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Debugging

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The debugger is in the bin\win32 or bin\dos directories (the GDB.EXE file), for the Windows and DOS versions respectively. It usually comes already installed in most Linux distros.


(Note: all commands should be typed without quotes and then [return] must be pressed.)


  • Compile the source code for your program in debug mode
    • use the -g command-line option to add debugging support, e.g. fbc -g myapp.bas.

  • Load your compiled program in GDB
    • For example, in Windows/DOS: gdb myapp.exe
    • Note: under windows if the GDB command prompt seems not to work well (like backspace not working), try set TERM=#win32con before launching GDB.

  • Set any arguments you want to send to your application
    • For example: set args arg1 arg2 argn.
    • You can also run GDB and pass the arguments directly to the application been debugged: gdb --args myapp.exe arg1 arg2 arg3.

  • Ensure GDB can see your program's source code directory
    • If the executable isn't in the same directory of the source files where it was compiled, type: dir path/to/my/application/sources.

  • Set a breakpoint in your program
    • Place a breakpoint in the first line using: b main.
    • To place a breakpoint in a function called func use: b FUNC.
    • Note: fbc exports variable/function names to UPPERCASE. GDB is case sensitive by default, but you can use the set language pascal command to change GDB to case-insensitive mode.

  • Use GDB shortcuts to run your code or to step through it
    • Type r to start running the application.
    • Type n to step to the next line, stepping over function calls.
    • Keep pressing [return] to step forward to the next line.
    • Type s to step into function calls.
    • As above, keep pressing [return] to step through.
    • Type c to continue execution until the next breakpoint.

  • Use GDB shortcuts to inspect variables
    • Use print VAR_NAME to show the contents of the variable called var_name.
      • GDB supports pointer/pointer field dereferencing, indexing and arithmetics too, so print *MYPOINTER will also work.
      • (note: undeclared variables or the ones with suffixes like % & ! # $ can't be printed).
    • Use disp VAR_NAME to display the contents of a variable called var_name.
    • Use watch VAR_NAME to stop each time a variable called var_name is changed.

  • Additional commands:
    • Use r again to restart the application when it finishes.
    • Type q to quit.
    • Type help to see a list of commands, there are many others.