Understanding `Makevars` for linking to external C library in R package - rcpp

Question

I am working on a package which includes C code from third-party library (SUNDIALS). The package compiles and works (i.e., is able to solve a test ODE) with the following Makevars file performing static linking
CXX=clang++
PKG_CPPFLAGS = -I../inst/include
PKG_LDFLAGS = /usr/local/lib
PKG_LIBS= $(LAPACK_LIBS) $(BLAS_LIBS) $(FLIBS) $(PKG_LDFLAGS)/libsundials_cvode.a $(PKG_LDFLAGS)/libsundials_nvecserial.a
However, a slightly modified version (based on the example in R-Exts, i.e. -
PKG_LIBS = -L$(XML_DIR)/lib -lxml2) of Makevars (below) fails
CXX=clang++
PKG_CPPFLAGS = -I../inst/include
PKG_LDFLAGS = /usr/local/lib
PKG_LIBS= $(LAPACK_LIBS) $(BLAS_LIBS) $(FLIBS) -L$(PKG_LDFLAGS) -lsundials_cvode -lsundials_nvecserial -lm
fails with the following error message.
Error: package or namespace load failed for ‘Rcppsbmod’ in dyn.load(file, DLLpath = DLLpath, ...):
unable to load shared object '/Library/Frameworks/R.framework/Versions/3.4/Resources/library/Rcppsbmod/libs/Rcppsbmod.so':
dlopen(/Library/Frameworks/R.framework/Versions/3.4/Resources/library/Rcppsbmod/libs/Rcppsbmod.so, 6): Library not loaded: libsundials_cvode.3.dylib
Referenced from: /Library/Frameworks/R.framework/Versions/3.4/Resources/library/Rcppsbmod/libs/Rcppsbmod.so
Reason: image not found
Error: loading failed
Execution halted
ERROR: loading failed
* removing ‘/Library/Frameworks/R.framework/Versions/3.4/Resources/library/Rcppsbmod’
* restoring previous ‘/Library/Frameworks/R.framework/Versions/3.4/Resources/library/Rcppsbmod’
Exited with status 1.
I am not sure why it is looking for the libraries in another location when I am specifying PKG_LDFLAGS as /usr/local/lib.
As an aside, the test example which comes which the SUNDIALS package compiles and works with the following command
gcc -Wall cvRoberts_dns.c -o cvRoberts_dns.exe -I/usr/local/include -L/usr/local/lib/ -lsundials_cvode -lsundials_nvecserial -lm
So, I know that the library is installed properly and correct files (for linking) are available at /usr/local/lib location.
The entire package source code can be found at - https://github.com/sn248/Rcppsbmod
Any help or guidance will be highly appreciated!

Answer 1

System-wide dynamic linking, as in your second use case which fails, requires the cooperation of the dynamic linker on your system.
That means after build and copyring the libraries to /usr/local/lib you must typically run sudo ldconfig to update the linker cache.
You can check if the libraries are know by grep'ing through the output of ldconfig -p. On my system, no sundials:
edd#rob:~$ ldconfig -p | grep sundials
edd#rob:~$
Relatedly you can (locally) use different directories by declaring them in
/etc/ld.so.conf.d/somefile.conf -- but that is of course not portable and would not help you with a package designated for CRAN.
The use of static libraries you build as part of your package as in your first example would work as it does not require any system assistance. It just takes longer the build the libraries each time.

Answer 2

I am fighting with similar issues, c.f. Runtime linking R-extension on MacOS. My current workaround is to set the rpath at compile time. In your case that would mean:
CXX=clang++
PKG_CPPFLAGS = -I../inst/include
PKG_LDFLAGS = /usr/local/lib
PKG_LIBS= $(LAPACK_LIBS) $(BLAS_LIBS) $(FLIBS) -L$(PKG_LDFLAGS) -lsundials_cvode -lsundials_nvecserial -lm -Wl,-rpath,$(PKG_LDFLAGS)
However, this does not fix your problems. Comparing the error messages I see one difference: In your case the library libsundials_cvode.3.dylib is not found, while in my case it is #rpath/libaf.3.dylib. This means that the library you installed identifies itself as libsundials_cvode.3.dylib. You can check this with
$ otool -L /usr/local/lib/libsundials_cvode.3.dylib
/usr/local/lib/libsundials_cvode.3.dylib:
/usr/local/opt/sundials/lib/libsundials_cvode.3.dylib (compatibility version 3.0.0, current version 3.1.0)
/usr/lib/libSystem.B.dylib (compatibility version 1.0.0, current version 1252.0.0)
In your case the second output line should not contain an absolute path but only the basename of the library. My installation uses brew, which typically uses absolute paths as library names. In some trivial tests I had no problem with linking an R extension with these libraries.
I see several possibilities:
Try SUNDIAL from brew.
Adjust the library path in your installed libraries with
install_name_tool -id /usr/local/lib/libsundials_cvode.3.dylib /usr/local/lib/libsundials_cvode.3.dylib
to use absolute paths.
Adjust the library path in your installed libraries with
install_name_tool -id '#rpath/libsundials_cvode.3.dylib' /usr/local/lib/libsundials_cvode.3.dylib
and set rpath as above.
Adjust the name of the library your R extension is looking for with this addition to Makevars
all: $(SHLIB)
#if command -v install_name_tool; then install_name_tool -change libsundials_cvode.3.dylib /usr/local/lib/libsundials_cvode.3.dylib $(SHLIB); fi