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eigen
Sten Roar Snare
Sten Roar Snare

Reputation: 1

Including <Eigen/Dense> when using /openmp

I am pretty new user of Eigen and have run into a weird problem. I am adding to a C++ project that uses OpenMP (Visual Studio 2012 compiler, /openmp set). I get a compilation error:

include\eigen\src/Core/products/Parallelizer.h(34): error C3861: 'omp_get_max_threads': identifier not found

I have tried to google around for an answer, but have failed to find a solution. We have another project, not using openmp, where Eigen has been used successfully for a while. Adding /openmp to that project did not trigger the problem. I also tried to disable openmp in Eigen, using the EIGEN_DONT_PARALLELIZE preprocessor directive. The problem persists. All suggestions to solve the problem are more than welcome.

Upvotes: 0

Views: 1097

Answers (1)

Avi Ginsburg
Avi Ginsburg

Reputation: 10596

Long comment, not really an answer: Something appears to be broken in your project. I'm using Eigen 3.2.9 as a reference, as you haven't specified which version you're using. In Eigen/Core (133) we have

#if (defined _OPENMP) && (!defined EIGEN_DONT_PARALLELIZE)
  #define EIGEN_HAS_OPENMP
#endif

#ifdef EIGEN_HAS_OPENMP
#include <omp.h>
#endif

So, if you properly defined EIGEN_DONT_PARALLELIZE in your project, EIGEN_HAS_OPENMP shouldn't be defined and omp.h shouldn't be included. Additionally, in Parallelizer.h(30):

#ifdef EIGEN_HAS_OPENMP
if(m_maxThreads>0)
  *v = m_maxThreads;
else
  *v = omp_get_max_threads();
#else
*v = 1;
#endif

So if you had properly defined EIGEN_DONT_PARALLELIZE, you would not be getting the error you are getting.

Regarding the C3861 error, it means that the compiler is not able to find a declaration for omp_get_max_threads (called in Parallelizer.h). As that code is called within a #ifdef EIGEN_HAS_OPENMP as is the line #include <omp.h> in Core, and omp_get_num_threads is only wrapped in an #if defined( __cplusplus) you could add a check in Core or omp.h to make sure that the code is active

// This is in Eigen/Core
#ifdef EIGEN_HAS_OPENMP
static_assert(0, "OMP FILE IS INCLUDED IN CORE...");
#include <omp.h>
#endif

and 

// This is in omp.h
static_assert(0, "OMP FILE IS PROPERLY INCLUDED...");
_OMPIMP int _OMPAPI
omp_get_num_threads(
    void
    );

You should get both as errors if omp is loaded correctly.

Upvotes: 1

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