How can I return a PChar from a DLL function to a VB6 application without risking crashes or memory leaks?

Question

I have to create a DLL which is used by a VB6 application. This DLL has to provide several functions, some of them must return strings.

This is the VB6 declaration:<

Answer 1

I'm not familiar with Dephi, but here are the two main options when using strings with a non-COM DLL and VB6.

Option 1. Use "ANSI" strings.

'DLL routine expecting to be passed pointers to ANSI strings '
'VB6 will allocate and deallocate the strings '
'Its vital that VB6 allocates sufficient space for the return string '
Declare Sub MyProc Lib "mylib.dll" (ByVal Param As String, _ 
  ByVal OutVal As String) 

Function DoMyProc(ByVal Param As String) As String
  Dim sResult As String
  sResult = Space$(255)  ' create 255 bytes of space for the return string '
  Call MyProc(Param, sResult) 
  DoMyProc = sResult
End Function

Option two. Use BSTRs.

'DLL routine expecting to be passed two BSTRs. It will modify the second one. '
'VB6 "owns" both BSTRs and will deallocate them when it has finished with them. '
Declare Sub MyProc(ByVal lpParam As Long, ByVal lpOutVal As Long)

Function DoMyProc(ByVal Param As String) As String
  Dim sResult As String
  Call MyProc(StrPtr(Param), StrPtr(sResult)) 
  DoMyProc = sResult
End Function

I'd also suggest looking at the Microsoft advice on writing C DLLs to be called from VB. Originally released with VB5 but still relevant to VB6.