$ cat hello.c
#include <stdio.h>
#include <omp.h>
int main( void)
{
  #pragma omp parallel
  {
    int ID = omp_get_thread_num();
    printf( "hello(%d) ",ID);
     #ifdef WAIT
     #pragma omp barrier // synchronize the threads
     #endif
    printf( "world(%d)\n",ID);
  }
  return 0;
}
$ make
gcc -std=c11 -pedantic -Wall -fopenmp -o hello hello.c
gcc -std=c11 -pedantic -Wall -fopenmp -o hello_wait -DWAIT hello.c
$ lscpu | grep '^CPU(s)'
CPU(s):                2
$ env | grep OMP
$ ./hello
hello(1) world(1)
hello(0) world(0)
$ ./hello_wait
hello(0) hello(1) world(0)
world(1)
$ export OMP_NUM_THREADS=1
$ ./hello
hello(0) world(0)
$ export OMP_NUM_THREADS=4
$ ./hello
hello(1) world(1)
hello(3) world(3)
hello(2) world(2)
hello(0) world(0)
$ ./hello_wait
hello(1) hello(0) hello(2) hello(3) world(1)
world(2)
world(0)
world(3)
$
% cat loop.c
#include <stdio.h>
#include <omp.h>
int main( void)
{
  #pragma omp parallel for
  for( int i = 0; i < 10; ++i)
  {
    int ID = omp_get_thread_num();
    printf( "proc %d, i = %d\n", ID, i);
  }
  return 0;
}
$ make loop
gcc -std=c11 -pedantic -Wall -fopenmp -o loop loop.c
$ export OMP_NUM_THREADS=2
$ ./loop
proc 1, i = 5
proc 1, i = 6
proc 1, i = 7
proc 1, i = 8
proc 1, i = 9
proc 0, i = 0
proc 0, i = 1
proc 0, i = 2
proc 0, i = 3
proc 0, i = 4
$ export OMP_NUM_THREADS=4
$ ./loop
proc 0, i = 0
proc 0, i = 1
proc 0, i = 2
proc 1, i = 3
proc 1, i = 4
proc 1, i = 5
proc 3, i = 8
proc 3, i = 9
proc 2, i = 6
proc 2, i = 7
$