My reading of example 10.2 (in the 3.0 standard) is that the command array has 2 elements, (same as the value of count), but that an extra blank element is used to signal the end of the argument lists for each command in the array array_of_argv. That seems reasonable since the number of arguments would, in general, be different for each command. Here is how you do it in Fortran: CHARACTER*25 commands(2), array_of_argv(2, 3) commands(1) = ’ ocean ’ array_of_argv(1, 1) = ’ -gridfile ’ array_of_argv(1, 2) = ’ ocean1.grd’ array_of_argv(1, 3) = ’ ’ commands(2) = ’ atmos ’ array_of_argv(2, 1) = ’ atmos.grd ’ array_of_argv(2, 2) = ’ ’ call MPI_COMM_SPAWN_MULTIPLE(2, commands, array_of_argv, ...) Self-documentation of the declaration would be clearer if the command argument were declared command(count) instead of command(*). I suspect that the (*) declaration of the command dummy argument was done that way out of habit. Cheers, Bill

On Feb 9, 2018, at 12:26 AM, Gilles Gouaillardet <[email protected]> wrote:

Bill,

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That being said, the standard does not state that, and it is unclear whether 'array_of_commands' should be

NULL-terminated (so to speak) or not.

Fortran characters are NOT null terminated.

I was referring the array and not the characters, so let me clarify this :

In example 10.2, the array of commands is

CHARACTER*25 commands(2), commands(1) = ' ocean ' commands(2) = ' atmos '

By reading the standard literally, it was unclear to me whether this should have been

CHARACTER*25 commands(3), commands(1) = ' ocean ' commands(2) = ' atmos ' commands(3) = ' '

FWIW, Open MPI currently expects the latter form (see https://github.com/open-mpi/ompi/issues/4788) I initially thought it was a simple bug, but on second thought, I am still on the fence whether this should be regarded as a simple bug or a correct/literal interpretation of the standard.

Cheers,

Gilles

On Fri, Feb 9, 2018 at 7:07 AM, Bill Long <[email protected]> wrote:

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On Feb 8, 2018, at 3:44 PM, Jeff Squyres (jsquyres) <[email protected]> wrote:

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And the Fortran binding is

MPI_COMM_SPAWN_MULTIPLE(COUNT, ARRAY_OF_COMMANDS, ...) INTEGER COUNT CHARACTER*(*) ARRAY_OF_COMMANDS(*)

Note that the character*(*) syntax is archaic (as of 25+ years ago). The normal way of writing this is character(*).

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Intuitively, it seems obvious that 'array_of_commands' is an array of 'count' commands.

That is what I would assume. The user would be responsible for declaring the array elements large enough to hold the longest command. (All the elements of the array have the same length).

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That being said, the standard does not state that, and it is unclear whether 'array_of_commands' should be

NULL-terminated (so to speak) or not.

Fortran characters are NOT null terminated.

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Back to the Fortran binding, the following prototype, which is somehow consistent with 'array_of_argv',

could (help to) clear the confusion. The same comments also apply to the Fortran 2008 binding.

CHARACTER*(*) ARRAY_OF_COMMANDS(COUNT)

This is also fine, and clearer in terms of the expected minimum size of ARRAY_OF_COMMANDS.

Cheers, Bill

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Bill Long [email protected] Principal Engineer, Fortran Technical Support & voice: 651-605-9024 Bioinformatics Software Development fax: 651-605-9143 Cray Inc./ 2131 Lindau Lane/ Suite 1000/ Bloomington, MN 55425

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Bill Long [email protected] Principal Engineer, Fortran Technical Support & voice: 651-605-9024 Bioinformatics Software Development fax: 651-605-9143 Cray Inc./ 2131 Lindau Lane/ Suite 1000/ Bloomington, MN 55425