Is there any C/C++ library that I can use to write GIMP xcf files? I want a standalone library that does not depend on the GIMP source (because it is to much work do separate things out from this huge program)?
Some KDE applications can read .xcf files - so I suspect that if you use KDE libraries, you cna get to read, not shure if it is possible to write .XCF files.
Otherwise, GIMP developers usually advise against separate implementations of access to .xcf file, so that they keep the freedom to change the file format as the program evolution requires it.
An open file format specification for multi-layer graphic images is "Open Raster" - There is a Python gimp plug-inf or reading and writing open-raster, it is the default file format for the MyPaint application - I don't know if there are any C or C++ libraries that can currently deal with OpenRaster files - the specification can be found here:
http://create.freedesktop.org/wiki/OpenRaster/Specification
(it is basically a .zip archive, with the raster data stored in .png s and XML for the relationship between the png files and metadata)