Interfacing with DLLs

Using EiffelStudio, there are two different ways to call C routines exported in DLLs. This is because on the Windows platform, there are two frequently-used calling conventions for C routines found in DLLs:

• _cdecl: referred to as the standard calling convention
• __stdcall referred to as the Pascal calling convention

Calling conventions define 1) how arguments are placed on the stack, and 2) which entity (the caller, or the callee) is responsible for returning the call stack to its previous state when the routine has completed. These two methods are NOT compatible. Therefore if you have a routine in a DLL you wish to call, one of the first things to find out about it is which calling convention it expects. DLL routines that use the _cdecl calling convention require the use of the dll32 sub-language option. For __stdcall, use the dllwin32 sub-language option. Here is an example:

my_cdecl_routine (a: INTEGER): POINTER -- Encapsulation of a dll function with the `_cdecl' call mechanism. external "C [dll32 %"my_cdecl_dll.dll%"] (int): EIF_POINTER" end my_stdcall_routine (a: INTEGER): POINTER -- Encapsulation of a dll function with the `_stdcall' call mechanism. external "C [dllwin32 %"my_stdcall_dll.dll%"] (int): EIF_POINTER" end

Note: Most OS services provided by the Win32 API use the __stdcall calling convention.

Warning: Getting dll32 and dllwin32 reversed will cause your application's call stack to be corrupted. This will cause your application to malfunction, can cause it to crash, and can potentially crash your whole system. So getting this right is crucial to smooth implementation of external DLL calls. For more information please consult your C compiler documentation.