how to pass struct from vb to dll? - dll

Question

I'm trying to pass a struct from vb to dll im facinf isues on that.
//Datalink layer.cpp
#include"DataLinkLayer.h"
#include<stdio.h>
#include <windows.h>
int __stdcall DataLink_TellTale_Encode (struct telltalelib *st_telltale, LPSTR * rtnFramePassedPtr)
{
sprintf(*rtnFramePassedPtr,"<%d,%d,%d,%d,%d,%d,%d,%d,%d,%d,%d>",NORMAL,STREAM,VEHICLEOVERALL,NA,PERCENTAGE,TELLTALEID,COMPIDNA,TELLTALELENGTH,st_telltale.Telltaleid,st_telltale.color,st_telltale.glowLevel,st_telltale.onOff);
return 0;
}
//Datalink Layer.h
#define MAXFRAMELEN 20
#define MAXPOSSOPTFIELDS 50
#define RESERVED 0
#define COMPIDNA 0
#define NA 1
#define SPEEDLENGTH 1
#define RPMLENGTH 1
#define FUELLENGTH 1
#define TELLTALELENGTH 3
struct telltalelib
{
int Telltaleid;
int color;
int glowLevel;
int onOff;
};
int __stdcall DataLink_TellTale_Encode (struct telltalelib *st_telltale, char* rtnFramePassedPtr);
enum priority{ LOW=1 , NORMAL, HIGH, CRITICAL,FATAL };
enum frameType{ REQUEST=1, RESPONSE, ERROR, STREAM, NOTIFICATION, STATEINFORMATIONCHANGE, PERIODIC };
enum domain{MODULEPOWER=1,MODULESTATEMACHINE,VEHICLEOVERALL,ENGINE,TRANSMISSION,BRAKE,SAFETYSYSTEMS,LOCK,DRIVER,AC,DOOR,TPMS,LIGHTS,WINDOW, ANTENNA,SEATS,VEHICLEATTITUDE,WIPER,ORVM,DISPLAY,VEHICLECONSTANTS,TELEPHONY,INFOTAINMENT,FOTA };
enum Parameter{ SPEED=2 ,RPM ,FUEL, ODO ,TEMPERATURE, PRESSURE, HUMIDITY,BATTERYLEVEL, ONOFF , GLOWLEVEL, COLOR, OPENPOSITION, RAMPSTYLE, CMDSHUTDOWN, CMDREBOOT, NOTIFICATIONTEXT, CURRENT, DRIVEMODE, GEARLEVEL, MASS, VOLUME, DATAPACKET ,SLIDELEVEL ,RECLINELEVEL, DRIVERID, DRIVINGSCORE, MINLEVELSETTING, MAXLEVELSETTING, TIME, ROLL, PITCH, YAW,MODE};
enum Unit{ RAW=2, SECONDS, CM ,M ,KM, MILE, KMPERHOUR, MILESPERHOUR, X100RPM, X1000RPM, PERCENTAGE, RGB, PSI, BAR, KPA, DEGCELCIUS, DEGFARANHEIT ,FULLYOPENORFULYLCLOSED, ENUMERATEDCODES ,ENUMERATEDFIELDID, V ,MA, A, DEGREE , KG ,CM3, CC, PLAINTEXT ,DB ,EPOCHTIME ,OKNOTOK };
enum Component{TYREID=2 ,ACVENTID,DOORID, WINDOWID, LIGHTID, ANTENNAID, TELLTALEID, ACTUATORID,GENERICDATAFIELDID ,DATAFORMATID, WIPERID } ;
VB Code:
Private Type struct_telltalelib
Telltaleid As Integer
color As Integer
glowLevel As Integer
onOff As Integer
End Type
Private Declare Function DataLink_TellTale_Encode Lib "C:\MinGW\bin\DataLinkLayer.dll" (ByRef st_telltale As struct_telltalelib, ByRef rtnFramePassedPtr As String) As Integer
Dim vb_telltale As struct_telltalelib
Dim str_data As String * 20
Dim s2 As String
Dim stringToBeTrasmitted As String
Private Sub Check1_Click()
vb_telltale.Telltaleid = 8
vb_telltale.color = -1
vb_telltale.glowLevel = -2
vb_telltale.onOff = 1
dummy = DataLink_TellTale_Encode(str(vb_telltale), str_data)
stringToBeTrasmitted = Trim(Mid(Replace(str_data, Chr(0), " "), 1, InStr(str_data, Chr(0))))
Timer1.Enabled = True
End Sub
Private Sub Form_Load()
UART1.PortOpen = True
End Sub
Private Sub Timer1_Timer()
Timer1.Enabled = False
UART1.Output = stringToBeTrasmitted
End Sub
I'm getting error message like only user defined in public object modules can be corrected to or from varient or passed to late -bound functions.

Answer 1

I see the following problems:
The struct should be passed as vb_telltale rather than str(vb_telltale).
The string should be declared as ByVal.
The string buffer is likely too small and will be overrun.

Answer 2

The error is telling you the VB Type variable must be declared as Public in a BAS module. Also, in C an int is equivalent to a VB6 Long, not an Integer, so you'll likely have size and alignment mismatches. There may be other issues but those stand out.