To use it, you must compile your code with the -g option CXXFLAGS - - PDF document

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• To use it, you must compile your code with the -g option

CXXFLAGS += -g g++ -g debug.cpp -o debug

• To run, type in “gdb ./debug”
• After typing in “gdb ./debug” you will enter the debugger
• Type run and it will run your program from the beginning
• If your program takes command line arguments you can type:

run <command line arguments>

• eg. run x y
• Runs all the way through, so it’s not very interesting
• You want to add breakpoints to stop the program at strategic

places to inspect things

• Type in break main to add a breakpoint at the start of the main

function

Ways to add breakpoints

• 1. Add a breakpoint at a function:

¡ break <function name>

• eg. break main
• 2. Add a breakpoint at a line in the current file:

break <line number>

• eg. break 6

• 3. Add a breakpoint at a line in a specified file:

break <file>:<line number>

• eg. break debug.cpp:6
• Breakpoints are numbered based on when they were created
• To see all the breakpoints:

info breakpoints

• To delete a breakpoint, you need to know its number

delete 1

• To delete all breakpoints:

delete

Other commands

• print <variable>: prints the value of a variable eg. print i
• step: execute the next line of code and goes into the function call
• next: execute the next line of code but doesn’t go into the function call
• continue: resume execution from where you stopped. Note: run runs the

program from the beginning

• quit: quits gdb
• bt: prints the call stack of functions value of a variable
• up: moves up the call stack
• down: moves down the call stack

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Running through debug.cpp

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int main(int argc, char **argv){ char *bad; std::string input; First, start the gdb. Type in int i = 0; “break main” and then run. print_message(); std::cin >> bad; input = bad;

Type in “step” until you get to this line

for(i = input.length(); i >= 0; i--){ std::cout << input[i]; } std::cout << std::endl; return 0; }

14

int main(int argc, char **argv){ char *bad; std::string input; Type in “print i”. The variable i int i = 0; should be 0 print_message(); std::cin >> bad; input = bad;

Type in “print input”. The variable input should be an empty string.

for(i = input.length(); i >= 0; i--){ std::cout << input[i]; } std::cout << std::endl; return 0; }

15

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int main(int argc, char **argv){ char *bad; std::string input; int i = 0; Type in “step” to go into

print_message(). Once inside print_message(), type in “bt” to

print_message(); see the call stack. std::cin >> bad; input = bad; for(i = input.length(); i

Restart the program and step

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r- in

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){

message() this time

std::cout << } input[i];

by using “step” instead of “next”.

std::cout << std::endl; return 0; }

16

int main(int argc, char **argv){ char *bad; std::string input; Get to the std::cin line and type int i = 0; “step” again. print_message(); std::cin >> bad; input = bad;

At this point, the debugger is expecting you to type something in, so type in a

for(i = input.length(); i >= 0; i--){

string.

std::cout << input[i]; } std::cout << return 0;

The program

std::endl;

will crash at this point

}

17

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What went wrong?

• No memory allocated for the bad array
• Fix by declaring bad to be:

char bad[100];

• And not

char* bad;

• Now restart the program, put a breakpoint at line 16 and run
• Type in “step” to make sure the line doesn’t crash
• You can inspect the input array

– Type in print input to see the whole string – Type in print input[i] (notice i is a variable) to see the ith element in the array

• Note: hitting return re-executes the

previous command

• If you get the following error message:

Missing separate debuginfos, use: debuginfo-install boost-date-time-1.41.0-17.el6_4.i686

• To fix, type in:

sudo debuginfo-install –nogpgcheck –enablerep=debug glibc-2.12

1.80.el6_3.7.i686 libgcc-4.4.6-4.el6.i686 libstdc++-4.4.6-4.el6.i686

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Other resources

If you want to find out more about gdb commnds:

• http://www.yolinux.com/TUTORIALS/GDBCommands. html
• http://web.cecs.pdx.edu/~jrb/cs201/lectures/handouts/gdbcomm.txt