CS202: Lab 1: C review, gdb, software skills

This lab will reinforce your prior experience in the C programming language and give you practice using certain command-line tools. The overall comfort you’ll gain (or reinforce) with the Unix computing environment will be helpful for future labs.

Getting started

You will perform the labs on a virtual devbox (a machine with all of the required development software installed on it), and pull/push code with git and GitHub. It's time for you to set up that infrastructure:

Section 1: C review

All of the projects in this class will be done in C or C++, for two main reasons. First, some of the things that we want to implement require direct manipulation of hardware state and memory, which are operations that are naturally expressed in C. Second, C and C++ are widely used languages, so learning them is a useful thing to do in its own right. You have some experience in C from CS201, and you will need to build on that here.

If you are interested in why C looks like it does, we encourage you to look at Ritchie's history. Here, perhaps, is the key quotation: "Despite some aspects mysterious to the beginner and occasionally even to the adept, C remains a simple and small language, translatable with simple and small compilers. Its types and operations are well-grounded in those provided by real machines, and for people used to how computers work, learning the idioms for generating time- and space-efficient programs is not difficult. At the same time the language is sufficiently abstracted from machine details that program portability can be achieved."

You will do short exercises in C, as a warmup, and to help review some of what you learned in CS201 about C. Enter the devbox (vagrant ssh, per the setup page) and then:

$ cd cs202/lab1/mini

(Depending on your CS201 section, you may have seen very similar, or the same, exercises; that’s fine. If you’re not absolutely fluent in C programming, then doing them again will be useful.)

For Exercises 1 through 3, you can test with:

$ make
$ build/part1

You should have seen make in CSO. Recall that it is an automatic way to build software. In this case, it invokes the compiler and links your compiled code to a small test harness that we provide.

$ ./build/part1
part1: set_to_fifteen OK
part1: array_sum OK

If your part1.c is correctly implemented, you will see OK, as above.

You will need to remove the assert(0); line. Remove this as you implement functions in future exercises; it is a reminder that you have yet to implement a particular function.

As you write your code and improve it (fixing bugs, adding functionality), you should get in the habit of syncing your changes to the master copy of your labs repository on GitHub, for example:

$ git commit -am 'my solution for lab1 exercise1'
Created commit 60d2135: my solution for lab1 exercise1
 1 files changed, 1 insertions(+), 0 deletions(-)
$ git push origin

The commit keeps the history of changes to your code, and so allows you to revert to an older version if you find that a change causes a regression. The push serves to back up your code on GitHub’s servers, so you won’t lose work if your local working copy is corrupted or lost.

As above, you compile and test with:

$ make

Now you should see

$ ./build/part2
part2: set_point OK
part2: point_dist OK

Note that you can keep track of your changes by using the git diff command. Running git diff will display the changes to your code since your last commit, and git diff origin/main will display the changes relative to the initial code supplied for this lab. Here, origin/main is the name of the git branch with the initial code you downloaded from our server for this assignment.

Test it:

$ make
$ ./build/part3
part3: list_insert OK
part3: list_end OK
part3: list_size OK
part3: list_find OK
part3: list_remove OK

$ gdb build/part3 core
(gdb) bt

assert(head != NULL);

Advice on debugging common problems encountered in doing this lab

• Remember to recompile changed code Whenever you’ve changed a file, always type make to re-compile before executing again.

• Write your own simple test code. Don’t rely solely on the lab’s testing infrastructure (i.e. ./grade-lab) to test the correctness of your code. We don’t distribute the source of the harness code; this makes it hard to debug. So you should write your own tester. Let’s say you want to test the array_sum function in the file part1.c. To write your own test code, create a file (e.g. called test-part1.c) with a main function that invokes array_sum in various ways to test its correctness.

  An example test-part1.c might look something like this.

  Compile your test code by typing:

    $ gcc -pedantic -Wall -std=c11 -g -o test_part1 test_part1.c part1.c

• Use gdb As we noted above, and as will be covered below.

C programs from scratch

In the rest of Section 1 of the lab, you will get practice writing and compiling C programs from scratch. Knowing how to create a standalone C program is useful in its own right, and it will be helpful context for lab2.

We will quickly walk through how to write and compile a program in C, and then you will write and compile several programs of your own.

Using a text editor, create a file called fun.c. In this file, type:

#include <stdio.h>

int main(int argc, char** argv)
{
    char* first_arg; 
    char* second_arg; 

    /* this checks the number of arguments passed in */
    if (argc != 3) {
        printf("usage: %s <arg1> <arg2>\n", argv[0]);
        return 0;
    }

    first_arg = argv[1];
    second_arg = argv[2];

    printf("My program was given two arguments: %s %s\n",
           first_arg, second_arg);

    return 0;

}

This is a complete C program. The #include at the top tells the compiler to use the header files of "standard I/O" (the standard input/ouput functions of the C library; these functions include printf). Also, argc contains the number of arguments passed to the program (including the program name itself) while argv[0] contains the name of the program that was invoked.

You can compile this program using:

$ gcc -pedantic -Wall -std=c11 -g -o fun fun.c

You can now run this program using:

$ ./fun

You should see:

usage: ./fun <arg1> <arg2>

You can also do:

$ ./fun abc def
My program was given two arguments: abc def

Note the pattern here:

• you create a C file and compile it with

  $ gcc [flags] -o <output-file-name> <input-file-name>

• You run your program using whatever was after the -o flag.

• Inside the program, you gain access to the arguments using the argv array. (As we’ll see in this semester, the OS syscall exec() takes an argv; the OS takes that array and passes it to the new program's main() function.)

$ ./first3 a b
./first3: error: one or more arguments have fewer than 3 characters

$ ./first3 abcd wxy
abcwxy

$ ./countas abbba
2

$ ./countas aaaaa
5

$ ./countas bcdefghwxyz
0

$ ./countas
usage: countas <arg>

$ ./countas aaaaa bbbbbb
usage: countas <arg>

Section 2: Debugging

This part of the lab will give you practice debugging.

Put your answers for this section and in the next in the supplied answers.txt file.

Navigating to syntax errors

Go to the dbg directory in lab1:

$ cd ~/cs202/lab1/dbg

Now type:

$ make

The code has a syntax error; thus, it cannot be compiled.

$ vi +<number> <filename>

After you fix the syntax error, the code will compile. Use make to see this:

$ make

Now, try testing the code itself, using the small test_linked_list utility:

$ ./test_linked_list
Segmentation fault (core dumped)

Aha! Our code compiled, but it was not correct (core dumps are bad). Specifically, the segmentation fault means that our program issued an illegal memory reference, and the operating system ended our process. Making matters worse, we have no idea what the problem in the code is. In the following section, you will learn how to use gdb to debug this kind of problem.

Run gdb: Use the GNU debugger, or gdb to run the program:

$ gdb test_linked_list
(gdb)

Set breakpoints: One thing that you might want to do is to set a breakpoint before the program begins executing. Breakpoints are a way of telling gdb that you want it to execute your program and then stop, or break, at a place that you define. Use the following command to set a breakpoint at the main function:

(gdb) b main
Breakpoint 1 at 0x400963: file test_linked_list.c, line 43.

Then use gdb's command run to actually start the program (this is the general pattern in gdb: one invokes the debugger, perhaps sets a breakpoint, and then starts the program with run):

(gdb) run

The program will be stopped when it reaches the breakpoint (advanced topic: how does gdb conspire with the hardware to make this work??). At this point, you will be presented with gdb's command prompt again. To see the “call stack” (or stack trace), which is the list of functions that have called this one – literally, the stack frames on top of the current one – you issue backtrace or bt for short:

(gdb) bt

Experienced developers will often ask for a stack trace as step 0 or 1 of understanding a code problem. Get in the habit of asking gdb to give you a backtrace.

To make the program continue running after a breakpoint, use continue, or c for short:

(gdb) c

Step through the code: Of course, if you just c every time you hit a breakpoint, then you will lose control of the program. You often want the command next, or n:

(gdb) n

This "executes" the next line of code, for example executing an entire function. (The command step executes a single line of C code. There is little difference between step and next unless you are about to enter a function. step steps into the function; next "steps over" the function.)

Inspect the values of variables: In gdb's command prompt, the program is stalled. You can query the program's current global and local variables with the print command, or p for short.

At this breakpoint, determine the value of the integer id:

(gdb) print id
$1 = 1

This means that variable id holds the integer 1.

Aside: you can check local variables' names using:

(gdb) info local

Core dump: If a program terminated abnormally (for example, test_linked_list), the state of the program will be recorded by the OS and (if core dumps are enabled) saved in a so-called core dump. gdb can use a core dump to inspect a crash situation.

To debug using core dumps, you must first enable core dumps, and then point gdb at the relevant file. We'll do this in several steps:

# enable core dumps
$ ulimit -c unlimited 
$ ./test_linked_list
$ ls -l core
$ gdb ./test_linked_list core

The idea here is that the core file gives gdb enough information to recover the memory and CPU state of the program at the moment of the crash. This will allow you to determine which instructions experienced the error.

Section 3: Software development skills and general productivity

This part of the lab will walk through general software development skills as well as some tools to enhance your productivity. You may feel at first that these tools are slowing you down ("how do I enter characters in vi???"), but comfort and fluency with them will pay off in the long term. It's worth slowing down and "learning proper technique."

A good text editor is essential

$ vi myfile.txt 

$ emacs myfile.txt

Navigating code: ctags

We will investigate ctags with the Linux code base as a running example.

Create a new directory (not inside cs202, since we don't need the Linux code to be part of your later submission):

$ mkdir ~/learn-ctags
$ cd ~/learn-ctags

Now download and unpack the Linux source code archive. We are interested in the kernel source and header files:

$ wget https://cdn.kernel.org/pub/linux/kernel/v5.x/linux-5.2.9.tar.xz
# the line above will take a few minutes
$ tar -xf linux-5.2.9.tar.xz linux-5.2.9/include linux-5.2.9/kernel
$ cd linux-5.2.9
$ ls 
include/ kernel/

Now, you are going to use the ctags program. Type:

$ ctags --recurse=yes * 
# this could take time

The ctags utility searches a body of code and creates a tag file, which is an index of programming language objects, such as function and variable definitions. The command above invokes ctags recursively on all items in the entire directory (*) in order to create a single tags file that includes information from all files in the repository.

The rest of this section will use vi to interact with tags. However, emacs also supports tags.

Using vi, open the header file for the linux scheduler:

$ vi include/linux/sched.h

We will now walk through some uses of tags. When we type :something, this refers to a command given in vi's command mode. To get into command mode in vi, you type the escape key (vi by default starts in command mode; the editor enters insert mode when you type i, e, a, etc.). Once in command mode, typing :something does the "something". (For example, try pressing escape and then :q.)

Now, let's get started with tags. Type:

:tag sched_attr

This will take you to the definition of sched_attr. To return to your previous position, press ctrl-t.

The tag file that we created indexed all of Linux's source. We can jump to a tag in another file and it will automatically be opened, as we illustrate in the following exercise.

:tag block_device_operations

You can use ctrl-] to jump to the definition of the object underneath the cursor. Go to line 1001 in include/linux/sched.h (navigate there with :1001, since :<number> in vi takes you to the specified line number). Move your cursor over the token futex_pi_state. Press ctrl+].

A tag may have multiple definitions. type:

:tselect list_head

and note that a menu pops up giving you the choice of which definition to view.

Tags can also be used with regular expressions. For example, to look up all tags containing proc_sched, enter the following:

:tselect /proc_sched

And the following command shows tags that contain proc_sched and task (in that order):

:tselect /proc_sched.*task

For more information about tags, this resource is good.

General productivity: find

We now cover a very powerful Unix tool: find.

At a high level, find by default recurses through directories, "looking" at files, to see which files match a provided predicate. Here is the pattern for its use:

$ find [flags] [path...] [expression]

Now cd to the root directory of the linux kernel that you downloaded earlier. Type this command:

$ find .
.
./kernel
./kernel/panic.c
./kernel/Kconfig.locks
./kernel/params.c
./kernel/.gitignore
./kernel/memremap.c
./kernel/freezer.c
./kernel/sysctl_binary.c
.....

This recursively prints all files in the current directory (it will print the names of tens of thousands of files).

You can also search on file name patterns:

$ find . -name "sched.h"
./kernel/sched/sched.h
./include/uapi/linux/sched.h
./include/linux/sched.h
./include/linux/sunrpc/sched.h
./include/asm-generic/bitops/sched.h
./include/xen/interface/sched.h
./include/trace/events/sched.h

And you can limit the search to particular directories:

$ find include/linux include/asm-generic -name "sched.h"
include/linux/sched.h
include/linux/sunrpc/sched.h
include/asm-generic/bitops/sched.h

As with most Unix commands, wildcards can be used:

$ find . -name "sched.*"
./kernel/sched/sched.h
./include/uapi/linux/sched.h
./include/linux/sched.h
./include/linux/sunrpc/sched.h
./include/asm-generic/bitops/sched.h
./include/xen/interface/sched.h
./include/trace/events/sched.h

This returns all files named "sched" with any extension.

Where find really starts to be powerful is that you can execute commands on the files that are returned. For example:

$ find . -name "sched.h" -exec grep foobar {} \;

This says, "return all file names that match sched.h and grep each of them for the string foobar."

More information on find can be found here.

We will cover other tools (such as grep) as the semester goes on.

Handin Procedure

Handing in consists of three steps:

1. Executing this checklist:

   • Fill out the top of the answers.txt file, including your name and NYU Id
   • Make sure you’ve answered every question in answers.txt
   • Create a file called slack.txt noting how many slack days you have used for this assignment. (This is to help us agree on the number that you have used.) Include this file even if you didn’t use any slack days.

2. Push your code to GitHub, so we have it:

   $ cd ~/cs202/lab1     
   $ git commit -am "hand in lab1"
   $ git push origin 
   
   Counting objects: ...
   ....
   To ssh://github.com/nyu-cs202/labs-21sp-<username>.git
     7337116..ceed758  main -> main

3. Actually submit, by timestamping and identifying your pushed code:

   • Decide which git commit you want us to grade, and copy its id (you will paste it in the next sub-step). A commit id is a 40-character hexadecimal string. Usually the commit id that you want will be the one that you created last. The easiest way to obtain the commit id for the last commit is by running the command git log -1 --format=oneline. This prints both the commit id and the initial line of the commit message. If you want to submit a previous commit, there are multiple ways to get the commit id for an earlier commit. One way is to use the tool gitk. Another is git log -p, as explained here, or git show.
   • Now go to NYU Classes; there will be an entry for this lab. Paste only the commit id that you just copied.
   • You can submit as many times as you want; we will grade the last commit id submitted to NYU Classes.

NOTE: Ground truth is what and when you submitted to NYU Classes. Thus, a non-existent commit id in NYU Classes means that you have not submitted the lab, regardless of what you have pushed to GitHub. And, the time of your submission for the purposes of tracking lateness is the time when you upload the id to NYU Classes, not the time when you executed git commit.

This completes the lab.