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  • # OpenCoarrays
    
    ## Introduction
    
    Coarray Fortran (CAF) is an extension of Fortran language and offers a simple interface for parallel processing and memory sharing.
    The advantage is that only small changes are required to convert existing Fortran code to support a robust and potentially efficient parallelism.
    
    A CAF program is interpreted as if it was replicated a number of times and all copies were executed asynchronously.
    The number of copies is decided at execution time. Each copy (called *image*) has its own private variables.
    The variable syntax of Fortran language is extended with indexes in square brackets (called *co-dimension*), which represents a reference to data distributed across images.
    
    By default, the CAF is using Message Passing Interface (MPI) for lower-level communication, so there are some similarities with MPI.
    
    Read more on <http://www.opencoarrays.org/>
    
    
    ## Coarray Basics
    
    
    ### Indexing of coarray images
    
    Indexing of individual images can be shown on the simple *Hello World* program:
    
    ```fortran
        program hello_world
          implicit none
          print *, 'Hello world from image ', this_image() , 'of', num_images()
        end program hello_world
    ```
    * num_images() - returns the number of all images
    * this_image() - returns the image index - numbered from 1 to num_images()
    
    ### Co-dimension variables declaration
    
    Coarray variables can be declared with the **codimension[*]** attribute or by adding trailing index **[*]** after the variable name.
    Notice, the ***** character always has to be in the square brackets.
    
    ```fortran
        integer, codimension[*] :: scalar
        integer :: scalar[*]
        real, dimension(64), codimension[*] :: vector
        real :: vector(64)[*]
    ```
    
    ### Images synchronization
    
    Because each image is running on its own, the image synchronization is needed to ensure, that all altered data are distributed to all images.
    Synchronization can be done across all images or only between selected images. Be aware, that selective synchronization can lead to the race condition problems like deadlock.
    
    Example program:
    
    ```fortran
        program synchronization_test
          implicit none
          integer :: i          ! Local variable
          integer :: numbers[*] ! Scalar coarray
    
          ! Genereate random number on image 1
          if (this_image() == 1) then
            numbers = floor(rand(1) * 1000)
            ! Distribute information to other images
            do i = 2, num_images()
              numbers[i] = numbers
            end do
          end if
    
          sync all ! Barrier to synchronize all images
    
          print *, 'The random number is', numbers
        end program synchronization_test
    ```
    
    * sync all - Synchronize all images between each other
    * sync images(*) - Synchronize this image to all other
    * sync images(*index*) - Synchronize this image to image with *index*
    
    !!! note
        **number** is the local variable while **number[*index*]** accesses the variable in a specific image.  
        **number[this_image()]** is the same as **number**.
    
    
    ## Compile and run
    
    Currently, version 1.8.10 compiled with OpenMPI 1.10.7 library is installed on Cluster. The OpenCoarrays module can be load as follows:
    
    ```console
    $ ml OpenCoarrays/1.8.10-GCC-6.3.0-2.27
    ```
    
    ### Compile CAF program
    
    
    The preferred method for compiling a CAF program is by invoking the *caf* compiler wrapper.
    The above mentioned *Hello World* program can be compiled as follows:
    
    
    ```console
    $ caf hello_world.f90 -o hello_world.x
    
    ```
    
    !!! warning
        The input file extension has to be **.f90** or **.F90** to be interpreted as *Fortran 90*.  
        If the input file extension is **.f** or **.F** the source code will be interpreted as *Fortran 77*.
    
    Another method for compiling is by invoking the *mpif90* compiler wrapper directly:
    
    
    ```console
    $ mpif90 hello_world.f90 -o hello_world.x -fcoarray=lib -lcaf_mpi
    
    ### Run CAF program
    
    A CAF program can be run by invoking the *cafrun* wrapper or directly by the *mpiexec*:
    
    ```console
    $ cafrun -np 4 ./hello_world.x
    
        Hello world from image            1 of           4
        Hello world from image            2 of           4
        Hello world from image            3 of           4
        Hello world from image            4 of           4
    
    
    $ mpiexec -np 4 ./synchronization_test.x
    
        The random number is         242
        The random number is         242
        The random number is         242
        The random number is         242
    ```
    
    
    where **-np 4** is number of images to run. The parameters of **cafrun** and **mpiexec** are the same.
    
    For more information about running CAF program please follow [Running OpenMPI](../mpi/Running_OpenMPI.md)