When your program crashes Why Debug a Program? When your program crashes Finding out where it crashed Examining program memory at that point When a bug occurs in your program Watching program variables Identifying where a problem occurs (and why) Tracing through a program’s behaviors Learning what a program does Good basic testing approach

Debugging on Various Platforms We’ll use Visual Studio’s graphical debugger Much more on this as we go through the semester Set command line arguments in the project properties It’s also good to know about common text-based tools gdb Debugger provided with the g++ compiler (e.g., on Linux) Using –g switch embeds debug info during compilation dbx Debugger provided with Sun CC Also requires –g switch to embed debug symbols

Some Essential Debugging Commands start (or restart) execution in the debugger Visual Studio: F5 (Start Debugging) gdb: run (also give its command line arguments) execute one line, stepping into any functions Visual Studio: F11 (Step Into) gdb: step execute one line, stepping over any functions VS: F10 (Step Over) gdb: next set breakpoint at a line or function entry point VS: F9 (Toggle Breakpoint) gdb: break (can give file and line, function, etc.)

“Core” Dumps A program dumps a memory image when it crashes creates a file (called “core” on Linux) and then exits this happens when a program receives certain signals: Segmentation fault accessed memory it does not own or dereferenced a 0 pointer Bus error divided by zero corrupted stack Debuggers are very useful to examine “core” files When your program crashes within Visual Studio On Linux: gdb myprogram core

Debugging an Example Program A debugger helps us observe a program’s behavior Step through (and into) functions Watching the call stack and variable values But, before we start using the fancy tools… What do we expect the program to do? How might the 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

What We Expect the Example Program To Do 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 same result different order 1 1 + + 2 3 2 5 8 + + 10 4 5 3 4 9 8 10 9

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) Try this on your own in the studio, for practice 1 + 2 3 8 + 4 ??? 9

Example Program Header File // prefix_adder.h // // author: Chris Gill cdgill@cse.wustl.edu // 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 */

Example Program Source File / Usage Function // prefix_adder.cc // // author: Chris Gill cdgill@cse.wustl.edu // 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; }

Example Program Main Function int 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;

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 1; } // 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;