OpenMP with CMake

README.md

OpenMP simple C example with CMake

For macOS, first do:

brew install libomp

and set in ~/.zshrc

export OpenMP_ROOT=$(brew --prefix)/opt/libomp

export LDFLAGS="$LDFLAGS -L${OpenMP_ROOT}/lib"

CMakeLists.txt

cmake_minimum_required(VERSION 3.19)

project(OpenMPdemo LANGUAGES C)

find_package(OpenMP COMPONENTS C REQUIRED)

add_executable(hello hello_openmp.c)
target_link_libraries(hello PRIVATE OpenMP::OpenMP_C)

hello_openmp.c

/******************************************************************************
// https://computing.llnl.gov/tutorials/openMP/samples/C/omp_hello.c
* FILE: omp_hello.c
* DESCRIPTION:
*   OpenMP Example - Hello World - C/C++ Version
*   In this simple example, the master thread forks a parallel region.
*   All threads in the team obtain their unique thread number and print it.
*   The master thread only prints the total number of threads.  Two OpenMP
*   library routines are used to obtain the number of threads and each
*   thread's number.
* AUTHOR: Blaise Barney  5/99
* LAST REVISED: 04/06/05
******************************************************************************/
#include <omp.h>
#include <stdio.h>
#include <stdlib.h>

int main (void)
{
int nthreads, tid;

/* Fork a team of threads giving them their own copies of variables */
#pragma omp parallel private(nthreads, tid)
  {

  /* Obtain thread number */
  tid = omp_get_thread_num();
  printf("Hello World from thread = %d\n", tid);

  /* Only master thread does this */
  if (tid == 0)
    {
    nthreads = omp_get_num_threads();
    printf("Number of threads = %d\n", nthreads);
    }

  }  /* All threads join master thread and disband */
  return EXIT_SUCCESS;
}

linux.txt

$ cmake -B build
-- The C compiler identification is Clang 16.0.6
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Check for working C compiler: /usr/bin/clang - skipped
-- Detecting C compile features
-- Detecting C compile features - done
-- Found OpenMP_C: -fopenmp=libomp (found version "5.0") 
-- Found OpenMP: TRUE (found version "5.0") found components: C 
-- Configuring done
-- Generating done
-- Build files have been written to: openmp_c/build

$ cmake --build build
[2/2] Linking C executable hello

$ build/hello 
Hello World from thread = 0
Hello World from thread = 56
Hello World from thread = 2
Hello World from thread = 3
Hello World from thread = 6
Hello World from thread = 12
Hello World from thread = 26
Hello World from thread = 19
Hello World from thread = 20
Hello World from thread = 31
Hello World from thread = 14
Hello World from thread = 36
Hello World from thread = 48
Number of threads = 64
<... more messages from other threads>

macos.txt

% cmake -Bbuild --fresh
-- The C compiler identification is AppleClang 14.0.3.14030022
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Check for working C compiler: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/cc - skipped
-- Detecting C compile features
-- Detecting C compile features - done
-- Found OpenMP_C: -Xclang -fopenmp (found version "5.0") 
-- Found OpenMP: TRUE (found version "5.0") found components: C 
-- Configuring done (0.4s)
-- Generating done (0.0s)
-- Build files have been written to: openmp_c/build

% cmake --build build
[2/2] Linking C executable hello

% build/hello 
Hello World from thread = 0
Hello World from thread = 5
Hello World from thread = 4
Hello World from thread = 2
Hello World from thread = 7
Hello World from thread = 1
Hello World from thread = 6
Number of threads = 8
Hello World from thread = 3

Works great, thanks!

Thank you!

Thank you!

Great! Thanks a lot!

It works. Thanks!