8. Debugging

8.1. X11 server

If you are developing on a linux system, it is recommended that you reconfigure your X11 server with the following setup (see the file /etc/X11/XF86Config-4):

Section "ServerFlags"
        Option "AllowDeactivateGrabs" "true"   # Ctrl+Alt+Keypad *
        Option "AllowClosedownGrabs"  "true"   # Ctrl+Alt+Keypad /
EndSection

The two key bindings described above are used to release any grab that a GUI application might have. This is especially useful when debugging through gdb: it might happen that the breakpoint happens while such a grab is in place, and would therefore prevent any input (mouse or keyboard) to any application in your X11 session, in particular the debugger.

8.2. gtk+ library

It is also recommended that you recompile your own gtk+ library (on systems where this is easily doable such as Unix systems), with the following configure command:

./configure --with-debug=yes

In addition to providing the usual debugging information in the debugger, this also activates several environment variables which might be used to monitor the actions in gtk+ and its associated libraries.

These variables are the following:

export GTK_DEBUG=misc:plugsocket:text:tree:updates:keybindings;
export GDK_DEBUG=updates:nograbs:events:dnd:misc:xim:colormap:gdkrb:gc:pixmap:image:input:cursor;
export GOBJECT_DEBUG=objects:signals;

Some of the values for these variables can be omitted. The exact semantic (or even the exact list) of such variables depends on your version of gtk+, and you should therefore consult its documentation.

8.3. debugger

When debugging with gdb, it is recommended that you always specify the flag –sync to gps. This forces any gtk+ application, and in particular GPS, to process X11 events synchronously, and therefore makes it easier to debug possible problems.

If your application is printing some gtk+ warnings on the console, you should do the following in the debugger:

(gdb) set args --sync
(gdb) begin
(gdb) break g_log
(gdb) cont

This will stop the application as soon as the gtk+ warning is printed.