How to declare pointer-of-pointer using iso_c_binding?

Question

I am writing an iso_c_binding in Fortran to call a C-function with the below prototype

int zmat_run(
    const size_t inputsize,
    unsigned char *inputstr, 
    size_t *outputsize, 
    unsigned char **outputbuf,
    const int zipid, 
    int *ret, 
    const int iscompress
);

My question is how do I declare unsigned char **outputbuf, a pointer that is used inside the c-function to allocate output buffer, in this interface?

Also, what data type I should be using in Fortran as the real parameter to pass to this outputbuf parameter? should it be allocatable? (if it is allocated inside the c-function)?

I currently drafted this module, but haven't tested it (I doubt it will work).

module zmatlib
  use iso_c_binding, only: c_char, c_size_t, c_ptr, C_NULL_CHAR
  interface
    integer(c_int) function zmat_run(inputsize, inputbuf, outputsize, outputbuf, zipid, ret, level) bind(C, name="zmat_run")
      use iso_c_binding
      integer(c_size_t), value :: inputsize
      integer(c_int), value :: zipid, level
      integer(c_size_t),  intent(out) :: outputsize
      integer(c_int),  intent(out) :: ret
      character(kind=c_char),  intent(in)  :: inputbuf(*)
      character pointer(c_ptr),intent(out) :: outputbuf
    end function zmat_run
  end interface
end module

At the least character pointer(c_ptr) isn't a Fortran thing. Could you revisit the Fortran block you drafted to be more syntactically correct in what you think is appropriate? — francescalus
– francescalus, Commented May 25, 2020 at 0:17
Yes, I know that wasn't valid, just a place holder. Googled but only find examples of pointers, but not pointer of pointer. — FangQ
– FangQ, Commented May 25, 2020 at 2:19

M. S. B. · Accepted Answer · 2020-05-25 06:41:49Z

1

Try type (C_PTR), intent (out). Then you will need to use Fortran function c_f_pointer to associate the C pointer with a Fortran pointer. Probably of type C_CHAR.

answered May 25, 2020 at 6:41

M. S. B.

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5 Comments

FangQ Over a year ago

Thank you, it works! here is the interface declaration, and here is my test code. However, the only remaining question is how do I "free" the myout pointer on the fortran side? valgrind detects a memory leak in this test code.

FangQ Over a year ago

another follow-up question - is there any risk of using fortran native types vs iso_c_binding c-types? for example, for c_size_t, I used a integer(kind=8) on the fortran side, does this cause any side effect? similarly for character(kind=c_char) vs the native character type.

FangQ Over a year ago

Re my earlier question on freeing the c-allocated buffer, I added a c-function to call free, and added it to the interface. github.com/fangq/zmat/blob/master/fortran90/…

Steve Lionel Over a year ago

The risk of not using the kind values declared in ISO_C_BINDING is that the compiler's kind values may not match C's. Why not use C_SIZE_T? kind=8 is inherently non-portable.

M. S. B. Over a year ago

As you did, since the memory was allocated on the C side, it is best to free the memory on the C side. You can optionally nullify the Fortran pointer.

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