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Writing Ada Bindings for C Libraries, Part 3


Void Pointers

C has no generics. So whenever a subprogram parameter may take differently typed values, a void pointer is used. Usually, a void pointer value will be used in one of these ways:

  • It will be passed on to another subprogram that will know its type, cast it appropriately and do stuff with it.
  • It will be used to return data to the caller, and he has to know what to do with it.

Here's an example for the second case:

void * clGetExtensionFunctionAddress(const char * func_name);

Here, a void pointer is returned to the caller. The purpose of this function is to return a pointer to a subprogram specified with func_name. So there is a fixed set of accepted values for func_name, and for every value, the function may return a differently typed pointer to a subprogram.

There are several possibilities to wrap C functions taking void pointers in Ada:

Import it multiple times with different signatures

package C renames Interfaces.C;
type Func_Type1 is access function return;
pragma Convention (C, Func_Type1);
type Func_Type2 is access function (Param : return C.double;
pragma Convention (C, Func_Type2);
function Get_Extension_Function_Address
  (Func_Name : C.Strings.chars_ptr) return Func_Type1;
function Get_Extension_Function_Address
  (Func_Name : C.Strings.chars_ptr) return Func_Type2;
pragma Import (Convention => C, Entity => Get_Extension_Function_Address,
               External_Name => "clGetExtensionFunctionAddress");

The Import pragma will be applied to all functions that match the given name. While this works, it does not give us type safety: If the user calls the wrong function, he gets a function reference back that will not work as expected.

Wrap the C function

function Backend (Func_Name : C.char_array) return System.Address;
pragma Import (Convention => C, Entity => Backend,
               External_Name => "clGetExtensionFunctionAddress");

   type Return_Type is private;
   Function_Name : String;
function Get_Extension_Function_Address return Return_Type is
   function Convert is new Ada.Unchecked_Conversion (System.Address, Return_Type);
   return Convert (Backend (C.To_C (Function_Name)));
end Get_Extension_Function_Address;
function Get_Func1 is new Get_Extension_Function_Address
  (Func_Type1, "func1");
function Get_Func2 is new Get_Extension_Function_Address
  (Func_Type2, "func2");

Obviously, you want to expose just the last two functions to the caller. As you cannot implement a declaration made in a package specification by a generic instantiation, you have to use renames to do that:

function Get_Func1_Public return Func_Type1 renames Get_Func1;

Provide a generic interface

... so the caller can define the type he wants to use. This is useful in cases like this:

void registerCallback(void(*callback)(void* user_data), void* user_data);

Here, the C procedure lets the caller register a callback that, when called, will be passed a pointer to some data the caller provides. This is a pattern that is often used with callbacks in C. You can wrap it like this:

procedure Backend (Callback_Raw, User_Data : System.Address);
pragma Import (Convention => C, Entity => Backend,
               External_Name => "registerCallback");
   type User_Data_Type is private;
   type User_Data_Access is access User_Data_Type;
   type Callback is access procedure (User_Data : User_Data_Type);
procedure Register_Callback (Target : Callback; User_Data : User_Data_Access) is
   function Convert_User_Data is new Ada.Unchecked_Conversion
     (User_Data_Access, System.Address);
   function Convert_Callback is new Ada.Unchecked_Conversion
     (Callback, System.Address);
   Backend (Convert_Callback (Target), Convert_User_Data (User_Data));
end Register_Callback;

You may want to convert this code to a generic package that can define the types User_Data_Access and Callback itself based on the parameter User_Data_Type, particularly if there are multiple similar callback registering functions.

Be aware that this wrapper leaves it to the caller to make sure his callback function has the correct convention (one can also use the pragma Convention on subprograms that are implemented in Ada if they will be called by C code).

If you want to make your wrapper even thicker, you can define your own User_Data_Type and callback function, and embed the reference to the caller's function as well as the caller's data in your User_Data_Type. Your callback function can then extract the subprogram reference and user data from your container and call the callback the caller provided. This way, the caller does not need to apply any pragmas in his code.


If you want to wrap a void pointer, you usually declare it as System.Address and use Ada.Unchecked_Conversion in your wrapper. The lesser the caller needs to take care about Convention pragmas, the easier your wrapper is to use.


Bitfields are usually declared as numeric type like int in C. Then, a number of constants is defined that can be combined with bitwise OR to build a value of the bitfield. Example:

typedef cl_ulong            cl_bitfield;
typedef cl_bitfield         cl_device_type;

/* cl_device_type - bitfield */
#define CL_DEVICE_TYPE_DEFAULT                      (1 << 0)
#define CL_DEVICE_TYPE_CPU                          (1 << 1)
#define CL_DEVICE_TYPE_GPU                          (1 << 2)
#define CL_DEVICE_TYPE_ACCELERATOR                  (1 << 3)

cl_int clGetDeviceIDs(cl_platform_id   /* platform */,
                      cl_device_type   /* device_type */, 
                      cl_uint          /* num_entries */, 
                      cl_device_id *   /* devices */, 
                      cl_uint *        /* num_devices */);

Of course, you could just copy the constants to Ada and provide the same interface. But you can also wrap it with a record:

type Device_Type is record
   Default     : Boolean := False;
   CPU         : Boolean := False;
   GPU         : Boolean := False;
   Accelerator : Boolean := False;
end record;
for Device_Type use record
  Default     at 0 range 0 .. 0;
  CPU         at 0 range 1 .. 1;
  GPU         at 0 range 2 .. 2;
  Accelerator at 0 range 3 .. 3;
end record;
for Device_Type'Size use ULong'Size;
pragma Convention (C_Pass_By_Copy, Device_Type);

This way, the possible values are directly linked to the type. If you just provide constants and a numeric type, there is no explicit link between them.

Tags: ada programming