A Printing routine for QString in GDB

GDB supports the command print to print out the content of variables. This works pretty well with all basic types, but becomes annoying at best with complex types like QString. GDB has its own macro language, so it is possible to write routines for printing these types.

When GDB starts it reads in the file $HOME/.gdbinit to load macros that should be available at runtime. In the following two sections I show a macro to print QString objects. If you copy this macro into your .gdbinit file it will be automatically available each time you call gdb.

Example session:

(gdb) print myString
$2 = {static null = {<No data fields>}, static shared_null = {ref = {atomic = 39},
    alloc = 0, size = 0, data = 0x82174ca, clean = 0, simpletext = 0, 
    righttoleft = 0, asciiCache = 0, reserved = 0, array = {0}}, 
    static shared_empty = {ref = {atomic = 1}, alloc = 0, size = 0, 
    data = 0xf5f71aca, clean = 0, simpletext = 0, righttoleft = 0,
    asciiCache = 0, reserved = 0, array = {0}}, d = 0x8260b48, 
    static codecForCStrings = 0x0}
(gdb) printqstring myString
(QString)0x8217658 (length=26): "this is an example QString"
(gdb)

As you see above, print myString prints the QString as an object. Since the actual data is hidden in the element "d" it is not immediately visible - even print myString.d would not be very satisfactory, since QString stores its data in unicode format.

The form printqstring myString prints out a more readable version.

Qt 3.x

This macro was posted by David Faure to the KDE maillist in 2001:

define printqstring
    set $i=0
    while $i < $arg0.d->len
	print $arg0.d->unicode[$i++].cl
    end
end

It already prints out each character of the QString onto a single line.

A much refined version was posted by Arnaud de Muyser to the qt-interest list the same year:

define pqs
    set $i=0
    set $unicode=$arg0.d->unicode

    printf "Getting QString...\n"
    while $i < $arg0.d->len
        set $c=$unicode[$i++].cl
        if $c < 32 
          printf "\\0%o", $c
        else
          if $c <= 127
            printf "%c", $c
          else 
            printf "\\0%o", $c
          end 
        end
    end
    echo \n
end

Qt 4.x

The internal representation of QString changed for Qt 4.x: the length is now stored in d->size and it uses UCS-16 instead of UTF-8 for internal storage. The fact that QStrings are now implicitly shared does not matter in this context though.

So this is my adapted version of the macro:

define printqstring
    printf "(QString)0x%x (length=%i): \"",&$arg0,$arg0.d->size
    set $i=0
    while $i < $arg0.d->size
        set $c=$arg0.d->data[$i++]
        if $c < 32 || $c > 127
                printf "\\u0x%04x", $c
        else
                printf "%c", (char)$c
        end
    end
    printf "\"\n"
end

KDE

For KDE there is a collection of macros here: http://websvn.kde.org/trunk/KDE/kdesdk/scripts/kde-devel-gdb