The maximum number of threads in C++ OpenMP is always 1

Question

The maximum number of threads in C++ OpenMP is always 1

We set the task to write a couple of algorithms in C++ using OpenMP. Since I have a Macbook, I had to install the Clion IDE and work with it.

Code main.cpp:

#include <iostream>
#include "/usr/local/opt/libomp/include/omp.h"

#define THREAD_NUM 4
int main()
{
    omp_set_num_threads(THREAD_NUM);
    std::cout << omp_get_thread_limit << std::endl;
#pragma omp parallel for num_threads(4)
    {
        std::cout << "Number of available threads: " << omp_get_num_threads() << std::endl;
        std::cout << "Current thread number: " << omp_get_thread_num() << std::endl;
        std::cout << "Hello, World!" << std::endl;
    }
    return 0;
}

Code CMakeLists.txt:

cmake_minimum_required(VERSION 3.14)
project(OpenMPTest)

set(CMAKE_CXX_STANDARD 17)

add_executable(OpenMPTest main.cpp)

find_package(OpenMP REQUIRED)
target_link_libraries(${PROJECT_NAME} ${OpenMP_CXX_LIBRARIES})

When compiling, it always outputs the following:

1
Number of available threads: 1
Current thread number: 0
Hello, World!

I can't understand the reason why the number of threads doesn't increase according to the value I'm trying to set it. I must say that I have not worked with C++ or C ++ before. OpenMP.

OS: macOS Catalina Beta Version 10.15.6 (19G36e)

2

c++clion mac macos openmp

Author: EVGKZN, 2020-09-11

Source

1 answers

score 6 · Accepted Answer

You use the omp parallel for directive, but you don't have a for loop there. What do you expect to get like this? Remove the word for from here and you will be happy.

#include <iostream>
#include "/usr/local/opt/libomp/include/omp.h"
// #include <omp.h>

#define THREAD_NUM 4
int main()
{
    omp_set_num_threads(THREAD_NUM);
    std::cout << omp_get_thread_limit() << std::endl;
#pragma omp parallel num_threads(4)
    {
        std::cout << "Number of available threads: " << omp_get_num_threads() << std::endl;
        std::cout << "Current thread number: " << omp_get_thread_num() << std::endl;
        std::cout << "Hello, World!" << std::endl;
    }
    return 0;
}

The example was built using mingw-w64 under Win10; command line: g++ test2.cpp -o test2 -O2 -fopenmp

PS: also, you didn't write parentheses for the function omp_get_thread_limit() and because of this, you didn't get the corresponding number.