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Debug Tools

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Most times debugging without a debugger is really hard. In complex projects printing text and values of variables to the console is not very feasible for debugging anymore. Being able to handle a debugger like GDB or DDD makes (a coder's) life much easier.

GDB

GDB is the standard (console based) unix debugger.

Basic usage

To simply run the program and wait for a segfault follow the instructions below.

  1. Go to the binary directory (bin) and run gdb orxonox
  2. Type run
  3. Wait for the segfault to occur …
  • If you want to display the back trace ( list of function calls ) just type bt.
  • You can change the context (function to debug) by typing up and down.
  • If you want to display the value of a variable make sure you are in the right context (by using up and down) and type print varname
  • If you have an object pointer and you don't know the exact type of the class (e.g. pointer to WorldEntity? which is actually a SpaceShip) you can type x/wa addressOfObject and gdb will tell you what class type the object located in addressOfObject has by considering the vtables

Disabling mouse grabbing for in-game breakpoints

One problem with the method outlined above is that the game grabs the mouse when in-game, and does not release it when a breakpoint is reached, trapping you in the locked game. One solution to this is to disable mouse grabbing in the code. In the file src/libraries/core/input/InputManager.c, there is a function LoadDevices?. The part shown below grabs the mouse and keyboard:

if (CommandLineParser::getValue("keyboard_no_grab").get<bool>())
    paramList.insert(StringPair("x11_keyboard_grab", "false"));
else
    paramList.insert(StringPair("x11_keyboard_grab", "true"));
    paramList.insert(StringPair("x11_mouse_grab",  "true"));
    paramList.insert(StringPair("x11_mouse_hide", "true"));

Change all the values from "true" to "false" here to disable mouse/keyboard grabbing altogether. This makes the game awkward to play, but when it runs into a breakpoint, you can freely move your mouse focus out of the game and into the GDB window.

Setting Breakpoints

You can set breakpoints for

  • functions: break namespace::ClassName::functionName (e.g. break orxonox::SpaceShip::fire)
  • codelines: break fileName.cc::lineNr (e.g. break SpaceShip.cc:367)

If the program hasn't yet been run in gdb, it will not have loaded all our libraries (e.g. libcore.so, libnetwork.so) and thus will not be able to set the breakpoint immediately, but ask you whether it should set it at run time. Make sure you entered the address/function correctly and type yes.

  • To delete a breakpoint you can either use delete or clear. Type help / help gdb-function for more information.

Setting Watchpoints

Sometimes it's useful to know when the value of a variable changes.

  1. To make use of this feature set a breakpoint somewhere you can access the variable from (watchpoints for static variables can be set without doing this).
  2. Run the program and wait for the breakpoint to get triggered
  3. Type watch varname

Useful Commands

Below is a list of useful commands (type help command for more information)

x/wa examine the memory at a certain address (e.g. determine class of an object)
print print the value of a variable or function return value
break set a breakpoint
watch set a watchpoint
clear delete all/specific breakpoints
delete delete a breakpoint/watchpoint

DDD

DDD is a simple graphical debugger based on GDB.

Microsoft Visual Studio Debugger

Yet to come.

Last modified 4 years ago Last modified on May 21, 2013, 3:00:56 PM