What does `LinkingTo` do in an R package? [duplicate] - r

Question

This question already has an answer here:
Do I need to import RccpEigen in the DESCRIPTION file for an R package using it, or is "LinkingTo" enough?
(1 answer)
Closed 5 years ago.
I am building an R package whose description contains:
LinkingTo: Rcpp
The package has a fairly deep makefile structure. I know using R CMD build . creates and modifies variables such as CXX11FLAGS which must be passed down through subsequent layers of makefiles.
I suspect that LinkingTo also produces such a variable, which I must be aware of and pass along. I suspect this because, several layers down, I hit the error:
mycode.hpp:5:10: fatal error: Rcpp.h: No such file or directory
#include <Rcpp.h>
I'm not sure how to inform this file's makefile as to where Rcpp is. I suspect the hypothesized variable could be used, but I do not know that variable's name. Can anyone clarify?

Answer 1

Here is LinkingTo: from a smaller in-progress package of mine:
LinkingTo: Rcpp, RcppArmadillo
When compiling, these two packages are both uses via -I... switches:
edd#bud:~/git/rcppkalman(master)$ ./cleanup
edd#bud:~/git/rcppkalman(master)$ R CMD INSTALL .
* installing to library ‘/usr/local/lib/R/site-library’
* installing *source* package ‘RcppKalman’ ...
** libs
ccache g++ -I/usr/share/R/include -DNDEBUG -I"/usr/local/lib/R/site-library/Rcpp/include" -I"/usr/local/lib/R/site-library/RcppArmadillo/include" -fpic -g -O3 -Wall -pipe -Wno-unused -pedantic -Werror -march=native -c RcppExports.cpp -o RcppExports.o
ccache g++ -I/usr/share/R/include -DNDEBUG -I"/usr/local/lib/R/site-library/Rcpp/include" -I"/usr/local/lib/R/site-library/RcppArmadillo/include" -fpic -g -O3 -Wall -pipe -Wno-unused -pedantic -Werror -march=native -c expmMat.cpp -o expmMat.o
ccache g++ -I/usr/share/R/include -DNDEBUG -I"/usr/local/lib/R/site-library/Rcpp/include" -I"/usr/local/lib/R/site-library/RcppArmadillo/include" -fpic -g -O3 -Wall -pipe -Wno-unused -pedantic -Werror -march=native -c kfpredict.cpp -o kfpredict.o
ccache g++ -I/usr/share/R/include -DNDEBUG -I"/usr/local/lib/R/site-library/Rcpp/include" -I"/usr/local/lib/R/site-library/RcppArmadillo/include" -fpic -g -O3 -Wall -pipe -Wno-unused -pedantic -Werror -march=native -c kfupdate.cpp -o kfupdate.o
ccache g++ -I/usr/share/R/include -DNDEBUG -I"/usr/local/lib/R/site-library/Rcpp/include" -I"/usr/local/lib/R/site-library/RcppArmadillo/include" -fpic -g -O3 -Wall -pipe -Wno-unused -pedantic -Werror -march=native -c ltidisc.cpp -o ltidisc.o
ccache g++ -I/usr/share/R/include -DNDEBUG -I"/usr/local/lib/R/site-library/Rcpp/include" -I"/usr/local/lib/R/site-library/RcppArmadillo/include" -fpic -g -O3 -Wall -pipe -Wno-unused -pedantic -Werror -march=native -c rtssmooth.cpp -o rtssmooth.o
ccache g++ -I/usr/share/R/include -DNDEBUG -I"/usr/local/lib/R/site-library/Rcpp/include" -I"/usr/local/lib/R/site-library/RcppArmadillo/include" -fpic -g -O3 -Wall -pipe -Wno-unused -pedantic -Werror -march=native -c tfsmooth.cpp -o tfsmooth.o
g++ -Wl,-S -shared -L/usr/lib/R/lib -Wl,-Bsymbolic-functions -Wl,-z,relro -o RcppKalman.so RcppExports.o expmMat.o kfpredict.o kfupdate.o ltidisc.o rtssmooth.o tfsmooth.o -llapack -lblas -lgfortran -lm -lquadmath -L/usr/lib/R/lib -lR
installing to /usr/local/lib/R/site-library/RcppKalman/libs
** R
** demo
** inst
** preparing package for lazy loading
** help
*** installing help indices
** building package indices
** testing if installed package can be loaded
* DONE (RcppKalman)
edd#bud:~/git/rcppkalman(master)$
There is nothing else needed. Writing R Extensions says:
A package that wishes to make use of header files in other packages
needs to declare them as a comma-separated list in the field
'LinkingTo' in the 'DESCRIPTION' file. For example
LinkingTo: link1, link2
The 'LinkingTo' field can have a version requirement which is checked
at installation.
Specifying a package in 'LinkingTo' suffices if these are C++
headers containing source code or static linking is done at
installation: the packages do not need to be (and usually should not
be) listed in the 'Depends' or 'Imports' fields. This includes CRAN
package BH (https://CRAN.R-project.org/package=BH) and almost all
users of
RcppArmadillo (https://CRAN.R-project.org/package=RcppArmadillo) and
RcppEigen (https://CRAN.R-project.org/package=RcppEigen).
For another use of 'LinkingTo' see *note Linking to native routines
in other packages::.
and that is on point as can be seen from the (essentially empty) src/Makevars:
PKG_LIBS = $(LAPACK_LIBS) $(BLAS_LIBS) $(FLIBS)
which is the standard uses by RcppArmadillo to support the external linking for LAPACK and BLAS. Note that my package is a little behind as RcppArmadillo in its most recent release now uses this inst/skeleton/Makevars passes into every package created via RcppArmadillo.package.skeleton():
## optional
#CXX_STD = CXX11
PKG_CXXFLAGS = $(SHLIB_OPENMP_CXXFLAGS)
PKG_LIBS = $(SHLIB_OPENMP_CFLAGS) $(LAPACK_LIBS) $(BLAS_LIBS) $(FLIBS)
to also support OpenMP where available. That is the current recommended form.

Answer 2

I edited my Makefile to include the following dark magic:
VARS_OLD := $(.VARIABLES)
$(foreach v, \
$(filter-out $(VARS_OLD) VARS_OLD,$(.VARIABLES)), \
$(info $(v) = $($(v))))
This prints out all of the environmental variables passed to the make process by R when you run R CMD build ..
Digging in this revealed some very interesting variables:
ALL_CPPFLAGS = -I/usr/share/R/include -DNDEBUG -I"/home/myuser/.R/x86_64-pc-linux-gnu-library/3.4/Rcpp/include"
CLINK_CPPFLAGS = -I"/home/myuser/.R/x86_64-pc-linux-gnu-library/3.4/Rcpp/include"
R_INCLUDE_DIR = /usr/share/R/include
Note that these variables contain the -I flag and, thus, must be communicated to any part of the build process that relies on those.
By passing these between makefiles I was able to achieve compilation.