Pragma directive to get thread number not working Rcpp

Question

I am trying to parallelise a loop using pragma directives in Rcpp. Aside from a warning message during compilation that pragma is not recognised (although this appears to be a non-issue from what I have read), other pragma commands are not working. This is the minimal example I have been using (content of the for-loop is irrelevant):

sourceCpp(code = '
#include <Rcpp.h>
#include <omp.h>
using namespace Rcpp ; 

// [[Rcpp::export]]
int test() {

#pragma omp parallel for
int foo = omp_get_num_threads() ;
for(int i = 0; i < 2; i++) {
  Rcout << i ;
}
return foo ;

}')

Here is my error:

"C:/rtools40/mingw64/bin/"g++ -std=gnu++11  -I"C:/PROGRA~1/R/R-40~1.4/include" -DNDEBUG   -I"C:/Users/User/Documents/R/win-library/4.0/Rcpp/include" -I"C:/Users/User/AppData/Local/Temp/RtmpWW0LXx/sourceCpp-x86_64-w64-mingw32-1.0.4.6"        -O2 -Wall  -mfpmath=sse -msse2 -mstackrealign -c file2fe83fae2189.cpp -o file2fe83fae2189.o
file2fe83fae2189.cpp:9: warning: ignoring #pragma omp parallel [-Wunknown-pragmas]
 #pragma omp parallel for
 
C:/rtools40/mingw64/bin/g++ -std=gnu++11 -shared -s -static-libgcc -o sourceCpp_90.dll tmp.def file2fe83fae2189.o -LC:/PROGRA~1/R/R-40~1.4/bin/x64 -lR
C:/rtools40/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/8.3.0/../../../../x86_64-w64-mingw32/bin/ld.exe: file2fe83fae2189.o:file2fe83fae2189.cpp:(.text+0x106): undefined reference to `omp_get_num_threads'
collect2.exe: error: ld returned 1 exit status

I am on a Windows machine so the MacOS compiler issue should not apply, and my num_threads call is inside the pragma section. Any ideas on what is going wrong here?

Answer 1

While this stuff can be finicky, you clearly missed the fact that you must inform Rcpp that you want an OpenMP compilation: you do this via the plugin (or in a package, which is what you should probably use anyway, via the src/Makevars or src/Makevars.win variable).

Anyway, here is a worked example I just derived from an older C++ example I had hanging around.

Code

#include <Rcpp.h>
#include <omp.h>

// [[Rcpp::plugins(openmp)]]

// [[Rcpp::export]]
int foo() {
    
    int th_id, nthreads;

#pragma omp parallel private(th_id)
    {
        th_id = omp_get_thread_num();
        std::ostringstream ss;  // allow for better synchronization
        ss << "Hello World from thread " << th_id << std::endl;
        Rcpp::Rcout << ss.str();
#pragma omp barrier
#pragma omp master
        {
            nthreads = omp_get_num_threads();
            Rcpp::Rcout << "There are " << nthreads << " threads" << std::endl;
        }
    }
    return 0;
}

/*** R
foo()
*/

Output

On my machine with a hyperthreaded six-core cpu:

> Rcpp::sourceCpp("answer.cpp")

> foo()
Hello World from thread 0
Hello World from thread 1
Hello World from thread 8
Hello World from thread 10
Hello World from thread 4
Hello World from thread 9
Hello World from thread 11
Hello World from thread 7
Hello World from thread 3
Hello World from thread 5
Hello World from thread 6
Hello World from thread 2
There are 12 threads
[1] 0
>