Building shared libraries in subdirectories

Question

I am trying to build an R package that uses some C code. I have a C library that is compiled into an executable, that can be called from the command line. There is a Makefile as

Answer 1

callable.c

#include 

int main(int argc, char **argv) {
    printf("Hello World\n");
    return 0;
}

To get a .so file from the C file callable.c use

R CMD SHLIB callable.c

So now we have callable.so

R normally requires that all C arguments be pointers, but if your main method ignores argc, then we can get around this. So to call main from callable.so from R we can write a method like so.

main <- function() {
    dyn.load("callable.so")
    out <- .C("main", argc=0, argv="")
}

Calling that main function will run the C main function.