I ran into an unexpected seg fault, which took me too long to realize was because of the old-school "you forgot the ierr" mistake! I was expecting the compiler to complain, since we've had better checking for a while. E.g. as in the attached code to reproduce, my compiler indeed errors on this call PCSetType(pc, PCLU) but not this call PCFactorSetMatOrderingType(pc, MATORDERINGEXTERNAL) I'm not yet seeing what the difference is, but there is still plenty I don't understand about how the custom fortran interfaces work. E.g. both of those functions have custom interfaces in ftn-custom directories, accepting an additional "len" argument to be used with FIXCHAR(), but I'm not sure how that argument is ultimately populated.