The PAL library is a partial re-implementation of Pat Wallace's popular SLALIB library written in C using a Gnu GPL license and layered on top of the IAU's SOFA library (or the BSD-licensed ERFA) where appropriate. PAL attempts to stick to the SLA C API where possible.
https://ui.adsabs.harvard.edu/abs/2013ASPC..475..307J or https://ui.adsabs.harvard.edu/abs/2016ascl.soft06002J