How to create Fortran interface for type void ** p

Question

I am new to use Fortran, and for a c function like below:

cudaError_t cudaMalloc  (void** devPtr, size_t size)

Allocates size bytes of linear memory on the device and returns in *devPtr a pointer to the allocated memory. The allocated memory is suitably aligned for any kind of variable. The memory is not cleared. cudaMalloc() returns cudaErrorMemoryAllocation in case of failure.

Parameters:
devPtr  - Pointer to allocated device memory
size    - Requested allocation size in bytes

Returns:
cudaSuccess, cudaErrorMemoryAllocation

I want to create an Fortran interface to use this c function but how to fix void** ptr? Can anyone help me? Thanks in advance!

Answer 1

I have no idea whether it will work OK with the device pointers (i.e., if cudaMalloc is callable from non-CUDA C), but generally in Fortran-C interoperability you represent void* as type(c_ptr) from the iso_c_binding module. C interoperable procedures pass their arguments by reference by default, so this should work:

integer(c_int) function cudaMalloc(devPtr, size) bind(C,name="cudaMalloc")
  use iso_c_binding
  type(c_ptr) :: devPtr
  integer(c_size_t), value :: size
end function

The pointer is passed by reference, so that the C side sees a pointer to pointer and can change it to store the value of the device pointer.

With some more work you can also define the enumeration with the return codes.