Recently we have been trying to solve multiscale mechanics problems with CalculiX. For this, we need to read the stress tensor from preCICE. We currently have a working version of the adapter which reads the stress tensor in Add functionality for volumetric coupling of material models by kalupaika · Pull Request #118 · precice/calculix-adapter · GitHub
The relevant lines of code are these ones. preCICE cannot handle tensor data directly, so we split the stress tensor into two vectors while writing to preCICE, and assemble it again before handing over to CalculiX.
I hope this helps.