Revision as of 15:34, 28 March 2017

Programming Details

C is a general-purpose, imperative computer programming language. There are some derivations of the language due to the modernisation of the original specification in the form of C++ and C#.

There is direct support for openMP and openMPI.

Programming example

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

int main (int argc, char *argv[]){
        int nthreads, tid;

        /* Fork a team of threads giving them their own copies of variables*/
        #pragma omp parallel
        {
                /* 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 together and disband */

        return (0);
}

Compilation

The program would be compiled in the following way, optional Intel compiler available too:

[username@login01 ~]$  module add gcc/4.9.3
[username@login01 ~]$  gcc -o test2 -fopenmp test2.c (openMP support)
[username@login01 ~]$ ./test2

Hello World from thread = 0
Number of threads = 1

Modules Available

The following modules are available for C

• module add gcc/4.9.3 (GNU compiler)
• module add intel/compiler/64/2016.2.181 (Intel compiler)

OpenMP is a library directive within the compiler and does not require any additional module to be loaded.

Usage Examples

Batch example

#!/bin/bash

#SBATCH -J openmpi-single-node
#SBATCH -N 1
#SBATCH --ntasks-per-node 28
#SBATCH -o %N.%j.%a.out
#SBATCH -e %N.%j.%a.err
#SBATCH -p compute
#SBATCH --exclusive

echo $SLURM_JOB_NODELIST

module purge
module add gcc/4.9.3

export I_MPI_DEBUG=5
export I_MPI_FABRICS=shm:tmi
export I_MPI_FALLBACK=no

/home/user/CODE_SAMPLES/OPENMP/demo

[username@login01 ~]$ sbatch demo.job
Submitted batch job 289552

Further Information

• https://en.wikipedia.org/wiki/C_(programming_language)
• https://en.wikipedia.org/wiki/OpenMP
• http://www.openmp.org/
• https://computing.llnl.gov/tutorials/openMP/

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