Portable Makevars for R package using C, GSL and OpenMP with help of Rcpp

Question

I am constructing an R package where I have few third party C functions which use GSL and OpenMP, and I then call these from R using wrappers made with Rcpp (which just modify some arguments and call the appropriate C function). Everything works fine in my Windows machine, but I am not sure how to define the Makevars.win and Makevars file in a portable way. My Makevars.win looks like this:

## This assumes that the LIB_GSL variable points to working GSL libraries
PKG_CPPFLAGS=-I$(LIB_GSL)/include -I../inst/include
PKG_LIBS=-L$(LIB_GSL)/lib/x64 -lgsl -lgslcblas $(SHLIB_OPENMP_CFLAGS)
PKG_CFLAGS=$(SHLIB_OPENMP_CFLAGS)

It is basically copied from various sources. Like said, this compiles in my computer (using toolchain of from Rtools), and if I remove PKG_CFLAGS=$(SHLIB_OPENMP_CFLAGS) I can also compile without OpenMP (for some reason I don't understand I get error if I remove OpenMP flag in PKG_LIBS).

My Makevars file looks identical, but I don't have access to Unix platforms so I am not sure how to deal with that side. My guess is that I need to replace LIB_GSL with something?

EDIT:

Okay, I think I finally understand how configure etc. works and was able to get everything working.

My Makevars.win:

## This assumes that the LIB_GSL variable points to working GSL libraries
PKG_CPPFLAGS=-I$(LIB_GSL)/include -I../inst/include
PKG_LIBS="-L$(LIB_GSL)/lib/$(R_ARCH)" -lgsl -lgslcblas $(SHLIB_OPENMP_CFLAGS)
PKG_CFLAGS=$(SHLIB_OPENMP_CFLAGS)

My Makevars.in:

GSL_CFLAGS = @GSL_CFLAGS@
GSL_LIBS = @GSL_LIBS@

PKG_LIBS=$(GSL_LIBS) $(SHLIB_OPENMP_CFLAGS)
PKG_CFLAGS=$(GSL_CFLAGS) $(SHLIB_OPENMP_CFLAGS)

And my configure.ac:

AC_INIT([testpackage], 0.0.1)

## Use gsl-config to find arguments for compiler and linker flags
##
## Check for non-standard programs: gsl-config(1)
AC_PATH_PROG([GSL_CONFIG], [gsl-config])
## If gsl-config was found, let's use it
if test "${GSL_CONFIG}" != ""; then
    # Use gsl-config for header and linker arguments
    GSL_CFLAGS=`${GSL_CONFIG} --cflags`
    GSL_LIBS=`${GSL_CONFIG} --libs`
else
    AC_MSG_ERROR([gsl-config not found, is GSL installed?])
fi

# Now substitute these variables in src/Makevars.in to create src/Makevars
AC_SUBST(GSL_CFLAGS)
AC_SUBST(GSL_LIBS)

AC_OUTPUT(src/Makevars)

I then run autoconf in testpackage directory in order to get configure file, which in turn converts Makevars.in to Makevars when running R CMD INSTALL.

Answer 1

There are a few CRAN packages using the GSL, and/or our RcppGSL bindings. Here is what I do in one of these (my RcppZiggurat package):

PKG_CPPFLAGS = -I. -I../inst/include
## Use the R_HOME indirection to support installations of multiple R version
PKG_LIBS = `$(R_HOME)/bin/Rscript -e "RcppGSL:::LdFlags()"`

That is the entire /src/Makevars. You can trivially add the same OpenMP variable from R. This does of course create a dependency on RcppGSL (as I already use it in RcppZiggurat). Else you can look into the R/init.R of RcppGSL and see how it tries to talk to gsl-config and store those values. You can do the same in src/Makevars -- it is just basic Make usage and nothing Rcpp specific.