Undefined reference when trying to use external library

Question

I am trying to incorporate a C library into some Rcpp code.

I can use the C library in a C++ program easily. I 'make' the C library, which creates the .a and .dll files in the /lib folder. I can then use the package by including the header in the program and running something like this from command line:

cc myfile.cpp -o myfile -Ipath.to.header path.to.lib.a -lz

This essentially tells the compiler to take the .cpp program, include headers from -I, and to link to two libraries.

Getting this to work with Rcpp shouldn't be overly difficult if I understand makevars correctly (which I unfortunately don't seem to).

I add the library to a folder in my package, and in src I add a makevars and makevars.win that look like this:

PKG_CFLAGS=
# specify header location
PKG_CPPFLAGS=-Ipath.to.lib/include
# specify libs to link to
PKG_LIBS=path.to.lib/lib/file.a -lz
# make library
path.to.lib/lib/file.a:
            cd path.to.lib;$(MAKE)

This correctly 'makes' the .a and .dll files for the library, however none of the Rcpp magic runs (i.e. in the build I never see the g++ system call that compiles the files in src), so "no Dll was created".

I am fairly certain this is a problem in my makevars target that makes the library. When I remove that portion from the makevars, and 'make' the library from the command line myself before building the package, I get the proper g++ calls with my -I and -l statements, but I get errors about undefined references.

I notice that the -l statements are only included in the final g++ call where the final .dll is made, but isn't included in the earlier g++ calls where the files with the library headers are compiled.

So I have two problems:

• How do I fix my makevars so that it 'makes' the library, but doesn't stop Rcpp from compiling the files in src?

• How do I deal with the undefined references? The library is clearly not header-only, so I am guessing it needs the -l statement in the earlier g++ calls, but that may not even be possible.

Answer 1

The best approach is to avoid complicated src/Makevars file altogether.

One easy-ish approach around this: use configure to build your static library, then once you actually build just refer to it in src/Makevars.

I use that scheme in Rblpapi (where we copy an externally supplied library in) and in nloptr where we download nlopt sources and build it 'when needed' (ie when no libnlopt is on the system).