mwe_stepsAndArrayCount.jl

 using ADIOS2

function main()

    adios = adios_init_serial()

    # --- Write a file with two steps with a global array `anArray`
    #     For step 0, `anArray` will have 200 rows
    #     For step 1, `anArray` will have 100 rows
    iow = declare_io(adios, "Write_MWE")

    initialSize = 200
    anArray_io = define_variable(iow, "anArray", Float64, (initialSize,), (0,), (initialSize,);
                                      constant_dims=false)

    engine = open(iow, "mwe.bp4", mode_write)

    for step in 0:1
        begin_step(engine, step_mode_update, 100)

        arraySize = initialSize - step*100  # Step 0 has 200 rows; step 1 has 100 rows
        anArray = collect(1:arraySize)   # Fill the array with values from 1 to its size

        # Note that ADIOS2.jl doesn't have set_shape - I added it and it will be part of a pull request
        set_shape(anArray_io, (arraySize,))

        # Note that ADIOS2.jl doesn't have set_selection - I added it and it will be part of a pull request
        set_selection(anArray_io, (0,), (arraySize,))

        put!(engine, anArray_io, anArray)

        end_step(engine)
    end

    close(engine)

    ## --- Read the file
    ior = declare_io(adios, "Read_MWE")
    engine = open(ior, "mwe.bp4", mode_read)
    theArray_io = inquire_variable(ior, "anArray")

    for step in 0:1
        begin_step(engine, step_mode_read, 100)

        theStep = current_step(engine) ## Note that ADIOS2.jl has a bug for this function
                                       ## I fixed it and it will be part of a pull request

        println("Step $theStep: Shape is $(shape(theArray_io)[1]), Start is $(start(theArray_io)[1]), "*
                          "Count is $(count(theArray_io)[1]), selection_size is $(selection_size(theArray_io)[1])")

        # The line below solves the problem - why isn't the count set correctly by default?
        #set_selection(theArray_io, (0,), shape(theArray_io))

        # Make an empty array of size given by selection_size
        theArray = Array{Float64}(undef, selection_size(theArray_io))
        get(engine,theArray_io, theArray)

        end_step(engine)
    end
end

main()

Output is

Step 0: Shape is 200, Start is 0, Count is 200, selection_size is 200
Step 1: Shape is 100, Start is 0, Count is 200, selection_size is 200
[Fri Aug 12 18:38:28 2022] [ADIOS2 ERROR] <Helper> <adiosSystem> <ExceptionToError> : adios2_end_step: [Fri Aug 12 18:38:28 2022] [ADIOS2 EXCEPTION] <Toolkit> <format::bp::BP4Deserializer> <SetVariableBlockInfo> : selection Start Dims(1):[0] and Count Dims(1):[200] (requested) is out of bounds of (available) Shape Dims(1):[100] , when reading global array variable anArray in step 2, in call to Get