CSCE 313 Introduction to Computer Systems

Activity · 2

Four Bugs in a Client

CSCE 313 · Introduction to Computer Systems

Fall 2026 · Builds on Lab 2

Format
In class, supervised
Time
30 minutes

1. What you are doing#

client.cpp starts four worker programs with fork() and exec(), then collects them. It compiles cleanly and it is wrong in four places.

Every one of the four is a mistake people actually make in Lab 2. You are debugging someone else’s client so that next week you can debug your own.

There is no new material here. If you have started Lab 2 Task 1, you have met all four already.

2. Rules#

What you may use: the Lab 2 handout, your own Lab 2 work, the man pages, and your lecture notes. You may talk to the person next to you about ideas. The code you submit must be yours.

3. Get your repository#

  1. Sign in to classroom50.org with your GitHub account.
  2. Accept the activity: https://classroom50.org/CSCE-313-FA26/csce-313-fa26/assignments/activity-2/accept.
  3. Clone it and build:
git clone https://github.com/CSCE-313-FA26/<your-activity-2-repository>.git
cd <your-activity-2-repository>
make

Only g++ is needed. There is no clang and no LLVM IR in this activity.

4. What you are given#

Path What it is
client.cpp The only file you change. All four bugs are here.
worker.cpp The program the client starts. Correct — do not change it.
Makefile Builds both. Do not change it.

worker.cpp stands in for Lab 2’s finance, logging and file servers: a separate program, started with arguments, that reports who it is and then exits with a status. It reads one flag:

./worker -n 2        # prints its line, then exits with status 2

Read it. It is short, and it tells you exactly what the client has to get right.

5. What a correct run looks like#

$ make
$ ./client
client pid 4021 starting 4 workers
worker 0 pid 4022 parent 4021
worker 1 pid 4023 parent 4021
worker 2 pid 4024 parent 4021
worker 3 pid 4025 parent 4021
client reaped pid 4022 exit 0
client reaped pid 4023 exit 1
client reaped pid 4024 exit 2
client reaped pid 4025 exit 3
client done, reaped 4

Four things have to be true, and each one is a separate bug:

  1. The workers run at all.
  2. There are exactly four of them, and the client is still alive to reap them.
  3. Each worker is told its own index — 0, 1, 2, 3, not the same number four times.
  4. The client reaps every child — all four, not just the first — and reports the status each one exited with.

Your PIDs will differ. The lines may interleave in a different order — workers and the client run at the same time, so a reaped line can appear before another worker’s line. That is normal and not a bug. Only the four things above are checked.

6. How to work#

Run it first. Do not read for bugs — let the program tell you.

make
./client

Fix the first thing it complains about, run it again, and see what changes. Each fix makes the next symptom visible. All four are small; none needs more than a line or two.

7. Submitting#

Commit and push. The autograder runs on every push, so push as often as you like and read the result.

git add client.cpp
git commit -m "activity 2"
git push

A green check means the checks passed. A red X means they did not — open it and read the message. Points are awarded per bug fixed, so a partly-fixed client is worth partial credit.

What the grader says What it means
does not compile Fix the error g++ prints, then push again
no worker ever ran execvp could not find the program it was given
the client exec'd itself The exec is not guarded — the parent reached it too
produced N worker lines, expected 4 Some workers never started
worker indices were … Each worker must be told its own index
only N of 4 children were reaped One wait() collects one child
nothing was reaped The client never called wait()

8. If you finish early#

None of this is submitted — it is here if you have time left.