compiling openmp, macports gcc, and eclipse cdt

Question

I am newbie in openmp. The following is the environment.

OS : Mac OSX Mavericks
Compiler : gcc (MacPorts gcc48 4.8.2_0) 4.8.2
IDE : Eclipse Kepler CDT plugin

I wrote the following openmp program

#include < stdio.h>  
#include < omp.h>  

int main()  
{
#pragma omp parallel
{
    int i=omp_get_thread_num();
    printf("hello (%d)",i);
    printf("world (%d)",i);
  }
}

I compiled the above program and got the error that omp.h is not found and lgomp not found. Hence I added in the project properties an include path with /opt/local/lib/gcc48/gcc/x86_64-apple-darwin13/4.8.2/include and a library path /opt/local/lib/gcc48. The include path had the omp.h file and the library path had the file libomp.o.

I include the -fopenmp option in both the linker and the compiler option through project properties. It is compiling with gcc -I/opt/local/lib/gcc48/gcc/x86_64-apple-darwin13/4.8.2/include -O0 -g3 -Wall -c -fmessage-length=0 -fopenmp -MMD -MP -MF"src/OpenMPCourseExamples.d" -MT"src/OpenMPCourseExamples.d" -o "src/OpenMPCourseExamples.o" "../src/OpenMPCourseExamples.c" and linking with the command "gcc -L/opt/local/lib/gcc48 -fopenmp -o "OpenMPCourseExamples" ./src/OpenMPCourseExamples.o".

With the above command it compiles without an error but with a warning - "warning: unknown pragma ignored [-Wunknown-pragmas] #pragma omp parallel".

Also, I set an environment variable in the launch properties with OMP_NUM_THREADS=4. I ran the program that compiled with the above warning. I am getting only "hello (0)world (0)". I was under the impression that I should start four threads and should see the other outputs of "hello(1)world(1)hello(2)world(2)hello(3)world(3)" in some ordering as well. Now, here are my following questions.

Why am I getting the #pragma warning?
Is the compiler really detecting the openmp and building with openmp?
If everything is correct, why am I not seeing four different threads getting started?

Hristo Iliev Hristo Iliev · Accepted Answer · 2013-12-10T10:13:03

MacPorts configures the GCC build process with --program-suffix=-mp-${major} and therefore all compiler executables have the -mp-4.8 suffix. When you call gcc, you end up using Apple's Clang compiler, which does not support OpenMP and therefore does not recognise the -fopenmp option and #pragma omp ....

You have to do the following changes to the project settings:

Change the compiler command to gcc-mp-4.8
Change the linker command to gcc-mp-4.8
Remove the explicit specification of the include and library paths since the presence of -fopenmp adds them automatically.

compiling openmp, macports gcc, and eclipse cdt

2 Answers