How to set openMP for Multithreading in ubuntu 12.04 for an openCV project?

Question

I want to do multi threading in one of my for loop using #pragma omp parallel. So, i am writing the following code:

#pragma omp parallel for 
for(int i=0; i<square->total; i++)
{
......
}

My project have a CMakeList.txt and Makefile. I don't understand, how do i tell the compiler and linker to use openMP?

Updates:

I have edited my CMakeList.txt with following code

find_package(OpenMP)
if (OPENMP_FOUND)
    set (CMAKE_C_FLAGS "${CMAKE_C_FLAGS} ${OpenMP_C_FLAGS}")
    set (CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} ${OpenMP_CXX_FLAGS}")
endif()

and then i did cmake .. and got the following result at terminal:

-- The C compiler identification is GNU

-- The CXX compiler identification is GNU

-- Check for working C compiler: /usr/bin/gcc

-- Check for working C compiler: /usr/bin/gcc -- works

-- Detecting C compiler ABI info

-- Detecting C compiler ABI info - done

-- Check for working CXX compiler: /usr/bin/c++

-- Check for working CXX compiler: /usr/bin/c++ -- works

-- Detecting CXX compiler ABI info

-- Detecting CXX compiler ABI info - done

-- Try OpenMP C flag = [-fopenmp]

-- Performing Test OpenMP_FLAG_DETECTED

-- Performing Test OpenMP_FLAG_DETECTED - Success

-- Try OpenMP CXX flag = [-fopenmp]

-- Performing Test OpenMP_FLAG_DETECTED

-- Performing Test OpenMP_FLAG_DETECTED - Success

-- Found OpenMP: -fopenmp

-- Configuring done

-- Generating done

#pragma omp parallel for for(int i=0; i<square->total; i++) — herohuyongtao
yes, i tried...but same error that "i has not been declared in this scope"...but if i remove this "pragma" stuff then the program runs — skm
OP, if my attempt at an answer below doesn't help, please give us a complete but simple example (with main() and #includes and a (simple) loop body), including how you are trying to compile. — Nicu Stiurca

Andrey Smorodov Andrey Smorodov · Accepted Answer · 2014-02-11T21:03:59

@user2440724

I wrote the code sample as you ask:

#include <string>
#include <iostream>
#include <vector>
#include <omp.h>
using namespace std;
using namespace cv;
//----------------------------------------------------------
// MAIN
//----------------------------------------------------------
int main(int argc, char* argv[])
{
#ifdef _OPENMP 
#pragma omp parallel for
#endif
    for (int i = 0; i < 10; i++)
    {
      std::cout << "A" << std::endl;    
    }
    getchar();
    return 0;
}

in vs2010 you need enable openmp as shown below: enter image description here

Good tutorial for linux here: http://goulassoup.wordpress.com/2011/10/28/openmp-tutorial/ And GNU OpenMP docs: http://gcc.gnu.org/onlinedocs/libgomp/index.html#toc_Top

How to set openMP for Multithreading in ubuntu 12.04 for an openCV project?

2 Answers