Need a simple 3D vector and 3×3 Matrix utility in C? I just published one on GitHub providing basic functionalities like orthonormalizing, transposing, multiplying (etc.) matrices and vectors. It was supposed to be used in Nuttx drivers but certainly can be used in other contexts. For Nuttx, I had to add mathematical calibration to an acceleration sensor driver, so that x y and z axes can be re-specified no matter how the sensor is built in.