Monthly Archive for November, 2010

Start Emacs with GDB directly

Published on 24. November 2010 in Emacs. Closed

Hey folks out there,

as we all know, Emacs has a very powerful (graphical) interface to our beloved debugger GDB. Until five minutes ago, I used to start a program in debug mode by first starting emacs and then invoking M-x gdb, which can take a long time to navigate to the desired binary. Furthermore, the directory where the binary resides isn’t the actual runtime-path in many cases. Fortunately, Emacs can be started in GDB-mode directly, by calling it with the --eval parameter on the command-line. What we want to evaluate during startup is the (gdb) function. Its only parameter is a string describing how gdb should be invoked. Thus, we can write the following bash function:

debug () {
    emacs --eval "(gdb \"gdb --annotate=3 --cd=`pwd` $*\")" &
}

Here, the --annotate option tells gdb to generate annotations that can be interpreted by the Emacs gdb interface (the integer argument is the annotation-level). Furthermore, the --cd parameter instructs gdb to change its working dir to the specified one, which is the current working dir in our case. Thus we can call debug ./bin/prog, for instance.

Convert FLAC to MP3

Published on 14. November 2010 in Linux. Closed

These days I needed to convert some FLAC (Free Lossless Audio Codec) audio files to mp3. As usual when dealing with audio/video formats, there are lots of (useless) tools available on the internet, most of them being shareware or even voruses (windows). Fortunately, this is a strength of Linux … at least if you are used to the console. All you need is lame and the necessary codecs (use apt-cache search to find what you need).

for file in *flac ; do flac -dc "$file" | lame -h - "${file%flac}mp3" ; done

Print QStrings in GDB

Published on 5. November 2010 in Programming. Closed

If you will ever have to go through the hell of debugging Qt-applications, you might face the problem of printing the contents of QString objects. If you call print on a QString object in gdb, the output will most probably look like the following:

{static null = {<No data fields>}, static shared_null = {ref = {_q_value = 1}, 
alloc = 0, size = 0, data = 0x80e6d0a, clean = 0, simpletext = 0, righttoleft = 0, 
asciiCache = 0, capacity = 0, reserved = 0, array = {0}}, 
static shared_empty = {ref = {_q_value = 2}, alloc = 0, size = 0, data = 0xb734deb2, 
clean = 0, simpletext = 0, righttoleft = 0, asciiCache = 0, capacity = 0, 
reserved = 0, array = {0}}, d = 0x8105138, static codecForCStrings = 0x0}

This is (in most cases) not quite what we want to know about our QString object. Fortunately, GDB can interpret certain routines, so that we can implement a function that prints QStrings. These routines are stored in GDB’s init-file ~/.gdbinit. Unfortunately, the internal data representation of a string has changed in Qt4, such that we most certainly need two versions of the desired function. I will call the functions qprint3 and qprint4, respectively.

Qt3 Version

The Qt3 version was originally posted by David Faure to the KDE maillist in 2001:

define qprint3
    set $i=0
    while $i < $arg0.d->len
	print $arg0.d->unicode[$i++].cl
    end
end

Qt4 Version

define qprint4
    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

Further reading

The KDE source repository has some further pretty neat GDB macros.