C test functions for lmer()

Hi everyone, I am trying to use lmer() using only C; Does anyone have test
functions available that do not require R? My main routine needs to be C, but I
am having a hard time understanding the data types going back and forth between
R and lme4 C code. Any help would be appreciated,

Well, naturally enough, those structures are the internal
representations of R objects - vectors, matrices, functions, etc.  You
would need to read the "Writing R Extensions" manual to understand
their structure and even then you would need most of the R API to be
able to manipulate them.  If you really want to embark on this, and it
is definitely not trivial, I would recommend looking at the code in
the lme4a package instead, because it is based on C++ classes and the
Rcpp package.  I'm not sure that I see the point, though, as R is
already open source.  If you want to reimplement for another system
you would probably be better off examining the structure of the
calculations and reimplementing them instead of trying to adapt the
lme4 code.