Interoperability with C

Last updated on 2026-07-10 | Edit this page

Estimated time: 45 minutes

Overview

Questions

How can we write Fortran programs that interact with C in a portable way?
How can we make use of Fortran procedures from C code?

Objectives

Understand the use of the iso_c_binding intrinsic module.
Write C/Fortran programs that call Fortran/C procedures using iso_c_binding.

Fortran 2003 introduced facilities to allow a Fortran program to interact with C in a standardised way.

The intrinsic module `iso_c_binding`

Fortran has introduced the idea of interoperable entities, which have declarations which have an analogue in C. (Strictly C, not C++. However, as there is a subset of C which is also valid C++, one can also communicate with C++.)

Facilities for ensuring that data objects and procedures are interoperable are provided by the intrinsic module iso_c_binding

Numeric data types

For integer, real and complex types, iso_c_binding provides names for constants which are the relevant kind parameters for Fortran. For example, integer interoperable types include:

  !  declaration                                       ... interoperable with ...
  integer (c_int)                 :: i_int             ! C "int"
  integer (c_short)               :: i_short_int       ! C "short int"
  integer (c_long)                :: i_long            ! C "long int"
  integer (c_size_t)              :: i_size_t          ! C "size_t

The value can be -1 (no interoperable Fortran type) or -2 (no corresponding C type). Recall size_t is usually an unsigned type in C. There is still no direct analogue of unsigned types in Fortran, although an interoperable type is almost certainly available.

For real types, the full list is:

  ! declaration                                        ... interoperable with ...
  real (c_float)                  :: r_float           ! C "float"
  real (c_double)                 :: r_double          ! C "double"
  real (c_long_double)            :: r_long_double     ! C "long double"

If the value of the C name is -1 (e.g., c_long_double) then Fortran does not provide a precision equal to that of C type (long double). Other negative values indicate unavailabillity for different reasons.

For complex types, the full list is:

  ! declaration                                        ... interoperable with ...
  complex (c_float_complex)       :: z_float_complex   ! C "float _Complex"
  complex (c_double_complex)      :: z_double_complex  ! C "double _Complex"
  complex (c_long_double_complex) :: z_ldc             ! C "long double _Complex"

The precision for complex numbers relates to the precision for each of the real and imaginary parts.

Logical data types

There is one interoperable logical type:

  ! declaration
  logical (c_bool)                :: logical_c_bool    ! C "_Bool"

As logical operations in C are often performed using integer types, an integer type may be appropriate in a given context.

Example (1 minute)

Discussion

`iso_c_binding` symbols

A program is provided which prints out a full list of the symbols from iso_c_binding illustrated above.

$ ftn print_iso_c_binding.f90

Characters and strings

An interoperable character type is expected to be available:

  ! declaration                                  ... interoperable with ...
  character (kind = c_char, len = 1) :: char     ! C "char"

To pass strings to C, rather than single characters, it is usually necessary to think about a string in Fortran as being an assumed size array of characters of len = 1.

Named constants for the following C special characters are provided: c_null_char (\0), c_alert (\a), c_backspace (\b), c_form_feed (\f),c_new_line (\n), c_carriage_return (\r), c_horizontal_tab (\t), and c_vertical_tab (\v).

Fortran code receiving strings from C may wish to discard the c_null_char at the end of the string.

Calling C from Fortran

Suppose we wish to call the standard C library function

int atoi(const char * str);

from Fortran. To do this, we will write a Fortran interface to reflect the need for interoperable arguments. This might be:

  interface
    function c_atoi(str) bind(c, name = "atoi") result(i)
      use, intrinsic :: iso_c_binding, only : c_char, c_int
      character (kind = c_char, len = 1), intent(in) :: str(*)
      integer (kind = c_int)                         :: i
    end function c_atoi
  end interface

The bind(c) declaration indicates that this interface relates to a C function. If the name is present, it can be used to associate the C name with the Fortran interface declaration. If the name is not present, the C name will be taken to be the Fortran name (in lower case). Here, the function could be called from Fortran via, e.g.,

   integer (c_int) :: i
   i = c_atoi("-23")

The arguments of the function, and the return type, should all be relevant interoperable types.

A C function with void return type should have an interface defining a subroutine rather than a function.

Arguments passed by value

Many C functions have non-pointer dummy arguments, e.g.,

double hypot(double x, double y);

where actual arguments would be passed by value.

An appropriate Fortran interface can provide information on such scalar arguments via the value attribute:

  interface
    function c_hypot(x, y) bind(c, name = "hypot") result(z)
      use iso_c_binding, only : c_double
      real (c_double), value :: x, y
      real (c_double)        :: z
    end function c_hypot
  end interface

The value attribute takes the place of intent(in) (they cannot occur together). If a scalar argument does not have the value attribute, it means that C expects a pointer.

The value attribute can also be used in a normal Fortran context, where it is a signal to the compiler that the value should be copied in, whereon it may be changed, but not copied out again.

Example (10 minutes)

Challenge

Calling C from Fortran

Suppose we have a C function with prototype

int c_snprintf_float(char * str, size_t nsz, const char * format, float x);

which we wish to call from Fortran. The function uses the standard C library call snprintf() to write the value of the float argument to a string str with C format specification format. The maximum number of characters to be written is nsz. The return value is the number of characters actually written to the string (not including the \0 terminating character).

The C function is supplied in the current directory; it needs to be compiled (not linked) with the relevant C compiler, e.g. on ARCHER2,

$ cc -c c_snprintf.c

which will produce an object c_snprintf.o.

Write a program which includes an interface which allows the C function to be called with appropriate arguments. If the program is called f_snprintf.f90, we should be able to compile this with the C object via:

$ ftn c_snprintf.o f_snprintf.f90

Note this is the Fortran compiler.

What can you say about the length of the string returned in Fortran compared with the number of characters written indicated by the return value?

Show me the solution

You will need to provide an interface like so:

  interface
    function f_snprintf_float(str, sz, cformat, x) &
         bind(c, name = "c_snprintf_float")  result(nchar)
      use iso_c_binding, only : c_int, c_char, c_size_t, c_float
      character (kind = c_char, len = 1),        intent(out) :: str(*)
      integer   (kind = c_size_t),        value, intent(in)  :: sz
      character (kind = c_char, len = 1),        intent(in)  :: cformat(*)
      real      (kind = c_float),         value, intent(in)  :: x
      integer   (kind = c_int)                               :: nchar
    end function f_snprintf_float

    function f_snprintf_double(str, sz, cformat, x) &
         bind(c, name = "c_snprintf_double") result(nchar)
      use iso_c_binding, only : c_int, c_char, c_size_t, c_double
      character (kind = c_char, len = 1),        intent(out) :: str(*)
      integer   (kind = c_size_t),        value, intent(in)  :: sz
      character (kind = c_char, len = 1),        intent(in)  :: cformat(*)
      real      (kind = c_double),        value, intent(in)  :: x
      integer   (kind = c_int)                               :: nchar
    end function f_snprintf_double
  end interface

In order to make use of it, you should declare the variables that will be the actual arguments using the same interoperable types.

According to the Fortran, the returned string has length of 6, whereas the string legnth reported by the returned value from snprintf is 5. The extra is the null character used by C to terminate strings.

An example program called f_snprintf.f90 is provided.

Discussion

Calling C from Fortran (continued)

If you were to implement interfaces for both the c_snprintf_float() and c_snprintf_double() versions, you might wonder whether you could overload the specific names with a generic name. It seems like this should be possible, but all attempts currently fail with the compiler unable to resolve which specific interface it should use from the generic.

Arrays

Fortran arrays of non-zero size are interoperable if the data type is interoperable, and the array has an explicit shape or an assumed size (the * notation).

As an example of an explicit shape consider the C function with prototype

void c_array1(int nlen, double * data);

As usual, one would expect the array to be indexed from zero in C.

An appropriate Fortran interface might be

  interface
    subroutine c_array1(nlen, data) bind(c)
      use iso_c_binding, only : c_int, c_double
      integer (c_int), value         :: nlen
      real (c_double), intent(inout) :: data(nlen)
    end subroutine c_array1
  end interface

A Fortran allocatable array should be declared as assumed size in the interface. The relevant current size of the allocation will typically need to be passed as well, as above.

For arrays of rank 2, we must remember that the array element order is reversed in C relative to Fortran. That is, the Fortran declaration

   integer (c_int) :: array(m, n)

would need to correspond an array array[][m] or array[n][m] to be interoperable. The Fortran interface would specify dimension (m,*) or dimension(m,n), respectively.

Exercise (10 minutes)

Challenge

Passing arrays to C

The accompanying C file c_array.c holds a function with prototype

void c_array(int mlen, int nlen, int [][mlen]);

intended to be interoperable with a Fortran array declared (mlen,nlen). The C function simply prints out the values of the elements.

Write a Fortran program that passes a small array of shape (2,3) to the C function. Initialise the values consistent with Fortran array element order (e.g., indicative of increasing address). Does what you see make sense?

Show me the solution

If you declare the array in C as described and in your Fortran interface to the C function declare it as assumed size

integer (kind = c_int), intent(in) :: idata(mlen, *)

then you should receive output like the following:

OUTPUT

Element [0][0]  0  1
Element [0][1]  1  2
Element [1][0]  2  3
Element [1][1]  3  4
Element [2][0]  4  5
Element [2][1]  5  6

If we insist on visualising the array as a matrix, then in C it appears to be transposed. This is not actually correct – the memory layout is unchanged, and we reverse the order of indices while in C code in order to use it as we did in Fortran.

Sample solution code is available in the solutions directory in f_array.f90.

Pointers

Derived types c_ptr and c_funptr are provided for interoperability with C pointer types. These shouldn’t be assigned to directly, but instead a number of functions are provided to manage the translation of Fortran entities to and from these new types.

c_loc(x) returns the c_ptr type which C can use as the address of the argument. The argument can be a scalar, a contiguous array of non-zero size (or allocated non-zero size), or an associated pointer. The argument must be of interoperable type. The argument must be a pointer or a data object with target attribute.
c_funloc(p) can return the c_funptr address of an interoperable procedure.
c_associated(c_ptr1 [, c_ptr2]) is an analogue of the associated() intrinsic which returns .true. if the first argument is not c_null_ptr. If the second argument is present, the function will return .true. if both arguments are the same.
c_f_pointer(c_ptr, fptr [, shape]) provides functionality to translate a c_ptr type into a Fortran pointer. A rank 1 integer array shape is required if fptr is an array.
c_f_procpointer(c_funptr, fptr) provides similar functionality for a procedure pointer.

Derived types

To be interoperable, a Fortran derived type must map to a plain C struct with interoperable components. This means the Fortran type must have no type-bound procedures, cannot be extended, and cannot have components that have either the allocatable or pointer attributes,

The type should be declared as bind(c), e.g.,

  type, bind(c), public :: my_type
    integer (c_int) :: icomponent
    real (c_float)  :: fcomponent
  end type my_type

The presence of the bind(c) means the type cannot be extended.

Calling Fortran from C

Let us suppose we have a C program which defines a struct

typedef struct array_s {
  int nlen;
  float * data;
} array_t;

to aggregate the information on an array of float which is to be allocated by the program. Further, we wish to call a subroutine declared in C as

void f_subroutine(const array_t * a);

which we wish to write in Fortran.

A corresponding subroutine in Fortran requires the definition of the interoperable structure, i.e.,

  type, bind(c) :: array_t
    integer (c_int)  :: nlen
    type (c_ptr) :: data
  end type array_t

where we have used a c_ptr type to represent the C pointer component.

An interoperable subroutine declared bind(c) might be

  subroutine f_subroutine(a) bind(c)

    type (array_t), intent(in) :: a
    real (c_float), pointer    :: data(:)

    call c_f_pointer(a%data, data, [ a%nlen ])

    ! ... perform operations with data(:) ...

  end subroutine f_subroutine

The information that the C data is of type float appears in the declaration of the Fortran pointer used to access the array. This must be initialised by a call to c_f_pointer() before it can be used.

Exercise (10 minutes)

Challenge

Calling Fortran from C

Check you can construct a working example based on the above outline. Initialise some sample values and check you can recover the values in the Fortran subroutine.

Try using either the C or the Fortran compiler to perform the link stage.

Show me the solution

The Fortran type and subroutine can be placed in a module. Both need to be interoperable with C, meaning they should use bind(c). In the C code provide the struct, the extern void declaration of the Fortran subroutine, then write a brief program to set some values in the struct’s array. In the Fortran subroutine, you can print the array (after retrieving a usable pointer to it via c_f_pointer()) and check it’s correct.

Example solutions are available in exercises/14-interoperability-with-c/solutions as c_struct_to_fortran.c and f_array_t.f90.

Using the GCC compilers, you should be able to compile the code as follows:

OUTPUT

ftn -c f_array_t.f90
cc -c c_struct_to_fortran.c
ftn f_array_t.o c_struct_to_fortran.o

You can also compile in two steps as long as you tell the C compiler that it will need to use libgfortran.so in order to link the symbols from f_array_t.o.

OUTPUT

ftn -c f_array_t.f90
cc -lgfortran f_array_t.o c_struct_to_fortran.c

Key Points

The iso_c_binding module allows a programmer to write Fortran that is interoperable with C.
Care must be taken with pointers and arrays, and with variables which are to be passed to C by value.

Interoperability with C

Overview

Questions

Objectives

The intrinsic module iso_c_binding

Numeric data types

Logical data types

Example (1 minute)

iso_c_binding symbols

Characters and strings

Calling C from Fortran

Arguments passed by value

Example (10 minutes)

Calling C from Fortran

Show me the solution

Calling C from Fortran (continued)

Arrays

Exercise (10 minutes)

Passing arrays to C

Show me the solution

OUTPUT

Pointers

Derived types

Calling Fortran from C

Exercise (10 minutes)

Calling Fortran from C

Show me the solution

OUTPUT

OUTPUT

The intrinsic module `iso_c_binding`

`iso_c_binding` symbols