How use UEFI locate_protocol in a rust project with r_efi crate

In a cross-compiled educative project with rust and the crate r_efi and without the rust standard library, I wanna make a small program to UEFI systems. For the moment, the goal is to be able to use the graphical output protocol.

Through the use of r_efi crate, I start by locating the GOP with the system module :

r_efi::system::BootServices::locate_protocol

The definition is :

locate_protocol: extern "win64" fn(_: *mut Guid, _: *mut c_void, _: *mut *mut c_void) -> Status

My problem is this 2 parameters :

_: *mut c_void

and

_: *mut *mut c_void

I don't know how return core::ffi::c_void parameters like in a C code :

EFI_STATUS efi_main(EFI_HANDLE image, EFI_SYSTEM_TABLE *systemTable) {
      EFI_BOOT_SERVICES *bs = systemTable->BootServices;
      EFI_GRAPHICS_OUTPUT_PROTOCOL *graphicsProtocol;
      EFI_STATUS status;

      status = bs->LocateProtocol(&GraphicsOutputProtocolGUID, NULL, (void**)&graphicsProtocol);

Thank in advance for your explanations.

Solution

Well, here a code solution ( working on virtualbox ) to locate the graphical output protocol with the r_efi crate :

#[export_name = "efi_main"]
pub extern fn main(_h: efi::Handle, st: *mut efi::SystemTable) -> efi::Status {

     let mut gop: *mut efi::protocols::graphics_output::Protocol = ptr::null_mut();
     let mut guid_gop: efi::Guid = efi::protocols::graphics_output::PROTOCOL_GUID;

     let stats = unsafe { ((*(*st).boot_services).locate_protocol)(
            &mut guid_gop as *mut _,
            ptr::null_mut(),
            &mut gop as *mut _ as *mut *mut core::ffi::c_void,
     )};

     match stats {

        efi::Status::SUCCESS => {...},
        efi::Status::NOT_FOUND => {...},

     }

     efi::Status::SUCCESS
}