How to debug (line-by-line) Rcpp generated code in Windows?

Question

I am trying to debug Rcpp compiled code at run-time. I have been trying to get this to work unsuccessfully, for a very long time. A very similar question was as

Answer 1

It can be done with Visual Studio Code, since it can handle both R and C++. This allows you to step through your Rcpp code one line at a time in a GUI environment.

See this demo to get started.

Answer 2

The usual approach R -d gdb, which I also suggested in my original answer below, does not work on Windows:

--debugger=name
-d name

(UNIX only) Run R through debugger name. For most debuggers (the exceptions are valgrind and recent versions of gdb), further command line options are disregarded, and should instead be given when starting the R executable from inside the debugger.

https://cran.r-project.org/doc/manuals/r-release/R-intro.html#Invoking-R-from-the-command-line

Alternative:

1. Start R in debugger: gdb.exe Rgui.exe
2. Set break point: break TimesTwo2
3. Run R: run
4. Source file: Rcpp::sourceCpp("debug.cpp")
5. Use next, print, display to step through the code.

An alternative to step 1. would be to start R, get the PID with Sys.getpid(), attach debugger with gdb -p <pid>. You will than have to use continue instead of run.

---

I don't have a Windows machine right now, so the following was done on Linux. I hope it is transferable, though. Let's start with a simple cpp file (debug.cpp in my case) that contains your code:

#include <Rcpp.h>
using Rcpp::NumericVector;

// [[Rcpp::export]]
NumericVector timesTwo2(NumericVector x) {
  for(int ii = 0; ii <= x.size(); ii++)
  {
    x.at(ii) = x.at(ii) * 2;
  }
  return x;
}

/*** R
data = c(1:10)
data
timesTwo2(data)
*/

I can reproduce the error by calling R on the command line:

$ R -e "Rcpp::sourceCpp('debug.cpp')"

R version 3.5.1 (2018-07-02) -- "Feather Spray"
[...]

> Rcpp::sourceCpp('debug.cpp')

> data = c(1:10)

> data
 [1]  1  2  3  4  5  6  7  8  9 10

> timesTwo2(data)
Error in timesTwo2(data) : Index out of bounds: [index=10; extent=10].
Calls: <Anonymous> ... source -> withVisible -> eval -> eval -> timesTwo2 -> .Call
Execution halted

Next we can start R with gdb as debugger (c.f. Writing R Extensions as Dirk said):

$ R -d gdb -e "Rcpp::sourceCpp('debug.cpp')"
GNU gdb (Debian 8.2-1) 8.2
[...]
(gdb) break timesTwo2
Function "timesTwo2" not defined.
Make breakpoint pending on future shared library load? (y or [n]) y
Breakpoint 1 (timesTwo2) pending.
(gdb) run
[...]
> Rcpp::sourceCpp('debug.cpp')
[Thread 0xb40d3b40 (LWP 31793) exited]
[Detaching after fork from child process 31795]

> data = c(1:10)

> data
 [1]  1  2  3  4  5  6  7  8  9 10

> timesTwo2(data)

Thread 1 "R" hit Breakpoint 1, 0xb34f3310 in timesTwo2(Rcpp::Vector<14, Rcpp::PreserveStorage>)@plt ()
   from /tmp/RtmphgrjLg/sourceCpp-i686-pc-linux-gnu-1.0.0/sourcecpp_7c2d7f56744b/sourceCpp_2.so
(gdb)

At this point you can single step through the program using next (or just n) and output variables using print (or just p). A useful command is also display:

Thread 1 "R" hit Breakpoint 1, timesTwo2 (x=...) at debug.cpp:5
5   NumericVector timesTwo2(NumericVector x) {
(gdb) n
6     for(int ii = 0; ii <= x.size(); ii++)
(gdb) n
8       x.at(ii) = x.at(ii) * 2;
(gdb) display ii
2: ii = 0
(gdb) n
8       x.at(ii) = x.at(ii) * 2;
2: ii = 0

[...]

2: ii = 9
(gdb) 
46          inline proxy ref(R_xlen_t i) { return start[i] ; }
2: ii = 9
(gdb) 
6     for(int ii = 0; ii <= x.size(); ii++)
2: ii = 10
(gdb) 
8       x.at(ii) = x.at(ii) * 2;
2: ii = 10
(gdb) 
Error in timesTwo2(data) : Index out of bounds: [index=10; extent=10].
Calls: <Anonymous> ... source -> withVisible -> eval -> eval -> timesTwo2 -> .Call
Execution halted
[Detaching after fork from child process 32698]
[Inferior 1 (process 32654) exited with code 01]

BTW, I used the following compile flags:

$ R CMD config CXXFLAGS
-g -O2 -Wall -pedantic -fstack-protector-strong -D_FORTIFY_SOURCE=2

You might want to switch to -O0.