calling fortran from C

Hello, I had a question about calling some of R's fortran routines from C.
Specifically, I would like to call: dqrfit from some C code which will be
bundled as an R package. I was hoping someone knew of an example in some
of R's code which does something like this (any fortran call from R's C
would probably be a sufficient guide). So far I can only find locations
where R calls the Fortran directly, but this is not an option for me.

Also, I am trying to gauge the overhead of making this call, does anyone
have any knowledge of whether there might be some non-trivial constant
time penalty on making such a call.

Thanks in advance, Jim