CSE 232: C++ debugging in Visual Studio and emacs
C++ Debugging (in Visual Studio and emacs)
• We’ve looked at programs from a text-based mode– Shell commands and command lines– Text editors, compiler, linker– How the program receives input and generates output– General program structure and logic
• We’ve also looked at Visual Studio– In which all of these functions are integrated…– …within a nice graphical environment
• Today we’ll bring those perspectives together– Debug a simple example program in Visual Studio– Look at how do the same thing within emacs
CSE 232: C++ debugging in Visual Studio and emacs
Getting Started
• To set up your own environment for what we’ll cover– Log on and then ssh into grid.cec.wustl.edu– Create a new directory (e.g., mkdir prefix_adder)– Save files from course web page (or copy them from the
~cse232/.www-docs/ directory) into that directory:
Makefile prefix_adder.h prefix_adder.cc
• Also download those files to your Windows desktop – Create a new project in Visual Studio– Copy those files into the new project in Visual Studio– Build the project
CSE 232: C++ debugging in Visual Studio and emacs
Debugging an Example Program• Now we’ll use Visual Studio to debug a program
– Step through (and into) functions– Watching the call stack and variable values
• But, before we start using Visual Studio…– What are we trying to achieve?– What do we expect our program to do?– How might our program fail?– Can we make predictions and test them?
• Thinking: the most powerful way to debug– Scientific method should guide what you do
• hypothesis, prediction, experiment, analysis– Tools can help you follow this disciplined approach faster
CSE 232: C++ debugging in Visual Studio and emacs
What the Example Program Does• Called with command line arguments
./prefix_adder + 8 + 9 10• Calculates prefix addition expressions
+ 8 + 9 10 + + 8 9 10• These are equivalent to their infix versions
(8 + (9 + 10)) ((8 + 9) + 10)• Key idea: walk through expresion, calculate value
+
+8
9 10
1
2 3
4 5
+
+
8 9
10
1
2
3 4
5
same result
different order
CSE 232: C++ debugging in Visual Studio and emacs
How the Example Program Can Fail• Too few arguments in expression
./prefix_adder + 8 + 9• Cannot calculate result
+ 8 + 9 (needs another value to finish 2nd + operation)
• Exercise: try this on your own, for practice
+
+8
9
1
2 3
4 ???
CSE 232: C++ debugging in Visual Studio and emacs
Example Program: Header File // prefix_adder.h//// author: Chris Gill [email protected]//// purpose: Declarations for a simple prefix adder program,
which// takes the command line arguments as a prefix
addition // expression and computes an integer result.
#ifndef PREFIX_ADDER_H#define PREFIX_ADDER_H
// Function prototypes.void usage (char * program_name);int parse_and_compute (int & current_index, int last_index, char *argv[]);
#endif /* PREFIX_ADDER_H */
CSE 232: C++ debugging in Visual Studio and emacs
Example Program: Start of the Source File// prefix_adder.cc//// author: Chris Gill [email protected]//// purpose: definitions for a simple prefix adder program, which// takes the command line arguments as a prefix addition // expression and computes an integer result.
#include "prefix_adder.h"
#include <iostream> // For std output stream and manipulators.#include <string> // For standard C++ strings.#include <sstream> // For standard string streams.#include <cstring> // For C-style string functions
// Helper function to print out the program's usage message.void usage (char * program_name) {
cout << "Usage: " << program_name << " <argument> [<argument>]..." << endl << "Purpose: computes program arguments as prefix addition expression" << endl;}
CSE 232: C++ debugging in Visual Studio and emacs
Example Program: Main Functionint main (int argc, char *argv[]){ // A few useful constants for argument positions const int minimum_arguments = 2; const int starting_index = 1; const int program_name_index = 0;
if (argc < minimum_arguments || strcmp (argv[starting_index], "--help") == 0) {
usage (argv[program_name_index]); return 1; }
try {
// Pass the current and last index to use, and the array, to the // expression parsing function, and store the result. int current_position = starting_index; int value = parse_and_compute (current_position, argc - 1, argv);
// Print out the result, and return success value. cout << "The value calculated is " << value << endl; return 0; } catch (...) {
cout << "caught exception" << endl; return -1; }}
CSE 232: C++ debugging in Visual Studio and emacs
Example Program: Parsing Function// Helper function to parse the input symbols and compute a value.int parse_and_compute (int & current_index, int last_index, char *argv[]) {
// make sure we're still in the argument range if (current_index > last_index) { throw; }
// look for a single-symbol addition operator if (strlen (argv[current_index]) == 1 && *(argv[current_index]) == '+') {
int first_operand = parse_and_compute (++current_index, last_index, argv); int second_operand = parse_and_compute (current_index, last_index, argv); return first_operand + second_operand; }
// treat anything else as an integer else {
int result; istringstream i (argv[current_index++]); i >> result; return result;
}}
• Exercise:– Set a break point at the
first if statement in this function
– Watch how stack grows/shrinks if you have debug continue
CSE 232: C++ debugging in Visual Studio and emacs
Debugging in emacs
CSE 232: C++ debugging in Visual Studio and emacs
Open Makefile, Source, Header Files in emacs
CSE 232: C++ debugging in Visual Studio and emacs
Compile the Program: Esc x compile
CSE 232: C++ debugging in Visual Studio and emacs
Hit Enter to Run “make -k”
CSE 232: C++ debugging in Visual Studio and emacs
Compilation Succeeded
CSE 232: C++ debugging in Visual Studio and emacs
Launch Debugger: Esc x gdb
CSE 232: C++ debugging in Visual Studio and emacs
Run “gdb prefix_adder”
CSE 232: C++ debugging in Visual Studio and emacs
GDB Debugging Prompt
CSE 232: C++ debugging in Visual Studio and emacs
Setting Breakpoints
CSE 232: C++ debugging in Visual Studio and emacs
Running With Command Line Arguments
CSE 232: C++ debugging in Visual Studio and emacs
At Breakpoint in Main
CSE 232: C++ debugging in Visual Studio and emacs
Stepping Through the Program
CSE 232: C++ debugging in Visual Studio and emacs
What Options Does Emacs/GUD Provide?
CSE 232: C++ debugging in Visual Studio and emacs
Stepped into Function Call, now at a Breakpoint
CSE 232: C++ debugging in Visual Studio and emacs
Printing out Values of Variables
CSE 232: C++ debugging in Visual Studio and emacs
Continue to next Breakpoint, Look at Stack
CSE 232: C++ debugging in Visual Studio and emacs
Exercise• Build and debug a damaged version of the program
– Save another file from course web page into the project (or create a new project with same Makefile and header file)bad_prefix_adder.cc
– Build the new version of the program
• Debug what’s wrong with the program(no fair using diff to detect code differences ;-)– Run the program with different inputs– Observe what happens (how does it go wrong?)– Trace through the program in Visual Studio to narrow down
the possible cause(s) of the problem(s)• If you’d like to, also try the same thing in emacs
– Fix the error(s), rebuild, and re-run to observe its behavior