Debugging Lab                

A video introduction is available.                 

In this lab, you will learn to use debugging tools such as gdb, assert(), and -D_GLIBCXX_DEBUG. You will modify a program bad.cc, and turn in the modified version.                 

Consider the following program, bad.cc. It opens resolv.conf, and prints out the line that starts with “search”. It has several errors (don’t rob others of the joy of discovery—keep it to yourself):                 

#include <fstream>
#include <iostream>
#include <string>
#include <vector>
#include <cassert>

using namespace std;

int main() {
    // First, some stupid code just to introduce an error:
    vector<int> v = {2,42,65535,253,1003};
    cout << "My favorite number is " << v[10000003] << '\n';

    string filename = "\etc\resolv.conf";
    ifstream in(filename);
    assert(in.is_open());

    for (string s; getline(in, s); ) {
        string prefix = s.substr(1,6);
        if (prefix == "search")
            cout << s << '\n';
    }

    return 0;
}

When the program works, and is run on a CS Dept. computer, it prints something like this:                 

My favorite number is 253
search cs.colostate.edu colostate.edu

Handy gdb commands                

Here are some gdb commands that you may find useful:                 

Command	Description
r optional-arguments	run: run the program
q	quit: exit gdb
bt	backtrace: show what function called what
l	list: show the next 10 lines of source code
l line-number	list: show the source code around that line
b line-number	breakpoint: stop when you come to this line
p	print: show all local variables
p variable	print: show the value of that variable

For this lab, you should:                

1. Copy & paste the program to bad.cc.
   If you don’t know how to copy & paste, then this is a fine opportunity to learn.
2. Compile it, like this: g++ bad.cc
   (We’re not using -Wall in this lab, which should make you nervous, and that’s good, but just go with it.)
3. Execute it, observe the segmentation fault, obviously caused by v[100000003]
4. Compile it, with the C++ library debugging flag, like this:
   g++ -D_GLIBCXX_DEBUG bad.cc
   Yes, the underscores (_) matter. This flag tells the C++ containers (vector, string, etc.) to do more error-checking, at the cost of speed and size.
5. Execute it, observe the more helpful error message. It’s not perfect, but it beats “Segmentation fault”. At least it gives a hint as to which container is having problems.
6. Fix the offending line by changing [100000003] to [3].
7. Compile & execute, see the assert() failure.
8. Compile it, like this: g++ -DNDEBUG bad.cc
9. Execute it, observe that the assertion failure vanished (and the program still doesn’t work).
10. Compile for the debugger gdb: g++ -ggdb bad.cc
11. Execute the debugger: gdb ./a.out
12. Set a breakpoint at the assertion: b 16 (your line number may vary)
13. Run the program: r
14. Print the value of filename, like this: p filename
15. Figure out why filename has that value. Fix the source.
16. Recompile for the debugger.
17. Set a breakpoint at if (prefix == "search") line.
18. Run the program to that breakpoint, inspect s and prefix the same way that you printed the value of filename.
19. Fix the call to string::substr().
20. Recompile, see that the program works correctly.

For extra fame & glory (but no extra points):                 

1. Debug the program using ddd.

How to submit your work:                

In Canvas, check in the file bad.cc to the assignment “Lab06”. It’s due 11:59ᴘᴍ MT Saturday, with a 24-hour late period for a 25% penalty.                 

How to receive negative points:                

Turn in someone else’s work.