program main

!*****************************************************************************80
!
!! MAIN is the main program for HELLO.
!
!  Discussion:
!
!    HELLO is a "Hello, World" program for OpenMP.
!
!    But we demonstrate a few simple features of OpenMP, including:
!
!    How you get the "include" file.
!    How you measure wall clock time.
!    How you find out how many processors are available.
!    How you find out which thread you are in a parallel section.
!    How you find out how many threads are available in a parallel section.
!    How you set the number of threads.
!
!  Licensing:
!
!    This code is distributed under the GNU LGPL license. 
!
!  Modified:
!
!    20 July 2007
!
!  Author:
!
!    John Burkardt
!
  use omp_lib

  integer id
  integer proc_num
  integer thread_num
  double precision wtime
  double precision wtime1
  double precision wtime2

  write ( *, '(a)' ) ' '
  write ( *, '(a)' ) 'HELLO'
  write ( *, '(a)' ) '  FORTRAN90/OpenMP version'

  wtime1 = omp_get_wtime ( )
!
!  How many processors are available?
!
  proc_num = omp_get_num_procs ( )

  write ( *, '(a)' ) ' '
  write ( *, '(a)' ) '  The number of processors available:'
  write ( *, '(a,i8)' ) '  OMP_GET_NUM_PROCS () = ', proc_num
!
!  How many threads are available by default or environment setting?
!
!$omp parallel private ( id )

  id = omp_get_thread_num ( )
  thread_num = omp_get_num_threads ( )

  if ( id == 0 ) then

    write ( *, '(a)' ) ' '
    write ( *, '(a)' ) '  Calling OMP_GET_NUM_THREADS inside a '
    write ( *, '(a)' ) '  parallel region, we get the number of'
    write ( *, '(a,i3)' ) '  threads is ', thread_num

  end if

  write ( *, '(a,i8,a,i8)' ) '  This is process ', id, ' out of ', thread_num

!$omp end parallel
!
!  Request double the number of threads.
!
  thread_num = 2 * thread_num

  write ( *, '(a)' ) ' '
  write ( *, '(a,i8,a)' ) '  We request ', thread_num, ' threads.'

  call omp_set_num_threads ( thread_num )
!
!  Now how many threads do we see?
!
!$omp parallel private ( id )

  id = omp_get_thread_num ( )
  thread_num = omp_get_num_threads ( )

  if ( id == 0 ) then

    write ( *, '(a)' ) ' '
    write ( *, '(a)' ) '  Calling OMP_GET_NUM_THREADS inside a '
    write ( *, '(a)' ) '  parallel region, we get the number of'
    write ( *, '(a,i3)' ) '  threads is ', thread_num

  end if

  write ( *, '(a,i8,a,i8)' ) '  This is process ', id, ' out of ', thread_num

!$omp end parallel
!
!  Finish up by measuring the elapsed time.
!
  wtime2 = omp_get_wtime ( )
  wtime = wtime2 - wtime1

  write ( *, '(a)' ) ' '
  write ( *, '(a)' ) 'HELLO'
  write ( *, '(a)' ) '  Normal end of execution.'

  write ( *, '(a)' ) ' '
  write ( *, '(a,g14.6)' ) '  Elapsed wall clock time = ', wtime

  stop
end