char*	UserName = NULL;

UserName = NewUserName; //char* NewUserName  

int __stdcall CPP_Get_Registration_Information(char * MyUserName)
{ 
  MyUserName = UserName;
  return 0;
}

Private Declare Function CPP_Get_Registration_Information Lib "MyTestDll.dll" _
 (ByVal NewUserName As String) As Long
Private Sub CommandGetRegistrationInformation_Click()
  Dim NewUserName As String
  NewUserName = String$(255, 0)
  CPP_Get_Registration_Information NewUserName
  MsgBox NewUserName
End Sub

Declare Function CPP_Get_Registration_Information Lib "MyTestDll.dll" (ByVal lpBuffer As String, ByVal nSize As Long) As Long
Function GetRegName() As String
Dim RegName As String
    ' Receive a Registration Info from API's CPP_Get_Registration_Information  function
     RegName = String$(255, 0)
     GetRegName = Left(RegName, CPP_Get_Registration_Information(RegName, InStr(RegName, Chr(0)) - 1))
End Function

int		TrialDaysTotal		= -1,
		TrialDaysLeft		= -1;
...
	TrialDaysTotal = Total;
	TrialDaysLeft  = Left;
...
extern "C" {
int __stdcall CPP_Get_Trial_Days(int * Total, int * Left)
{ 
  * Total = TrialDaysTotal;
  * Left = TrialDaysLeft;
  return 0;
}

Private Declare Function CPP_Get_Trial_Days Lib "MyTestDll.dll" _
 (ByRef Total As Long, ByRef Left As Long) As Long

Private Sub CommandGetTrialDays_Click()
  Dim Total As Long
  Dim Left As Long
  
  CPP_Get_Trial_Days Total, Left
  
  MsgBox "Total=" & Total & " Left=" & Left
End Sub

//C++
extern "C" {

char * __stdcall CPP_Get_Registration_Information()
{ 
  return UserName;
}
...

'VB6
Private Declare Function CPP_Get_Registration_Information Lib "MyTestDll.dll"  () As Long

Private Sub CommandGetRegistrationInformation_Click()
  MsgBox PtrToString(CPP_Get_Registration_Information)
End Sub

Function PtrToString(Ptr As Long) As String
    Dim Data As Byte
    Dim str As String
    Dim i As Long
    str = ""
    If Ptr <> 0 Then
        For i = 0 To 10000 '255
            CopyMemory Data, ByVal Ptr + i, 1
            If Data = 0 Then Exit For 'конец строки \0
            str = str & Chr(Data)
        Next i
    End If
    PtrToString = str
End Function

char*	UserName			= NULL;

void __declspec(dllexport) __stdcall GetRegistrationInformation(char* NewUserName){

	UserName = NewUserName;
}

extern "C"  BSTR  __declspec(dllexport) __stdcall fnString()
{
	char* retVal = "Самодур";
	return ::SysAllocStringByteLen(retVal,strlen(retVal));	
}

int WINAPI GetWindowText(
  _In_   HWND hWnd,
  _Out_  LPTSTR lpString,
  _In_   int nMaxCount
);

Private Declare Function GetWindowText Lib "user32" Alias _
 "GetWindowTextA" (ByVal hwnd As Long, ByVal lpString As String, _
 ByVal nMaxCount As Long) As Long
...
  Dim buf As String * 256
  Dim Title As String
  Dim length As Long

  ' Get the window's title.
  length = GetWindowText(app_hwnd, buf, Len(buf))
  Title = Left$(buf, length)

char*	UserName			= NULL;
....
  UserName = NewUserName; //короче в char* точно есть строка "1-0-0" из 5 символов не считая \0
...

extern "C" {

int __stdcall LPTSTR_Get_Registration_Information(LPTSTR lpString, int nMaxCount)
{ 
  int lenth = (int) strlen(UserName);
  lpString = (LPTSTR)UserName;
  if (lenth < nMaxCount) {
    lpString[lenth] = 0;
    return lenth;
  }
  else {
    lpString[nMaxCount - 1] = 0;
    return nMaxCount - 1;
  }
}

Private Declare Function LPTSTR_Get_Registration_Information Lib "LicenseDll.dll" _
 (ByVal lpString As String, ByVal nMaxCount As Long) As Long

Private Sub CommandGetReg_Click()
  Dim buf As String * 256
  Dim RegInfo As String
  Dim length As Long
  
  length = LPTSTR_Get_Registration_Information(buf, Len(buf))
  MsgBox length
  RegInfo = Left$(buf, length)
  MsgBox RegInfo
End Sub

int __stdcall LPTSTR_Get_Registration_Information(LPTSTR lpString, int nMaxCount)
{ 
  int lenth = (int) strlen(UserName);
  strncpy(lpString, UserName, nMaxCount);

strncpy(lpString, UserName, nMaxCount);

strncpy_s(lpString, nMaxCount, UserName, nMaxCount);

extern "C" {

int __stdcall LPTSTR_Get_Registration_Information(LPTSTR lpString, int nMaxCount)
{ 
  int lenth = (int) strlen(UserName);
	strncpy_s(lpString, nMaxCount, UserName, nMaxCount);

  if (lenth < nMaxCount) {
    lpString[lenth] = 0;
    return lenth;
  }
  else {
    lpString[nMaxCount - 1] = 0;
    return nMaxCount - 1;
  }
}

Private Declare Function LPTSTR_Get_Registration_Information Lib "LicenseDll.dll" _
 (ByVal lpString As String, ByVal nMaxCount As Long) As Long

Private Sub CommandGetReg_Click()
  Dim buf As String * 256
  Dim RegInfo As String
  Dim length As Long
  
  length = LPTSTR_Get_Registration_Information(buf, Len(buf))
  RegInfo = Left$(buf, length)
  MsgBox RegInfo
End Sub

char*	UserName = NULL;

char*	UserName = "";

errno_t rc = strncpy_s(lpString, nMaxCount, UserName, length);

char*	UserName = NULL;

char*	UserName = NULL;
...
UserName = NewUserName; //НЕ ГАРАНТИРОВАНО
...

extern "C" {

int __stdcall LPTSTR_Get_Registration_Information(LPTSTR lpString, int nMaxCount)
{ 
  if (UserName != NULL) {
    int lenth = (int) strlen(UserName);
    strncpy_s(lpString, nMaxCount, UserName, nMaxCount-1);
    if (lenth < nMaxCount) {
      lpString[lenth] = 0;
    } else {
      lpString[nMaxCount - 1] = 0;
    }
    return lenth;
  }
  else 
    return 0;
}

    if (lenth < nMaxCount) {
     strncpy_s(lpString, nMaxCount, UserName, lenth);
     lpString[lenth] = 0;
   } else {
     strncpy_s(lpString, nMaxCount, UserName, nMaxCount-1);
     lpString[nMaxCount - 1] = 0;
   }

Private Sub CommandGetReg_Click()
  
  Dim buf As String
  Dim length As Long
  Dim RegInfo As String
    
  buf = Space(256)
  length = LPTSTR_Get_Registration_Information(buf, Len(buf))
  MsgBox length
  If length > 255 Then
    buf = Space(length + 1)
    length = LPTSTR_Get_Registration_Information(buf, Len(buf))
  End If
  RegInfo = Left$(buf, length)
  Text1.Text = RegInfo
End Sub

debug.print Left$("Пример",100)
Пример

DWORD WINAPI GetPrivateProfileString(
  _In_   LPCTSTR lpAppName,
  _In_   LPCTSTR lpKeyName,
  _In_   LPCTSTR lpDefault,
  _Out_  LPTSTR lpReturnedString,
  _In_   DWORD nSize,
  _In_   LPCTSTR lpFileName
);

Private Declare Function GetPrivateProfileString Lib "kernel32" Alias _
 "GetPrivateProfileStringA" (ByVal lpApplicationName As String, ByVal lpKeyName As Any, _
 ByVal lpDefault As String, ByVal lpReturnedString As String, ByVal nSize As Long, _
 ByVal lpFileName As String) As Long

Private Function ReadIni(Section As String, Key As String, Default As String, FileName As String) As String

  Dim RetVal As String, v As Long
  Dim retLen As Long
    
  Do
    retLen = retLen + 260 ' arbitrary - can be set higher/lower if desired
    RetVal = Space(retLen)
    v = GetPrivateProfileString(Section, Key, Default, RetVal, retLen, FileName)
  Loop Until v < retLen - 1
    
  ReadIni = Left$(RetVal, v)
  
End Function

strncpy_s(lpString, 0, UserName, -1);

errno_t rc = strncpy_s(lpString, nMaxCount, UserName, length);

char*	UserName = NULL;

Private Declare Function LPTSTR_Get_Registration_Information Lib "LicenseDll.dll" _
 (ByVal lpString As String, ByVal nMaxCount As Long) As Long

Private Sub CommandGetReg_Click()
  Dim buf As String * 256
  Dim RegInfo As String
  Dim length As Long
  
  length = LPTSTR_Get_Registration_Information(buf, Len(buf))
  RegInfo = Left$(buf, length)
  MsgBox RegInfo
End Sub