It can be very difficult to debug compiled code when linked within R. The goal of this section is to show how this process can be made easier with the use of some of the c++ tool chain. We'll cover the following:

• use assert to check catch issues in non production phase
• use gdb to find where a crash happens

A simple crashing routine

Let's write a simple RcppGSL function that performs a spline interpolation. The error will happen because we are going to ask for an evaluation that is outside the support.

#include<iostream>
using namespace std;

RcppExport SEXP cpp_lagrWithEffort( SEXP R_args){
  BEGIN_RCPP

  bool crashit = Rcpp::as<bool>(R_args);

  if (crashit) {
      int x = 7;
      int *p = 0;

      cout << "x = " << x;
      cout << "The pointer points to the value " << *p;
  } else {
      cout << "you are lucky." << endl;
  }
  return(NULL);
  
  END_RCPP
}

Using assert

I recommend using the following simple macro. Copy the code snippet into an assert.h file that you can then include in your code.

#ifndef DEBUG
#define ASSERT(x)
#else
#define ASSERT(x) \
 if (! (x)) \
 { \
    cout << "ERROR!! Assert " << #x << " failed\n"; \
    cout << " on line " << __LINE__  << "\n"; \
    cout << " in file " << __FILE__ << "\n";  \
 }
#endif

Then you can populate your code with various assertion commands.

Using gdb

Start R in debug mode using

R -d gdb -e "source('tmp.r')"

you will then get a prompt. Type run at the prompt. When the program crashes, you can display the stack by calling backtrace.

When you find out the segfault, declare some breakpoints right before it crashes, for example here I catcth the call to exit:

break exit

Then run again, the program will break at the breakpoint. You can use step to step through the program or use next without stepping into functions.

It is important to remember compiling the code with the flags -rdynamic -g to get correct information. Later on for speed the code should be recompiled without the -g file and probably using the -O3 flag.

You can type CXXFLAGS = -rdynamic -g into ~/.R/Makevars.

an example debugging session

tbd