For gdb to work, debugging symbols must be built into the code at compile time.

Otherwise, the machine instructions will not be mapped to locations in C++ source files.

This is easily accomplished by using the appropriate command-line switch (-g) when invoking the compiler:

g++ -g filename.cpp

This often results in a significantly larger executable file.

To get qmake to generate makefiles with the debugging options switched on, add the following line to your qmake project file:

CONFIG += debug

When the Qt library has been built with debugging symbols, you can step through the Qt source code just as easily as your own code.

You may need to build Qt with debugging symbols to debug programs that contain code called from the Qt library (most GUI programs, for example).