c_interface_module in Fortran Wiki

**NOTE:** Gfortran has a bug that prevents using aliases via "USE, ONLY" as done here. See below for a work-around.
~~~~~~~{: lang=fortran}
! FILE: c_interface_module.f
! PURPOSE: Supplement ISO-C-Binding to provide type aliases and interfaces
! to common ISO-C string functions to aid working with strings.
! AUTHOR: Joseph M. Krahn
! STATUS: Still in development. Reasonably complete, but somewhat limited testing.
!
! The idea is to provide type aliases for all ISO-C types, so that the
! Fortran interface code more explicitly defines the actual C interface.
! This should be updated to support F2008 variable-length allocatable
! strings.
!
! Entity names all have the "C_" prefix, as with ISO-C-Binding, with a
! few exceptions.
! 
module C_interface_module
 use, intrinsic :: ISO_C_Binding, &
 ! C type aliases for pointer derived types:
 C_ptr => C_ptr , &
 C_char_ptr => C_ptr, &
 C_const_char_ptr => C_ptr, &
 C_void_ptr => C_ptr, &
 C_const_void_ptr => C_ptr
 implicit none
!----------------------------------------------------------------------------
! C type aliases for intrinsic type KIND parameters:
! NOTE: a C enum may not always be a standard C int
 integer, parameter :: C_enum = C_int
! Defining off_t is difficult, because it may depend on "LARGEFILE" selection.
! integer, parameter :: C_off_t = ??
! C string terminator alais using the 3-letter ASCII name.
! The C_ prefix is not used because it is just an ASCII character.
 character(len=1,kind=C_char), parameter :: NUL = C_NULL_char
! NOTE: In C, "char" is distinct from "signed char", unlike integers.
! The plain "char" type is specific for text/string values, whereas
! "signed char" should indicate 1-byte integer data.
!
! Most ISO-C systems have wide chars "wchar_t", but Fortran compilers
! have limited support for different character kinds. UTF encoding
! adds more complexity. This should be updated as Fortran compilers
! include support for more character types.
! 
! Fortran does not (yet) support unsigned types.
 integer, parameter :: &
 C_unsigned = C_int, &
 C_unsigned_short = C_short, &
 C_unsigned_long = C_long, &
 C_unsigned_long_long = C_long_long, &
 C_unsigned_char = C_signed_char, &
 C_ssize_t = C_size_t, &
 C_uint8_t = C_int8_t, &
 C_uint16_t = C_int16_t, &
 C_uint32_t = C_int32_t, &
 C_uint64_t = C_int64_t, &
 C_uint_least8_t = C_int_least8_t, &
 C_uint_least16_t = C_int_least16_t, &
 C_uint_least32_t = C_int_least32_t, &
 C_uint_least64_t = C_int_least64_t, &
 C_uint_fast8_t = C_int_fast8_t, &
 C_uint_fast16_t = C_int_fast16_t, &
 C_uint_fast32_t = C_int_fast32_t, &
 C_uint_fast64_t = C_int_fast64_t, &
 C_uintmax_t = C_intmax_t
! Note: ptrdiff_t cannot be reliably defined from other types.
! When practical, it is larger than a pointer because it benefits
! from the full unsigned range in both positive and negative directions.
! Integer versions including 'int', where the 'int' is optional:
 integer, parameter :: &
 C_short_int = C_short, &
 C_long_int = C_long, &
 C_long_long_int = C_long_long, &
 C_unsigned_int = C_unsigned, &
 C_unsigned_short_int = C_short, &
 C_unsigned_long_int = C_long, &
 C_unsigned_long_long_int = C_long_long
 interface C_F_string
 module procedure C_F_string_ptr
 module procedure C_F_string_chars
 end interface C_F_string
 interface F_C_string
 module procedure F_C_string_ptr
 module procedure F_C_string_chars
 end interface F_C_string
!=======================================================================
! Some useful ISO C library string functions from <string.h>
! These are based on GCC header sections marked as NAMESPACE_STD
 interface
! Copy N bytes of SRC to DEST, no aliasing or overlapping allowed.
!extern void *memcpy (void *dest, const void *src, size_t n);
 function C_memcpy(dest, src, n) result(result) bind(C,name="memcpy")
 import C_void_ptr, C_size_t
 type(C_void_ptr) :: result
 type(C_void_ptr), value, intent(in) :: dest ! target=intent(out)
 type(C_void_ptr), value, intent(in) :: src ! target=intent(in)
 integer(C_size_t), value, intent(in) :: n
 end function C_memcpy
! Copy N bytes of SRC to DEST, guaranteeing correct behavior for overlapping strings.
!extern void *memmove (void *dest, const void *src, size_t n)
 function C_memmove(dest, src, n) result(result) bind(C,name="memmove")
 import C_void_ptr, C_size_t
 type(C_void_ptr) :: result
 type(C_void_ptr), value, intent(in) :: dest ! target=intent(out)
 type(C_void_ptr), value, intent(in) :: src
 integer(C_size_t), value, intent(in) :: n
 end function C_memmove
! Set N bytes of S to C.
!extern void *memset (void *s, int c, size_t n)
 function C_memset(s, c, n) result(result) bind(C,name="memset")
 import C_void_ptr, C_int, C_size_t
 type(C_void_ptr) :: result
 type(C_void_ptr), value, intent(in) :: s ! target=intent(out)
 integer(C_int), value, intent(in) :: c
 integer(C_size_t), value, intent(in) :: n
 end function C_memset
! Compare N bytes of S1 and S2.
!extern int memcmp (const void *s1, const void *s2, size_t n)
 pure &
 function C_memcmp(s1, s2, n) result(result) bind(C,name="memcmp")
 import C_int, C_void_ptr, C_size_t
 integer(C_int) :: result
 type(C_void_ptr), value, intent(in) :: s1
 type(C_void_ptr), value, intent(in) :: s2
 integer(C_size_t), value, intent(in) :: n
 end function C_memcmp
! Search N bytes of S for C.
!extern void *memchr (const void *s, int c, size_t n)
 pure &
 function C_memchr(s, c, n) result(result) bind(C,name="memchr")
 import C_void_ptr, C_int, C_size_t
 type(C_void_ptr) :: result
 type(C_void_ptr), value, intent(in) :: s
 integer(C_int), value, intent(in) :: c
 integer(C_size_t), value, intent(in) :: n
 end function C_memchr
! Copy SRC to DEST.
!extern char *strcpy (char *dest, const char *src)
 function C_strcpy(dest, src) result(result) bind(C,name="strcpy")
 import C_char_ptr, C_size_t
 type(C_char_ptr) :: result
 type(C_char_ptr), value, intent(in) :: dest ! target=intent(out)
 type(C_char_ptr), value, intent(in) :: src
 end function C_strcpy
! Copy no more than N characters of SRC to DEST.
!extern char *strncpy (char *dest, const char *src, size_t n)
 function C_strncpy(dest, src, n) result(result) bind(C,name="strncpy")
 import C_char_ptr, C_size_t
 type(C_char_ptr) :: result
 type(C_char_ptr), value, intent(in) :: dest ! target=intent(out)
 type(C_char_ptr), value, intent(in) :: src
 integer(C_size_t), value, intent(in) :: n
 end function C_strncpy
! Append SRC onto DEST.
!extern char *strcat (char *dest, const char *src)
 function C_strcat(dest, src) result(result) bind(C,name="strcat")
 import C_char_ptr, C_size_t
 type(C_char_ptr) :: result
 type(C_char_ptr), value, intent(in) :: dest ! target=intent(out)
 type(C_char_ptr), value, intent(in) :: src
 end function C_strcat
! Append no more than N characters from SRC onto DEST.
!extern char *strncat (char *dest, const char *src, size_t n)
 function C_strncat(dest, src, n) result(result) bind(C,name="strncat")
 import C_char_ptr, C_size_t
 type(C_char_ptr) :: result
 type(C_char_ptr), value, intent(in) :: dest ! target=intent(out)
 type(C_char_ptr), value, intent(in) :: src
 integer(C_size_t), value, intent(in) :: n
 end function C_strncat
! Compare S1 and S2.
!extern int strcmp (const char *s1, const char *s2)
 pure &
 function C_strcmp(s1, s2) result(result) bind(C,name="strcmp")
 import C_int, C_char_ptr, C_size_t
 integer(C_int) :: result
 type(C_char_ptr), value, intent(in) :: s1
 type(C_char_ptr), value, intent(in) :: s2
 end function C_strcmp
! Compare N characters of S1 and S2.
!extern int strncmp (const char *s1, const char *s2, size_t n)
 pure &
 function C_strncmp(s1, s2, n) result(result) bind(C,name="strncmp")
 import C_int, C_char_ptr, C_size_t
 integer(C_int) :: result
 type(C_char_ptr), value, intent(in) :: s1
 type(C_char_ptr), value, intent(in) :: s2
 integer(C_size_t), value, intent(in) :: n
 end function C_strncmp
! Return the length of S.
!extern size_t strlen (const char *s)
 pure &
 function C_strlen(s) result(result) bind(C,name="strlen")
 import C_char_ptr, C_size_t
 integer(C_size_t) :: result
 type(C_char_ptr), value, intent(in) :: s !character(len=*), intent(in)
 end function C_strlen
 end interface
! End of <string.h>
!=========================================================================
! Standard ISO-C malloc routines:
 interface
 ! void *calloc(size_t nmemb, size_t size);
 type(C_void_ptr) &
 function C_calloc(nmemb, size) bind(C,name="calloc")
 import C_void_ptr, C_size_t
 integer(C_size_t), value, intent(in) :: nmemb, size
 end function C_calloc
 ! void *malloc(size_t size);
 type(C_void_ptr) &
 function C_malloc(size) bind(C,name="malloc")
 import C_void_ptr, C_size_t
 integer(C_size_t), value, intent(in) :: size
 end function C_malloc
 ! void free(void *ptr);
 subroutine C_free(ptr) bind(C,name="free")
 import C_void_ptr
 type(C_void_ptr), value, intent(in) :: ptr
 end subroutine C_free
 ! void *realloc(void *ptr, size_t size);
 type(C_void_ptr) &
 function C_realloc(ptr,size) bind(C,name="realloc")
 import C_void_ptr, C_size_t
 type(C_void_ptr), value, intent(in) :: ptr
 integer(C_size_t), value, intent(in) :: size
 end function C_realloc
 end interface
 interface assignment(=)
 module procedure F_string_assign_C_string
 end interface assignment(=)
!==========================================================================
contains
 ! HACK: For some reason, C_associated was not defined as pure. 
 pure logical &
 function C_associated_pure(ptr) result(associated)
 type(C_ptr), intent(in) :: ptr
 integer(C_intptr_t) :: iptr
 iptr = transfer(ptr,iptr)
 associated = (iptr /= 0)
 end function C_associated_pure
! Set a fixed-length Fortran string to the value of a C string.
 subroutine F_string_assign_C_string(F_string, C_string)
 character(len=*), intent(out) :: F_string
 type(C_ptr), intent(in) :: C_string
 character(len=1,kind=C_char), pointer :: p_chars(:)
 integer :: i
 if (.not. C_associated(C_string) ) then
 F_string = ' '
 else
 call C_F_pointer(C_string,p_chars,[huge(0)])
 i=1
 do while(p_chars(i)/=NUL .and. i<=len(F_string))
 F_string(i:i) = p_chars(i)
 i=i+1
 end do
 if (i<len(F_string)) F_string(i:) = ' '
 end if
 end subroutine F_string_assign_C_string
! Copy a C string, passed by pointer, to a Fortran string.
! If the C pointer is NULL, the Fortran string is blanked.
! C_string must be NUL terminated, or at least as long as F_string.
! If C_string is longer, it is truncated. Otherwise, F_string is
! blank-padded at the end.
 subroutine C_F_string_ptr(C_string, F_string)
 type(C_ptr), intent(in) :: C_string
 character(len=*), intent(out) :: F_string
 character(len=1,kind=C_char), dimension(:), pointer :: p_chars
 integer :: i
 if (.not. C_associated(C_string)) then
 F_string = ' '
 else
 call C_F_pointer(C_string,p_chars,[huge(0)])
 i=1
 do while(p_chars(i)/=NUL .and. i<=len(F_string))
 F_string(i:i) = p_chars(i)
 i=i+1
 end do
 if (i<len(F_string)) F_string(i:) = ' '
 end if
 end subroutine C_F_string_ptr
! Copy a C string, passed as a char-array reference, to a Fortran string.
 subroutine C_F_string_chars(C_string, F_string)
 character(len=1,kind=C_char), intent(in) :: C_string(*)
 character(len=*), intent(out) :: F_string
 integer :: i
 i=1
 do while(C_string(i)/=NUL .and. i<=len(F_string))
 F_string(i:i) = C_string(i)
 i=i+1
 end do
 if (i<len(F_string)) F_string(i:) = ' '
 end subroutine C_F_string_chars
! Copy a Fortran string to an allocated C string pointer.
! If the C pointer is NULL, no action is taken. (Maybe auto allocate via libc call?)
! If the length is not passed, the C string must be at least: len(F_string)+1
! If the length is passed and F_string is too long, it is truncated.
 subroutine F_C_string_ptr(F_string, C_string, C_string_len)
 character(len=*), intent(in) :: F_string
 type(C_ptr), intent(in) :: C_string ! target = intent(out)
 integer, intent(in), optional :: C_string_len ! Max string length,
 ! INCLUDING THE TERMINAL NUL
 character(len=1,kind=C_char), dimension(:), pointer :: p_chars
 integer :: i, strlen
 strlen = len(F_string)
 if (present(C_string_len)) then
 if (C_string_len <= 0) return
 strlen = min(strlen,C_string_len-1)
 end if
 if (.not. C_associated(C_string)) then
 return
 end if
 call C_F_pointer(C_string,p_chars,[strlen+1])
 forall (i=1:strlen)
 p_chars(i) = F_string(i:i)
 end forall
 p_chars(strlen+1) = NUL
 end subroutine F_C_string_ptr
 pure &
 function C_strlen_safe(s) result(length)
 integer(C_size_t) :: length
 type(C_char_ptr), value, intent(in) :: s
 if (.not. C_associated_pure(s)) then
 length = 0
 else
 length = C_strlen(s)
 end if
 end function C_strlen_safe
 function C_string_value(C_string) result(F_string)
 type(C_ptr), intent(in) :: C_string
 character(len=C_strlen_safe(C_string)) :: F_string
 character(len=1,kind=C_char), dimension(:), pointer :: p_chars
 integer :: i, length
 length = len(F_string)
 if (length/=0) then
 call C_F_pointer(C_string,p_chars,[length])
 forall (i=1:length)
 F_string(i:i) = p_chars(i)
 end forall
 end if
 end function C_string_value
! Copy a Fortran string to a C string passed by char-array reference.
! If the length is not passed, the C string must be at least: len(F_string)+1
! If the length is passed and F_string is too long, it is truncated.
 subroutine F_C_string_chars(F_string, C_string, C_string_len)
 character(len=*), intent(in) :: F_string
 character(len=1,kind=C_char), dimension(*), intent(out) :: C_string
 integer, intent(in), optional :: C_string_len ! Max string length,
 ! INCLUDING THE TERMINAL NUL
 integer :: i, strlen
 strlen = len(F_string)
 if (present(C_string_len)) then
 if (C_string_len <= 0) return
 strlen = min(strlen,C_string_len-1)
 end if
 forall (i=1:strlen)
 C_string(i) = F_string(i:i)
 end forall
 C_string(strlen+1) = NUL
 end subroutine F_C_string_chars
! NOTE: Strings allocated here must be freed by the
! C library, such as via C_free() or C_string_free(),
 type(C_ptr) &
 function F_C_string_dup(F_string,length) result(C_string)
 character(len=*), intent(in) :: F_string
 integer, intent(in), optional :: length
 character(len=1,kind=C_char), pointer :: C_string_ptr(:)
 integer :: i
 integer(C_size_t) :: strlen
 if (present(length)) then
 strlen = length
 else
 strlen = len(F_string)
 end if
 if (strlen <= 0) then
 C_string = C_NULL_ptr
 else
 C_string = C_malloc(strlen+1)
 if (C_associated(C_string)) then
 call C_F_pointer(C_string,C_string_ptr,[strlen+1])
 forall (i=1:strlen)
 C_string_ptr(i) = F_string(i:i)
 end forall
 C_string_ptr(strlen+1) = NUL
 end if
 end if
 end function F_C_string_dup
! NOTE: Strings allocated here must be freed by the
! C library, such as via C_free() or C_string_free(),
 type(C_ptr) &
 function C_string_alloc(length) result(C_string)
 integer(C_size_t), intent(in) :: length
 character(len=1,kind=C_char), pointer :: C_charptr
 C_string = C_malloc(length+1)
 if (C_associated(C_string)) then
 call C_F_pointer(C_string,C_charptr)
 C_charptr = NUL
 end if
 end function C_string_alloc
 subroutine C_string_free(string)
 type(C_ptr), intent(inout) :: string
 if (C_associated(string)) then
 call C_free(string)
 string = C_NULL_ptr
 end if
 end subroutine C_string_free
end module C_interface_module
~~~~~~~
GFortran does not support multiple local-names for USE, ONLY: statements with the ISO_C_Binding module. This is a bug that only occurs with the intrinsic module. You can create a simple wrapper module that USEs ISO_C_BINDING, and the local-name aliases work. However, the IMPORT statements are still broken, so a similar work-around is required for every interface. Until this bug is fixed, it is easier just to avoid using the C_ptr aliases. The kind parameters are not affected, because they are written as separate parameters.
category: code