Interface To C Struct That Includes An Array

[rogermc@iinet.net.au]

 I'm having problems interfacing the following record to C.

   type API_String is record
      Length  : Interfaces.C.size_t := 0;
      Data    : API_String_Data_Array := (others => Interfaces.C.char'Val (0));
   end record;
   pragma Convention (C_Pass_By_Copy, API_String);

In general it seems OK, however, the following function in which Assimp_Path'Access is returned 
from a C function produces a strange result 

 Result := API_Get_Material_Texture (Material'Access, Tex_Type, unsigned (Tex_Index), Assimp_Path'Access);

The returned Assimp_Path is Length: 16, Data: \vB000phoenix.pcx0000
in which B is a series of four small zeros inside a rectangle.
I haven't been able to find a syntactical explanation for the four small zeros inside a rectangle.

The expected Assimp_Path is Length: 11, Data: phoenix.pcx
Memory inspection indicates that the Length 16 includes both the expected length and character string.

(gdb) print Assimp_Path
$8 = (length => 16, data => (0 => 11, 0, 0, 0, 112, 104, 111, 101, 110, 105, 120, 46, 112, 99, 120, 0 <repeats 1009 times>)

If I shift the Data characters left by four places and change Length to 12, the resultant Assimp_Path is
Length: 16, Data: \vBphoenix.pcx

The C interface is:

      function API_Get_Material_Texture (aMaterial : access API_Material_Tex;
                                         Tex_Type  : AI_Texture_Type;
                                         Index     : Interfaces.C.unsigned;
                                         Path      : access Assimp_Types.API_String := null;
                                         Mapping   : access AI_Texture_Mapping := null;
                                         UV_Index  : access Interfaces.C.unsigned := null;
                                         Blend     : access Interfaces.C.C_float := null;
                                         Op        : access AI_Texture_Op := null;
                                         Map_Mode  : access AI_Texture_Map_Mode := null)
                                         return Assimp_Types.API_Return;
      pragma Import (C, API_Get_Material_Texture, "aiGetMaterialTexture");

Any positive advice or explanation will be appreciated.
Roger