Re: [f2py] "natural" Fortran calling conventions?

While browsing the f2py manpage, I stubled upon the following
interesting option:
"""
-DF2PY_REPORT_ON_ARRAY_COPY=<int>
  To  send  a  message  to stderr whenever F2PY interface makes a copy
of an array. Integer
  <int> sets the threshold for array sizes when a message should be shown.
"""
Kevin


On Mon, May 25, 2009 at 5:09 AM, Kevin Mitchell<[email protected]> wrote:
>> I don't want my Fortran subroutine to have intent(c); if I did that,
>> I'd have to write an intent(c) wrapper for every Fortran routine I
>> want to call, which defeats the purpose.  Or if I write Fortran code
>> optimized for intent(c), it won't be idiomatic Fortran and hard to
>> maintain.
>
> You don't have to write a whole wrapper to do this, you can use the
> commenting method:
>
> subroutine f2py_trsps_f(n,m,l,h)
> !f2py intent(c) h
>  implicit none
>  integer :: n,m,l
>  real*8  :: h(n,m,l)
>  print*,h(1:8,1,1)
>  return
> end subroutine f2py_trsps_f
>
> If you f2py this with "f2py -c f2py_trsps_f.f90" (and as far as I know
> the same thing should work with fortran 77), you'll note that
>
> <<< import time,f2py_trsps_f
> <<< l,n,m=2**10,2**6,2**6
> <<< h=arange(l*n*m).reshape([l,n,m])
> <<< st=time.time();f2py_trsps_f.f2py_trsps_f(h);print 'c-order=',time.time()-st
>   0.0000000000000000        1.0000000000000000
> 2.0000000000000000        3.0000000000000000        4.0000000000000000
>       5.0000000000000000        6.0000000000000000
> 7.0000000000000000
> c-order= 0.0583457946777
> <<< h=asfortranarray(h)
> <<< st=time.time();f2py_trsps_f.f2py_trsps_f(h);print 'f-order=',time.time()-st
>   0.0000000000000000        1.0000000000000000
> 2.0000000000000000        3.0000000000000000        4.0000000000000000
>       5.0000000000000000        6.0000000000000000
> 7.0000000000000000
> f-order= 0.225140094757
>
> That is, things work as expected (and more efficiently) if you give it
> a python-natual c-order array. I agree that getting this behaviour
> from "inent(c)" is a little counter-intuitive, but I think its the
> closest thing to what you're looking for.
>
> Once you get into the Fortran subroutine everything works normally.
> You can forget that python or c was even involved once you get past
> the f2py comment which the compiler doesn't see anyway. So no, I
> wouldn't say this is idiomatic Fortran.
>
>> And I'd like that with an error message if it can't be done
>> efficiently
> As far as I know, this can't currently be done, but I would say it's a
> good idea.
>
> Kevin
>