ISO_C_BINDINGS: how to pass array of strings

[bbourdin]

Hi,

I am trying to understand better the C interoperability in fortran 2003. I have been able to write functions that pass scalar and arrays as arguments, but can;t figure out how to pass an array of string. More precisely, say that I have a C function

#undef __FUNCT__

#define __FUNCT__ "printconstcharlist"

void printconstcharlist(const char *const*list,int n) {

int i;

printf("\\nThis is %s\\n",__FUNCT__);

for (i=1; i

printf(" s[%i]=%s\\n",i,list);

}

}

What should the matching interface and calling sequence should be so that I can call it from fortran?

I am attaching a small example implementing the C functions and illustrating the interface in simpler cases.

Also, gfortran won;t compile this example and gives the following error message

TestF2003_C.f90:61.29:

Call cfunc_intarray(c_loc(i_array),n)

1

Error: Argument 'i_array' to 'c_loc' at (1) must be an associated scalar POINTER

while ifort works fine. Which one is right?

Thanks,

Blaise

[Steven_L_Intel1]

I am not a C expert, but I will comment that not all C constructs can be represented in Fortran using the Fortran 2003 C interoperability features.

Regarding the diagnostic you get from gfortran - an error at this point is appropriate but the one that gfortran gives is not what I would choose, since there are other possibilities for this argument.

C_LOC has several requirements for the arguments. Contrary to what Tim says in his private reply, the standard does not require that a compiler diagnose failure to meet these requirements, though certainly it is preferable if it does.

In your case, i_array fails the test of being either an interoperable type or a non-polymorphic scalar with no length attribute. A CHARACTER variable is interoperable only if it is length 1. It also fails because it is a pointer but is not scalar (as gfortran notes).

In the case of "an array of strings", the way to represent this in Fortran would be an array of type C_PTR to which you assign values using C_LOC.

[bbourdin]

Dear Steve and Timothy,

Thanks for your help.

I followed your advice and was able to pass scalar by address and value, and arrays of scalar. ABout arrays of strings, I followed Steve's advice and got the following code, but I am not quite sure that it is legal (I am worried about the dimension(1) in the cfunc_chararray interface). It compiles without warning and runs fine under ifort 11.1 and gfortran 4.6.0 under Mac OS 10.6.

Would you mind having a look at the attached program and comment?

Thanks,

Blaise

Module Mod1

Use, Intrinsic :: iso_c_binding

End Module Mod1

Module Mod2

Use, Intrinsic :: iso_c_binding

Interface

Subroutine cfunc_int(i) bind(c,name='printint')

Use Mod1

Integer(c_int), value :: i

End Subroutine

End Interface

Interface

Subroutine cfunc_intptr(i) bind(c,name='printintptr')

Use Mod1

Type(C_Ptr), value :: i

End Subroutine

End Interface

Interface

Subroutine cfunc_intarray(i,n) bind(c,name='printintarray')

Use Mod1

Type(C_Ptr), value :: i

Integer(c_int), value :: n

End Subroutine

End Interface

Interface

Subroutine cfunc_chararray(s,n) bind(c,name='printconstcharlist')

Use Mod1

Type(C_Ptr), dimension(1) :: s

Integer(kind=c_int), value :: n

End Subroutine cfunc_chararray

End Interface

End Module Mod2

Program main

Use Mod2

implicit none

Integer(kind=c_int) :: i, n

Integer(kind=c_int), target :: iptr

Integer(Kind=C_int), Dimension(:), Pointer :: ip_array

Integer(Kind=C_int), Dimension(:), Allocatable, Target :: it_array

Character(len=99,kind=C_char), dimension(:), Pointer :: sp_array

Type(C_Ptr), Dimension(:),Pointer :: S_Ptr

i = 12

Call cfunc_int(i)

iptr=13

Call cfunc_intptr(c_loc(iptr))

n = 5

Allocate(ip_array(n))

Allocate(it_array(n))

Do i = 1, n

ip_array(i) = i

End Do

it_array = ip_array

Call cfunc_intarray(c_loc(it_array),n)

DeAllocate(ip_array)

DeAllocate(it_array)

Allocate(sp_array(n))

Allocate(S_Ptr(n))

Do i = 1, n

Write(sp_array(i), 100) i,C_NULL_CHAR

End Do

100 format('*** sp_array /',I5,'/ ***',A)

Do i = 1, n

S_Ptr(i) = C_loc(sp_array(i))

End Do

Call cfunc_chararray(S_Ptr, n)

DeAllocate(sp_array)

DeAllocate(S_Ptr)

Write(*,*) 'All done!'

End Program main

[Steven_L_Intel1]

Use (*) instead of (1).

[bbourdin]

Of course!

Thanks a lot

Blaise