Saturday, October 26, 2013

QtCreator's Python Debug Visualizers

Peter Lohrmann wrote QtCreator debug visualizers in Python for some key classes used by the Linux OpenGL debugger project we've been working on together. He recently blogged about the details here.

So far we've got visualizers for our dynamic_string and vector classes. (Like many/most game devs, we use our own custom containers to minimize our reliance on the C++ runtime and "standard" libraries, but that's another story.) Before, to visualize the contents of vectors in QtCreator, I've had to muck around in the mud with the watch window and type in the object's name, followed by the pointer and the # of elements to view. Our dynamic_string class uses the small string optimization (not the super optimized version that Thatcher describes here, just something basic to get the job done). So it's been a huge pain to visualize strings, or basically anything in the watch/locals window.

The below pic shows the new debug visualizers in action on a vector of vectors containing dynamic_strings.  Holy shit, it just works!

I'm not a big fan of Python, but this is valuable and cool enough to make it worth my while to learn it.

Here's the code. Almost all of this is Peter's work, I've just tweaked the vector dumper to fix some things. I'm a total Python newbie so it's possible I screwed something up here, but this is working much better than I expected already. It's amazing how something simple like this on Linux can make me so happy.

You can find a bunch of QtCreator's debug visualizer code here: ~/qtcreator-2.8.0/share/qtcreator/dumper

In my ~/.gdbinit file:


And here's my /home/richg/dev/raddebugger/src/crnlib/ file:


# This file contains debug dumpers / helpers / visualizers so that certain crnlib

# classes can be more easily inspected by gdb and QtCreator.

def qdump__crnlib__dynamic_string(d, value):

    dyn = value["m_dyn"]
    small = value["m_small"]
    len = value["m_len"]
    small_flag = small["m_flag"]
    buf = dyn["m_pStr"]
    if small_flag == 1:
        buf = small["m_buf"]
    p = buf.cast(lookupType("unsigned char").pointer())
    strPrefix = "[%d] " % int(len)
    str = "'" + p.string(length=len) + "'"
    d.putValue(strPrefix + str)
    with Children(d):
        d.putSubItem("m_len", len)
        with SubItem(d, "m_small"):
            d.putValue( str if small_flag == 1 else "<ignored>")
            with Children(d):
                d.putSubItem("m_flag", small_flag)
                with SubItem(d, "m_buf"):
                    d.putValue(str if small_flag == 1 else "<ignored>")
        with SubItem(d, "m_dyn"):
            d.putValue("<ignored>" if small_flag == 1 else str)
            with Children(d):
                with SubItem(d, "m_buf_size"):
                    d.putValue("<ignored>" if small_flag == 1 else dyn["m_buf_size"])
                with SubItem(d, "m_pStr"):
                    d.putValue("<ignored>" if small_flag == 1 else str)

def qdump__crnlib__vector(d, value):

    size = value["m_size"]
    capacity = value["m_capacity"]
    data = value["m_p"]
    maxDisplayItems = 100
    innerType = d.templateArgument(value.type, 0)
    p = gdb.Value(data.cast(innerType.pointer()))
    d.putValue( 'Size: {} Capacity: {} Data: {}'.format(size, capacity, data ) )
    numDisplayItems = min(maxDisplayItems, size)
    if d.isExpanded():
         with Children(d, size, maxNumChild=numDisplayItems, childType=innerType, addrBase=p, addrStep=p.dereference().__sizeof__):
             for i in range(0,numDisplayItems):
                 d.putSubItem(i, p.dereference())
                 p += 1

1 comment:

  1. your string visualizers have a small problem.
    If you display a string with double quotes in it e.g.: 'hello"world'
    qtcreator will only show: 'hello'
    it seems you have to base64 encode the strings to display first and give the correct encoding, at least that is my solution for now:

    def putstr(d, s):
    d.putValue( base64.b64encode(s), encoding=1 )

    if you don't want surrounding " " use encoding=5