Set number of threads using omp_set_num_threads() to 2, but omp_get_num_threads() returns 1

Question

I have the following C/C++ code using OpenMP:

    int nProcessors=omp_get_max_threads();     if(argv[4]!=NULL){         printf("argv[4]: %s\n",argv[4]);         nProcessors=atoi(argv[4]);         printf("nProcessors: %d\n",nProcessors);     }     omp_set_num_threads(nProcessors);     printf("omp_get_num_threads(): %d\n",omp_get_num_threads());     exit(0); 

As you can see, I'm trying to set the number of processors to use based on an argument passed on the command line.

However, I'm getting the following output:

argv[4]: 2   //OK nProcessors: 2   //OK omp_get_num_threads(): 1   //WTF?! 

Why isn't omp_get_num_threads() returning 2?!!!

---

As has been pointed out, I'm calling omp_get_num_threads() in a serial region, hence the function returns 1.

However, I have the following parallel code:

#pragma omp parallel for private(i,j,tid,_hash) firstprivate(firstTime) reduction(+:nChunksDetected)     for(i=0;i<fileLen-CHUNKSIZE;i++){         tid=omp_get_thread_num();         printf("%d\n",tid);         int nThreads=omp_get_num_threads();         printf("%d\n",nThreads); ... 

which outputs:

0   //tid 1   //nThreads - this should be 2! 0 1 0 1 0 1 ... 

Answer 1

The omp_get_num_threads() call returns 1 in the serial section of the code. See Link

So you need to have parallel code to get the correct value, here how your code should look like:

#include <iostream> #include <omp.h>  int main (int argc, const char * argv[]) {     int nProcessors = omp_get_max_threads();      std::cout<<nProcessors<<std::endl;      omp_set_num_threads(nProcessors);      std::cout<<omp_get_num_threads()<<std::endl;  #pragma omp parallel for      for(int i = 0; i < 5; i++){         int tid = omp_get_thread_num();         std::cout<<tid<<"\t tid"<<std::endl;         int nThreads = omp_get_num_threads();         std::cout<<nThreads<<"\t nThreads"<<std::endl;     }      exit(0); } 

This code produces:

2

1 0    tid 2    nThreads 0    tid 2    nThreads 0    tid 2    nThreads 1    tid 2    nThreads 1    tid 2    nThreads 

It seems that you have either open mp not enabled or your loop is not in the form that can be parallized by openmp