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I'm trying to get started with using basic OpenMP functionality in C. My basic understanding of 'omp parallel for' leads me to believe the following should distribute the following iterations of the loop between threads and should execute concurrently. The output I am getting is as follows. Code Below. Is there something subtle I am missing in my hello world example?
Hello World from omp thread 0 Hello World from omp thread 0 Hello World from omp thread 0 Hello World from omp thread 0 Hello World from omp thread 0 Hello World from omp thread 0 etc..
int HelloFunc()
{
int i;
int numthreads = 8;
#pragma omp parallel for default(none) num_threads(numthreads) private(i)
for (i = 0; i < 100; i++)
{
int tid = omp_get_thread_num();
printf("Hello world from omp thread %d\n", tid);
}
return -1;
}
int main()
{
int result = HelloFunc();
}
360
EffectThe omp_get_thread_num routine returns the thread number of the calling thread, within the team that is executing the parallel region to which the routine region binds. The thread number is an integer between 0 and one less than the value returned by omp_get_num_threads, inclusive.
#pragma omp parallel spawns a group of threads, while #pragma omp for divides loop iterations between the spawned threads.
Purpose. The omp parallel directive explicitly instructs the compiler to parallelize the chosen block of code.
Purpose. The omp for directive instructs the compiler to distribute loop iterations within the team of threads that encounters this work-sharing construct.
#include <omp.h>
#include <stdio.h>
int HelloFunc()
{
int i;
int numthreads = 8;
#pragma omp parallel for default(none) num_threads(numthreads) private(i)
for (i = 0; i < 100; i++)
{
int tid = omp_get_thread_num();
printf("Hello world from omp thread %d\n", tid);
}
return -1;
}
int main()
{
HelloFunc();
return 0;
}
then compile :
gcc t.c -fopenmp -Wall
and run :
./a.out
output :
Hello world from omp thread 7
Hello world from omp thread 7
Hello world from omp thread 7
Hello world from omp thread 7
Hello world from omp thread 7
Hello world from omp thread 7
Hello world from omp thread 7
Hello world from omp thread 7
Hello world from omp thread 7
Hello world from omp thread 7
Hello world from omp thread 7
Hello world from omp thread 7
Hello world from omp thread 3
Hello world from omp thread 3
Hello world from omp thread 3
Hello world from omp thread 3
Hello world from omp thread 3
Hello world from omp thread 3
Hello world from omp thread 3
Hello world from omp thread 3
Hello world from omp thread 3
Hello world from omp thread 3
Hello world from omp thread 3
Hello world from omp thread 3
Hello world from omp thread 3
Hello world from omp thread 0
Hello world from omp thread 0
Hello world from omp thread 0
Hello world from omp thread 0
Hello world from omp thread 0
Hello world from omp thread 0
Hello world from omp thread 0
Hello world from omp thread 0
Hello world from omp thread 0
Hello world from omp thread 0
Hello world from omp thread 0
Hello world from omp thread 0
Hello world from omp thread 0
Hello world from omp thread 5
Hello world from omp thread 5
Hello world from omp thread 5
Hello world from omp thread 5
Hello world from omp thread 6
Hello world from omp thread 6
Hello world from omp thread 6
Hello world from omp thread 6
Hello world from omp thread 6
Hello world from omp thread 6
Hello world from omp thread 6
Hello world from omp thread 6
Hello world from omp thread 6
Hello world from omp thread 6
Hello world from omp thread 6
Hello world from omp thread 6
Hello world from omp thread 5
Hello world from omp thread 5
Hello world from omp thread 5
Hello world from omp thread 5
Hello world from omp thread 5
Hello world from omp thread 5
Hello world from omp thread 5
Hello world from omp thread 5
Hello world from omp thread 2
Hello world from omp thread 2
Hello world from omp thread 4
Hello world from omp thread 4
Hello world from omp thread 4
Hello world from omp thread 4
Hello world from omp thread 2
Hello world from omp thread 1
Hello world from omp thread 4
Hello world from omp thread 2
Hello world from omp thread 1
Hello world from omp thread 4
Hello world from omp thread 2
Hello world from omp thread 2
Hello world from omp thread 4
Hello world from omp thread 1
Hello world from omp thread 1
Hello world from omp thread 1
Hello world from omp thread 1
Hello world from omp thread 1
Hello world from omp thread 1
Hello world from omp thread 1
Hello world from omp thread 2
Hello world from omp thread 4
Hello world from omp thread 1
Hello world from omp thread 2
Hello world from omp thread 4
Hello world from omp thread 1
Hello world from omp thread 1
Hello world from omp thread 4
Hello world from omp thread 2
Hello world from omp thread 1
Hello world from omp thread 4
Hello world from omp thread 4
Hello world from omp thread 2
Hello world from omp thread 2
Hello world from omp thread 2
Hello world from omp thread 2
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