[Proj] DHDN / Gauss Krueger to WGS84

[Oliver Eichler]

Hi,

looks like this question has been posted in the past, but blimey, I
could not extract a simple answer that I do understand. Thus  I would
like to rephrase the question in the hope to get an easy to use answer
for someone not deep in geo math at all :)

I do have a coordinate in easting/northing [m] from a raster map  using
Gauss Krüger projection and Potsdam datum. Now I would like to get the
lon/lat coordinate on a WGS84 ellipsoid. And vice versa. I would like to
use the proj4 C API not the command line.

Can any kind soul take me by the hand and point me to some tutorial or
give me a quick guided tour?

Thanks,

Oliver