Select in Co-array Fortran

Our example demonstrates the use of multiple image synchronization in Co-array Fortran. This is similar to selecting across multiple channels in other languages.

program select_example
  use iso_fortran_env
  implicit none

  integer :: i
  character(len=10) :: msg[*]
  
  ! Simulate two different operations
  if (this_image() == 1) then
    call sleep(1)
    msg = "one"
  else if (this_image() == 2) then
    call sleep(2)
    msg = "two"
  end if

  ! Synchronize all images
  sync all

  ! Print messages as they arrive
  if (this_image() == 1) then
    do i = 1, 2
      if (msg[i] /= "") then
        print *, "received ", msg[i]
      end if
    end do
  end if

end program select_example

In this example, we use Co-array Fortran’s parallel execution model to simulate concurrent operations. Each image (similar to a goroutine) performs a different operation.

1. We define a co-array msg that can hold a string message.

2. Depending on the image number, we simulate different operation times using the sleep subroutine and set different messages.

3. We use sync all to ensure all images have completed their operations before proceeding.

4. Finally, the first image checks and prints the messages from all images.

To run the program:

$ gfortran -fcoarray=lib select_example.f90 -o select_example
$ ./select_example
received one
received two

Note that the total execution time will be approximately 2 seconds, as both the 1 and 2 second sleep operations execute concurrently on different images.

This example demonstrates how Co-array Fortran can be used to perform concurrent operations and synchronize between different images, similar to selecting across multiple channels in other languages.