</code>.</li>
<li><code>cd</code> into the unbundled directory.</li>
<li>Run <code>$ git log</code> and verify that you see the commits you expect to.
<li>Copy the commit hash of the starter code commit.</li>
<li>Run <code>$ git diff [hash]</code> to make sure they see all changes you want to submit.</li>
</ol></p>
<li><b>How do I debug?</b></li>
<p><code>gdb</code> is a great tool for debugging the labs! Check out the <a href="https://web.stanford.edu/class/archive/cs/cs107/cs107.1202/resources/gdb">CS107 guide</a> if you need a refresher.<br>
To use <code>gdb</code> to debug a test:
<ol>
<li>Install <code>gdb</code> (on the VM, <code>sudo apt-get install gdb</code>).</li>
<li>Start <code>gdb</code> on the executable corresponding to the test you want to debug (from the <code>build</code> directory, <code>ls tests</code> to see the executables). e.g. from the build directory: <code>gdb tests/byte_stream_one_write</code>.</li>
<li>The output of the test failure will show you the part of the test that failed. To set a breakpoint at that part of the test, break on the line in the test file where the test harness for that part is created. The test files are in <code>sponge/tests</code> (not <code>sponge/build/tests</code>). e.g. if you're failing the <code>write-pop2-end</code> test in <code>byte_stream_one_write</code>, <code>break 83</code> (i.e. where the <code>write-pop2-end</code> test harness is created).</li>
<li>You can set breakpoints on your functions using the function names, as usual.</li>
</ol>
Other notes:
<ul>
<li>We don't recommend modifying any files in the <code>libsponge/util</code> directory, since messing up the sponge library will make debugging difficult.</li>
<li>If a test is timing out, but you want to check if it passes without the timeout, run the test executable individually, which won't enforce the timeout. e.g. from the build directory: <code>./tests/byte_stream_one_write</code>.</li>
<li><code>gdb</code> may help debug timeouts. While running the test in <code>gdb</code>, if it appears to hang (meaning it may be executing a slow portion of code), ctrl-C and backtrace to pause and see which code was executing.</li>
</ul></p>
</ol>
<h4>Lab 0</h4>
<ol>
<li><b>What should the behaviour of my program be if the caller tries to peek or pop with a <code>len</code> greater than what is available?</b></li>
We don't have any preference on the behavior if the caller tries to peek or pop more than is available in the stream, as long as you behave reasonably and don't crash in that situation. If you want to peek/pop the maximum available, that is fine with us. If you want to throw an exception (either for an overlength peek or an overlength pop), that is also fine with us.
<li><b>How fast should my code be?</b></li>
Here's a reasonable rule of thumb: if any of your tests are
taking longer than 2 seconds, it's probably good to keep
exploring other ways of representing the ByteStream.
This lab doesn't require any sophisticated data structures,
but it might still take some trial-and-error to find a
reasonable approach. Sometimes "big-O" analysis can be
misleading about the performance of real programs—the
constant factors can matter a lot!
<p>
</ol>
<!--
<h4>Lab 1</h4>
<ol>
<li><b>Which bytes should be accepted in a <code>push_substring</code> call?</b></li>
Those whose index is within <code>capacity</code> indices of the first unread byte in the re-assembled stream.
Pictorally:
<image src="images/reassembler.png" />
<li><b>Is it okay for our re-assembly data structure to store overlapping substrings?</b></li>
It's possible to implement an interface-correct reassembler which stores overlapping substrings.
However, allowing the re-assembler to do this undermines the notion of <code>capacity</code> as a memory limit, so we'll consider the storage of overlapping substrings to be a style violation when grading.
</ol>
<h4>Lab 2</h4>
<ol>
<li><b>Can we use <code>static_cast</code> in functions for <code>WrappingInt32</code>?</b></li>
Yes, these kinds of functions (those that explicitly convert between
numeric types) present exactly the kind of situation in which
<code>static_cast</code> is appropriate. Note also that
you don't
<em>need</em> to use <code>static_cast</code> if you don't
want to—you can
explicitly construct a WrappingInt32 with syntax
like <code>WrappingInt32{x}</code>.<p>
<li><b>Should <code>TCPReceiver::segment_received()</code> return
true if <em>any</em> part of the segment is inside the window,
even if both the beginning and end of the segment are outside
the window?</b></li>
Good question—please
see <a href="https://piazza.com/class/k0u7fdam6082kb?cid=136">our
Piazza note</a> where we clarify this. The short answer is
that we've deleted a test in the lab2 starter code in order to
make both answers to this question (yes and no) acceptable.
</ol>
<h4>Lab 4</h4>
<ol>
<li><b>In <code>TCPConnection::segment_received</code>, what
are the three conditions in which the TCPConnection needs
to make <b>sure</b> that the segment receives at least one
ACK segment in reply, and may need to force the TCPSender
to spit out an empty segment to make this happen?</b><br>
<ol>
<li>If the incoming segment occupies any sequence numbers
(<code>length_in_sequence_space() > 0</code>)
<li>If the <code>TCPReceiver</code> thinks the segment
is unacceptable (<code>TCPReceiver::segment_received()</code> returns <code>false</code>)
<li>If the <code>TCPSender</code> thinks the ackno is invalid
(<code>TCPSender::ack_received()</code> returns <code>false</code>)
</ol>
<p>
<li><b>Okay but hang on, what happens if
the TCPConnection wants to send an ACK
segment but the TCPReceiver's ackno is
missing (e.g. because it hasn't gotten an incoming SYN
yet)?</b><br>
If the ackno is missing, then you can't send an ACK segment.
Don't send one in this case.<p>
<li><b>When is the connection fully over (i.e., when should
<code>active()</code> start returning false?</b><p>
There are two ways a connection can fully end:
<ul>
<li><i>[Unclean shutdown]</i> Any segment with the RST flag is sent or received, <b>or</b>
<li> <i>[Clean shutdown]</i> The sender is totally done (meaning, the sender's input stream is at EOF with no bytes in flight) <b>AND</b> the receiver is totally done (meaning, the receiver's output stream has ended) <b>AND either</b>
<ul>
<li> <code>_linger_after_streams_finish</code> is false (no need to linger), or
<li><code>time_since_last_segment_received() >= 10 * _cfg.rt_timeout</code> (we've lingered long enough)
</ul>
</ul><p>
<li><b>I think I may be having some trouble with my
logic about when a connection is fully over (i.e., when to stop
returning <code>true</code> from <code>active()</code>). Some of my
tests are timing out. How can I debug this?</b><p>
Check out <a href="https://piazza.com/class/k0u7fdam6082kb?cid=277">this Piazza
thread</a> for a debugging approach that may help. It may be easier to
run the client and server manually (as described there) if you want to figure
out where things are going awry.<p>
<li><b>The test suite keeps talking about the state of the TCPReceiver and TCPSender—what are these states?</b><br>
The test suite (and
the <a href="https://github.com/CS144/sponge/blob/lab4-startercode/libsponge/tcp_helpers/tcp_state.hh">tcp_state.hh</a>
and <a href="https://github.com/CS144/sponge/blob/lab4-startercode/libsponge/tcp_helpers/tcp_state.cc">tcp_state.cc</a>
files) uses the public interface of your TCPSender and
TCPReceiver to put them in broad categories or states. The
tests make sure that your TCPSender/TCPReceiver are in the
right state at the right time, given the actions taken.<p>
<hr>
Here is a diagram of the expected evolution (and definition of each
state) for the TCPReceiver. The receiver starts with no ISN (and
therefore no ackno) (<b>LISTEN</b>). Then it moves to a
state <b>SYN_SENT</b> where it has received a SYN segment (so there is
an ackno) but hasn't assembled a FIN segment (so the stream hasn't
ended). This is where most of the connection happens. Eventually the
receiver assembles a FIN segment and ends the stream
(<b>FIN_RECV</b>).
<p><p>
<img src="images/receiver-evolution.svg">
<p>
<hr>
And here is a diagram of the expected evolution (and definition of
each state) for the TCPSender. The sender starts with nothing sent
(<b>CLOSED</b>), then moves to a state <b>SYN_SENT</b> where it has
sent something but nothing has been acknowledged. At some point, some
bytes are acknowledged (<b>SYN_ACKED</b>). This is where most of the
connection happens. Eventually, the stream ends and the sender sends a
FIN segment (<b>FIN_SENT</b>), and some time after that, the stream is
over and everything has been acknowledged (<b>FIN_ACKED</b>).
<p>
<p>
<img src="images/sender-evolution.svg"> -->
</ol>
</div>
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