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Calling Fortran functions from C code

Some useful recommendations:

1. The 2003 Fortran standard defined a way to interface Fortran procedures with functions written in C. This solution is recommended because it ensures portability and allows the linking of any Fortran type with those of the C language. For more information, refer to Chapter 9 of the Fortran Expert course: "Interoperability between C and Fortran entities";
2. Parameter passing in Fortran is done by address, while in C it is done by value;
3. It is recommended to link with the Fortran compiler. If it is the ifort compiler, the -nofor-main option is necessary to indicate that the main function is not a Fortran function.

Be aware that if you do not adopt the solution proposed by Fortran 2003, you will be subject to the following restrictions:

1. When C activates a Fortran procedure, the arguments provided during the call must be addresses of basic type objects;
2. During the compilation of the Fortran program, the -assume nounderscore option of the ifort compiler (-fno-underscoring if it is gfortran) must be specified so that it does not add the _ character when generating the external references corresponding to the calls made in C. Obviously, if this character is indicated as a suffix during these calls, this option should not be specified;
3. As Fortran generates external references in lowercase, this must be respected when making calls in C.

Example of a C call

appelf.c

#include <stdio.h>
int main ()
{
  int   lg_chaine  = 0;
  float reel       = 0.0;
  char  chaine[]   = "chaîne_C";

  lg_chaine = strlen(chaine);
  concat( &lg_chaine, &reel, chaine);

  printf("chaîne finale                = %sn", chaine);
  printf("longueur de la chaîne finale = %dn", lg_chaine);
  printf("réel passé par adresse       = %fn", reel);

  return 0;
}

Example of definition in Fortran 2003/2008

concat.f90

module m
  use ISO_C_BINDING, only : C_CHAR, C_INT, C_FLOAT
contains
  SUBROUTINE concatenation( lg_chaine, reel, tab_car) bind(C,name='concat')
    INTEGER(kind=C_INT)                          :: lg_chaine
    REAL(kind=C_FLOAT)                           :: reel
    CHARACTER(kind=C_CHAR), dimension(lg_chaine) :: tab_car
    !
    CHARACTER(kind=C_CHAR,len=lg_chaine)         :: chaine
    !write( chaine, '(*(a))' ) tab_car ! Format => Fortran 2008
    ! ou bien
    do i=1,lg_chaine
      chaine(i:i) = tab_car(i)
    end do
    !
    chaine    = TRIM(chaine)//' et chaîne_Fortran'
    lg_chaine = LEN_TRIM(chaine)
    reel      = 100.0
  END SUBROUTINE concatenation
end module m

# compilation
icc -c appelf.c
ifort -c concat.f90
ifort -nofor-main appelf.o concat.o -o appelf.exe

# execution
$ appelf.exe
chaîne finale                = chaîne_C et chaîne_Fortran
longueur de la chaîne finale = 26
réel passé par adresse       = 100.000000

Example of definition in Fortran 90/95

concat.f90

SUBROUTINE concat( lg_chaine, reel, chaine)
  INTEGER,                  INTENT(INOUT) :: lg_chaine
  REAL,                     INTENT(INOUT) :: reel
  CHARACTER(LEN=lg_chaine), INTENT(INOUT) :: chaine
  chaine    = TRIM(chaine)//' et chaîne_Fortran'
  lg_chaine = LEN_TRIM(chaine)
  reel      = 100.0
END SUBROUTINE concat

# compilation
icc -c appelf.c
ifort -c -assume nounderscore concat.f90
ifort -nofor-main appelf.o concat.o -o appelf.exe

# execution
$ appelf.exe
chaîne finale                = chaîne_C et chaîne_Fortran
longueur de la chaîne finale = 26
réel passé par adresse       = 100.000000