Delphi how to call c++ dll function with typedef void * parameter?

Question

I have a dll that is written in C. My delphi wrapper is calling functions from c++ dll. This is C++ code: ``` typedef enum EFTDeviceControlAction { EFT_DCA_CR_CARD_RETRACT = 0x01, EFT_DCA_CR_CARD_REPOSITION = 0x02, EFT_DCA_CR_SHUTTER_OPEN = 0x03, EFT_DCA_CR_SHUTTER_CLOSE = 0x04, EFT_DCA_CR_CARD_EJECT = 0x05, } typedef enum EFT_PrintOptions { poPrintState = 0, poPrintFirst = 1, poPrintSubsequent = 2, poPrintFinal = 3, poPrintAbort = 9 } EFT_PrintOptions; typedef void * EFT_HANDLE; int EFT_CreateSession(EFT_HANDLE * h); int EFT_DestroySession(EFT_HANDLE h); int EFT_ReadProperty(EFT_HANDLE h, int table, int index, char * pValue, unsigned int maxLength); int EFT_WriteProperty(EFT_HANDLE h, int table, int index, char * pValue); ... ``` And this is delphi code : ``` EFTDeviceControlAction = ( EFT_DCA_CR_CARD_RETRACT = $01, EFT_DCA_CR_CARD_REPOSITION = $02, EFT_DCA_CR_SHUTTER_OPEN = $03, EFT_DCA_CR_SHUTTER_CLOSE = $04, EFT_DCA_CR_CARD_EJECT = $05, ); EFT_PrintOptions = ( poPrintState = 0, po…

Accepted Answer

```
typedef void * EFT_HANDLE;
```

The name of the declared type is `EFT_HANDLE` and it is an alias for `void*`. And `void*` is simply an untyped pointer.

So, in Delphi you define it like this:

```
type
  EFT_HANDLE = Pointer;
```

Which is exactly what you already did.

The rest of your translations look basically quite reasonable. I have these comments:

- Are you sure the calling convention is `stdcall`? The C++ code you show does not specify a calling convention, and that invariably means `cdecl`.

- Use `PAnsiChar` rather than `PChar` so that your code is correct on Unicode Delphi as well as old non-Unicode Delphi.

The obvious place for an access violation is the null-terminated string. It would be helpful to see the code that you have which calls `EFT_ReadProperty`. It will need to look like this:

```
var
  prop: AnsiString;
....
SetLength(prop, 128); // for example, not sure what value is needed here
retval := EFT_ReadProperty(handle, index, PAnsiChar(prop), Length(prop)+1);
// the +1 is for the null-terminator, but the library will specify exactly 
// how that is handled and it could equally be that the +1 is omitted
```

Delphi how to call c++ dll function with typedef void * parameter?

Solution

Problem