Blog from October, 2011

module test
use iso_c_binding
implicit none
integer, parameter :: MAXSTRLEN = 512
contains

! Utility functions
! fortran character(len=*) are not compatible with c
! To be compatible with c, strings sould be copied to a c_char array
function char_array_to_string(char_array, length)
  integer(c_int) :: length
  character(c_char) :: char_array(length)
  character(len=length) :: char_array_to_string
  integer :: i
  do i = 1, length
     char_array_to_string(i:i) = char_array(i)
  enddo
end function char_array_to_string

! C ends strings with a \0 character. Add this so it is received correctly in c compatible languages
function string_to_char_array(s, length)
  integer(c_int) :: length
  character :: s(*)
  character(c_char) :: string_to_char_array(length)
  integer :: i
  do i = 1, length
     string_to_char_array(i:i) = s(i)
  enddo
  string_to_char_array(i+1:i+1) = C_NULL_CHAR
end function string_to_char_array



! 1 int
integer(c_int) function oneint(arg1) bind(C, name="oneint")
  integer(c_int), intent(inout) :: arg1
  arg1 = 111
  oneint = 123
end function oneint

! 1 double
integer(c_int) function onedouble(arg1) bind(C, name="onedouble")
  real(c_double), intent(inout) :: arg1
  arg1 = 1.11d0
  onedouble = 123
end function onedouble


! 10by10 double
integer(c_int) function twobytwodouble(x) bind(C, name="twobytwodouble")
  real(c_double),intent(inout)  :: x(2,2)
  x = 4
  x(2,1) = 21
  x(1,2) = 12
  twobytwodouble = 123
end function twobytwodouble


! 10by10 double
integer(c_int) function twobythreedouble(x) bind(C, name="twobythreedouble")
  real(c_double),intent(inout)  :: x(2,3)
  x = 6
  x(2,1) = 21
  x(1,3) = 13
  twobythreedouble = 123
end function twobythreedouble

integer(c_int) function twobytwodoublepointer(ptr) bind(C, name="twobytwodoublepointer")
  type(c_ptr), intent(inout) :: ptr
  real(c_double), target, save  :: x(2,2)

  x = 4
  x(2,1) = 21
  x(1,2) = 12
  ptr=c_loc(x)
  twobytwodoublepointer = 123
end function twobytwodoublepointer


! 10by10 double pointer
integer(c_int) function twobythreedoublepointer(ptr) bind(C, name="twobythreedoublepointer")
  type(c_ptr), intent(inout) :: ptr
  ! Save is required here for the memory to remain available after the function call
  real(c_double), target, save  :: x(2,3)
  x = 6
  x(2,1) = 21
  x(1,3) = 13
  ptr = c_loc(x)
  twobythreedoublepointer = 123
end function twobythreedoublepointer

! character
integer(c_int) function letter(arg1) bind(C, name="letter")
  character(kind=c_char), intent(inout) :: arg1
  arg1 = 'W'
  letter = 123
end function letter

! string in (string in length is not fixed but internally you need to set a fixed string length)
integer(c_int) function stringin(arg1) bind(C, name="stringin")
  character(kind=c_char), intent(in) :: arg1(*)
  character(len=MAXSTRLEN) :: string
  string = char_array_to_string(arg1, MAXSTRLEN)
  write(*,*)string
  stringin = 123
end function stringin

! string out (requires fixed number of letters)
integer(c_int) function stringout(arg1) bind(C, name="stringout")
  ! Output string has to be fixed
  character(kind=c_char), intent(out) :: arg1(MAXSTRLEN)
  character(len=MAXSTRLEN) :: string
  string = "Hello from fortran"
  arg1 = string_to_char_array(string, len(trim(string)))
  stringout = 123
end function stringout


end module test

#!/usr/bin/env python
import numpy as np
from ctypes import (CDLL, POINTER, ARRAY, c_void_p,
                    c_int, byref,c_double, c_char,
                    c_char_p, create_string_buffer)
from numpy.ctypeslib import ndpointer
import os

dllpath = os.path.abspath("test.dylib") # or .dll or .so
libtest = CDLL(dllpath)

# Define some extra types
# pointer to a double
c_double_p = POINTER(c_double)
# pointer to a integer
c_int_p = POINTER(c_int)

shape2x2=(2,2)
# Pointer to a 2x2 double in fortran layout
c_double2x2_c = ndpointer(shape=shape2x2, dtype="double", flags="C")
c_double2x2_f = ndpointer(shape=shape2x2, dtype="double", flags="FORTRAN")
# Pointer to a pointer to a 10x10 double in fortran layout
c_double2x2_f_p = POINTER(c_double2x2_f)
c_double2x2_c_p = POINTER(c_double2x2_c)
shape3x2=(3,2)
shape2x3=(2,3)
# Pointer to a 2x3,3x2 double in fortran layout
c_double2x3_c = ndpointer(shape=shape2x3, dtype="double", flags="C")
c_double2x3_f = ndpointer(shape=shape2x3, dtype="double", flags="FORTRAN")
c_double3x2_c = ndpointer(shape=shape3x2, dtype="double", flags="C")
c_double3x2_f = ndpointer(shape=shape3x2, dtype="double", flags="FORTRAN")
# Pointer to a pointer to a 2x3,3x2 double in fortran layout
c_double2x3_f_p = POINTER(c_double2x3_f)
c_double2x3_c_p = POINTER(c_double2x3_c)
c_double3x2_f_p = POINTER(c_double3x2_f)
c_double3x2_c_p = POINTER(c_double3x2_c)

# Pointer to a character pointer
c_char_p_p = POINTER(c_char_p)
MAXSTRLEN=512
# Character array (Fortran can only return c_char arrays in c compatible mode)
c_char_array = ARRAY(c_char,MAXSTRLEN)
# Pointer to a character array
c_char_array_p = POINTER(c_char_array)

# oneint
f = libtest.oneint
f.argtypes=[c_int_p]
arg1 = c_int(1)
rc=f(byref(arg1))
print arg1.value

# onedouble
f = libtest.onedouble
f.argtypes=[c_double_p]
arg1 = c_double(1)
rc=f(byref(arg1))
print arg1.value

# 2x2
f = libtest.twobytwodouble
f.argtypes=[c_double2x2_f]
arg1 = np.zeros(shape2x2, order="F")
rc=f(arg1)
arr = np.array(arg1)
print arr
print arr.flags


# 2x2 p 
f = libtest.twobytwodoublepointer
f.argtypes=[c_double2x2_c_p]
arg1 = c_double2x2_c()
rc=f(byref(arg1))
arr = np.array(arg1)
print arr
print arr.flags

# 2x3 
f = libtest.twobythreedouble
f.argtypes=[c_double2x3_f]
arg1 = np.zeros(shape2x3,order="F")
rc=f(arg1)
arr = np.array(arg1)
print arr
print arr.flags


# 2x3 corresponds to 3x2 p in C order, reversed from F.
f = libtest.twobythreedoublepointer
f.argtypes=[c_double3x2_c_p]
arg1 = c_double3x2_c()
rc=f(byref(arg1))
arr = np.array(arg1, order="C")
print arr
print arr.flags


# Exchange one letter
f = libtest.letter
f.argtypes=[c_char_p]
arg1 = c_char('H')
rc=f(byref(arg1))
print arg1.value

# Exchange a string (in)
f = libtest.stringin
f.argtypes=[c_char_array_p]
arg1 = create_string_buffer('Hello from python',MAXSTRLEN)
rc=f(byref(arg1))

# Exchange a string (out)
f = libtest.stringout
f.argtypes=[c_char_array_p]
arg1 = create_string_buffer('',MAXSTRLEN)
rc=f(byref(arg1))
print arg1.value


del libtest

\bibliographystyle{jcr}	% (uses file "jcr.bst")
\bibliography{myrefs}		% expects file "myrefs.bib"

latex jcr.dbj

\usepackage{times}
\renewcommand{\refname}{Literature cited} % custom heading for JCR
\renewcommand{\keywordsname}{ADDITIONAL INDEX WORDS:} % custom named index words