How to debug an R package (with C code) in Emacs using GDB?

Question

I am currently writing an R package and using compiled C++ code through the Rcpp package in R (Rcpp makes the interaction of R and C++ code easier for a non-programmer like me, IMHO).

I want to debug a few errors in my C++ program using gdb. I have googled and found mainly a few resources on debugging R within emacs, R-FAQ, a few mails here, and definitely the R's Writing R Extension Manual.

However, I am doing this for the first time, I could not go too far. Could anyone give me a few pointers on how to debug R packages (or extensions with C++/C code) within emacs. Specifically, I want to take advantages of using ESS with R and gdb with Emacs (as the R-FAQ talks about).

Please note, I am ok on how to use gdb using only C or C++ programs. But I could not translate this knowledge to using gdb with R and extensions.

Answer 1

You can leverage your existing knowledge of debugging C++ programs by turning the problem into a pure C++ development and debugging task using RInside (a great companion to Rcpp).

Write a main() C++ function that creates an R instance using RInside, executes R code (or sources an R script) that sets up the test case, and then call the function under test from main(), e.g.

#include <Rcpp.h>
#include <RInside.h>
#include "function_under_test.h"

int main(int argc, char *argv[]) 
{
    using namespace std;
    using namespace Rcpp;

    RInside R(argc, argv);

    string evalstr = R"(
        a <- matrix(c(1,1,1, 1,1,1, 1,1,1), nrow = 3, ncol=3)
    )";
    R.parseEvalQ(evalstr);

    SEXP a = R["a"];

    R["b"] = function_under_test(a);

    evalstr = R"(
        print(b)
    )";
    R.parseEvalQ(evalstr);

    return 0;
}

Then proceed as usual when debugging a C++ program with gdb by setting breakpoints in function_under_test() etc.

This way you avoid switching between R and C++ development environments and having to re-install the R package.

Answer 2

It's not all that easy, unfortunately. You need to jump between ESS, gdb (ie gud in Emacs) and R. The best description is probably still win Writing R Extensions, however there was a recent thread on the ESS mailing list that discusses this too (and note that some replies came in outside the thread so do look at the mailing list archive too).