GDAL - https://www.osgeo.org/projects/gdal/ - is a nice codebase that features lots of hooks for extensibility, is relatively performant, and handles the sorts of complex data and algorithms you might encounter in SDR.
The GDAL library is a present from the gods to the geospatial community (well, Frank Warmerdam mostly). I read the parts of the source code a while ago and the C component was fairly readable. I had troubles following the C++ code. That's the beauty of C, even if you don't use it frequently, you can still read and write half-decent code with it.
