Re: Interface To C Struct That Includes An Array

[rogermc@iinet.net.au]

On Monday, 17 September 2018 22:01:43 UTC+10, rog...@iinet.net.au  wrote:
> 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:  B000phoenix.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>)
> 
CORRECTION:
> If I shift the Data characters left by four places and change Length to 12, the resultant Assimp_Path is
> Length: 12, Data:  Bphoenix.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