[R] Using .Fortran in R - how can I use file i/o flexibly OR arrays of variable length as arguments?

[Peter Green]

Thank you, Ivan, it's reassuring in a way to learn from your reply that 
I am not missing something very obvious.

It's unfortunate that a computation that could be done cleanly, using 
dynamic allocation of memory, within either R or Fortran separately, 
cannot be split between the two in a way that is optimal for the 
capabilities of each language, because of the .Fortran bottleneck. I 
wondered whether you (or anyone) thought there was a way to do this 
using C wrapper routines in some way? Unfortunately, the ultimate size 
of the output is something the algorithm would only learn very late in 
the computation, so pre-calculating that size is not a practical option 
(else indeed I would do that within R before use of .Fortran).

Any thought on the other route I suggested, that is, binary file i/o 
from with the Fortran code (e.g. full use of the connections 
functionality), again perhaps via C?

Although I mentioned CRAN, that was really just to stress I wanted a 
platform-independent solution ideally.

Peter Green

On 05/02/2022 08:09, Ivan Krylov wrote:
> On Fri, 4 Feb 2022 10:21:11 +0000
> Peter Green <mapjg using bristol.ac.uk> wrote:
>
>> the output I want to return to R is copious, and variable in length,
>> the length being computed within the Fortran code (it is a
>> variable-dimension simulation)
> Is there a way to separate the calculation of the output size from the
> rest of the subroutine? If yes, the simplest option would be to pass
> pre-allocated vectors to the .Fortran() call.
>
> Unfortunately, .Fortran() is not expressive enough for more complicated
> use cases. Memory that you intend to return to R must be allocated
> using its own allocator/garbage collector system, which (like any other
> call into R from user code) may longjmp() away from your functions on
> interrupt, error or allocation failure, abandoning any resources the
> garbage collector doesn't know about. In turn, this requires the use of
> C and the PROTECT() macro. In theory, you could use Fortran 2003 "C
> interoperability" to call C entry points, but I expect that to be very
> inconvenient (at least due to the lack of CPP macros).
>
> Since you mention CRAN checks, this might be a better question for
> <r-package-devel using r-project.org>, not R-help.
>