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VST Module Architecture

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Introduction

VST-MA is a component model system which is used in all Steinberg host applications as the basic layer for plug-in support.

It is object-oriented, cross-platform and (almost) compiler-independent.
The basics are very much like Microsoft® COM, so if you are familiar with this technology, understanding VST-MA should be quite easy.

VST-MA is provided in C++ only. Interfaces in C++ are expressed as pure virtual class (which is a class with nothing but abstract methods). Unlike COM there is no support for C or other languages yet - simply because there has been no need for this so far. But all VST-MA interfaces can be transformed into different representations in case this should be inevitable some day.
It is currently available for Windows, Mac OS X and Linux.

The C++ files belonging to VST-MA are located in the following folders:

  • pluginterfaces/base
  • pluginterfaces/gui

Note: The name 'VST Module Architecture' has only little relation to the 'Virtual Studio Technology' itself.
It describes the basic layer for any plug-in category supported in Steinberg hosts. VST-MA existed long before it was used as a base for VST 3 itself.

Interfaces

FUnknown

Steinberg::FUnknown is the basic interface of VST-MA. All other interfaces are directly or indirectly derived from it.

IID/CID

Each interface has a unique identifier (IID) of type Steinberg::FUID. It is used to retrieve a new interface from another one (Steinberg::FUnknown::queryInterface). It is important to understand the difference between interface identifier and component identifier.
A component-ID or class-ID (CID) is used to identify a concrete implementation class and is usually passed to a class factory in order to create the corresponding component.
So a lot of different classes (with different class identifiers) can implement the same interfaces.

Direction

An interface may have a direction, meaning that the interface is expected to be implemented either in the plug-in or in the host. The nature of an interface is documented like this:

  • [host imp]: the host implements the interface
  • [plug imp]: the plug-in implements the interface

When neither of these is specified, the interface can be used in both ways.

Versioning and inheritance

Unlike C++ classes, interfaces do not use inheritance to express specializations of objects. Inheritance is used for versioning only. One of the strict rules is that once an interface has been released, it must never change again. Adding new functionality to an interface requires a new version (usually an ordinal number is added to its name in this case, for example IPluginFactory3 adds new features to IPluginFactory2).
A new version inherits the old version(s) of the interface, so the old and the new methods are combined in one interface. This is why specializations need to be modeled as separate interfaces! If a specialized interface were to inherit from the basic interface as well, an implementation class that needs to implement all of these interfaces would inherit the base interface twice, causing the compiler to run into ambiguities. So the specialization relation to a basic interface can only be expressed in the documentation.

  • ISpecialInterface [extends IBaseInterface] => means IBaseInterface::queryInterface (ISpecialInterface::iid, ...) can be used to retrieve the derived interface.

You can find some example code here: Interface Versions and Inheritance.

COM Compatibility

The first layer of VST-MA is binary-compatible to COM. The Vtable and interface identifier of Steinberg::FUnknown match with the corresponding COM interface IUnknown. The main difference is the organization and creation of components/plug-ins by a host application. VST-MA does not require any Microsoft® COM source file. You can find information about COM on pages like:

Basic Interfaces

Helper Classes

See also "How to derive a class from an interface".

Plug-ins

Module Factory

A module (Windows: Dynamic Link Library, MAC: Mach-O Bundle, Linux: package) contains the implementation of one or more components (e.g. VST 3 effects). A VST-MA module must contain a class factory where meta-data and create-methods for the components are registered.
The host has access to this factory through the Steinberg::IPluginFactory interface. This is the anchor point for the module and it is realized as a C-style export function named GetPluginFactory. You can find an export definition file in the SDK - public.sdk/source/main/winexport.def (public.sdk/source/main/macexport.exp) which can be used to export this function or you could use the macro SMTG_EXPORT_SYMBOL directly in cpp file (check public.sdk/source/main/dllmain.cpp for example).
GetPluginFactory is declared as follows:

SMTG_EXPORT_SYMBOL IPluginFactory* PLUGIN_API GetPluginFactory ();

In addition to the GetPluginFactory function the plug-in may has to export additional entry/exit functions depending on the platform:

On Windows
On Windows the entry/exit functions are named InitDll / ExitDll and are optional!

A Plug-In can export these functions and a host has to call the InitDll function directly after loading the plug-in via LoadLibrary and before calling GetPluginFactory. The ExitDll function must be called before the plug-in is unloaded via FreeLibrary or on program termination without FreeLibrary.

As Windows already has this feature (see DllMain in Microsofts documentation) the above functions are optional.

On macOS
On macOS the entry/exit functions are named BundleEntry / BundleExit and are required!

A plug-in must export these functions and a host has to call the BundleEntry function directly after loading the plug-in via CFBundleLoadExecutable and before calling GetPluginFactory.

The BundleExit function must be called before the plug-in is unloaded or on program termination.

As macOS does not have a standard entry function when loading a bundle the above functions are required and a host has to reject plug-ins not exporting these functions.

On Linux
On Linux the entry/exit functions are named ModuleEntry / ModuleExit and are required!

A plug-in must export these functions and a host has to call the ModuleEntry function directly after loading the plug-In via dlopen and before calling GetPluginFactory.

The ModuleExit function must be called before the plug-in is unloaded via dlclose or on program termination.

As Linux does not have a standard entry function when loading a dynamic library, the above functions are required and a host has to reject plug-ins not exporting these functions.

The entry function is intended for providing the plug-in with the platform specific instance handle which are needed for many platform APIs.

Plug-in developers should use these functions instead of using platform functions to get the instance handle.

Here an example when using def/exp files instead of SMTG_EXPORT_SYMBOL:

winexport.def file on Windows

EXPORTS
GetPluginFactory
InitDll
ExitDll

macexport.exp file on mac

_GetPluginFactory
_bundleEntry
_bundleExit

Locations

Component modules do not require registration like DirectX. The host application expects component modules to be located in predefined folders of the file system. These folders and their subfolders are scanned for VST-MA modules during application startup. Each folder serves a special purpose:

  • The application's Components subfolder (e.g. "C:\Program Files\Steinberg\Cubase 10\Components") is used for components tightly bound to the application. No other application should use it.
  • Components that are shared between all Steinberg hosts are located at:
    • Win: "/Program Files/Common Files/Steinberg/Shared Components"
    • Mac: "/Library/Application Support/Steinberg/Components/"
  • For special purpose plug-in types, additional locations are defined. Please refer to the corresponding documentation to find out if additional folders are used and where to find them. For VST 3, see VST 3 Locations/Format.

Categories

Any class that the factory can create is assigned to a category. It is this category that tells the host the purpose of the class (and gives a hint of which interfaces it might implement).
A class is also described with a name and it has a unique id.

  • For example, the category for import/export filters is "Project Filter" and for VST 3 audio plug-ins it is "Audio Module Class".
  • "Service" is a special category. The purpose of a class of this category is completely unknown to the host. It is loaded automatically during program start (provided that the user did not deactivate it).
  • Since the factory can create any number of classes, one component library can contain multiple components of any type.

IPluginBase

The entry-point interface for any component class is Steinberg::IPluginBase. The host uses this interface to initialize and to terminate the Plug-in component. When the host initializes the Plug-in, it has to pass a so called context. This context contains any interface to the host that the Plug-in will need to work.

Purpose-specific interfaces

Each plug-in category (VST 3 Effects, Project import/export Filters, Audio Codecs, etc...) defines its own set of purpose-specific interfaces. These are not part of the basic VST-MA layer.

Unicode

Beginning with version 5 of Cubase and Nuendo, the internal structure of the host was modified for better support of internationalization. Therefore, string handling was changed to utilize Unicode strings whenever strings are passed around. As a consequence, all the interfaces to plug-ins have changed from using ASCI to Unicode strings for call and return parameters. So in turn, all plug-ins must be adapted to support Unicode. This has major consequences in that:

  • Unicode hosts (Cubase 5 or later) will only work with Unicode plug-ins. When loading a plug-in, a Unicode host checks the plug-in's type and will not load any non-Unicode plug-ins.
  • Unicode plug-ins will not load in non-Unicode hosts. When loading, a Unicode plug-in requests information from the host and will not load if no Unicode host is detected. Therefore, if a plug-in is supposed to work with both older and newer hosts, it is best to provide two versions of the plug-in.

Plug-ins for Unicode hosts

Writing plug-ins that are supposed to work only with Unicode hosts is easy. Use a current version of this SDK and develop a plug-in as usual. Make sure that you only ever pass Unicode UTF-16 strings to interfaces that have strings as call parameters and also be prepared that strings returned by these interfaces are always UTF-16. Therefore, to make things easier, it is recommended that Unicode strings are used throughout the plug-in's implementation, in order to avoid back and forth conversions. Also, use the Steinberg::String and Steinberg::ConstString classes from the Base module, as they have been designed to work universally on both Mac and Win.

Migrating from non-Unicode to Unicode

In Steinberg SDKs released before Cubase 5, the interface functions were using pointers of type char for passing strings to and from the host. These have been changed now to using Steinberg's defined type tchar which is equivalent to char16, i.e. 16 bit character. In theory, there are many ways for representing 16 bit characters, but we chose to use the industry standard Unicode, so strings are expected to be encoded in UTF-16.
Accordingly, also the implementation of a plug-in needs to be adapted to deal correctly with Unicode-encoded strings, as well as only ever passing Unicode strings to the host.

Note
Changing a function from using 8 bit to 16 bit character pointers may seem as only a minor modification, but in interface design this is a major intrusion, because an interface is a contract to the outside world that is never to be changed. Therefore, classes that are changed to use Unicode strings are distinguished and also receive a new unique class ID.

SDK backward compatibility

Even with the current SDK it is still possible to develop non-Unicode plug-ins. In the file pluginterfaces/base/ftypes.h, the line "#define UNICODE_OFF" is commented out, but by uncommenting it you can revert all interfaces to using single byte ASCII strings. Alternatively, you can also specify UNICODE_OFF as a preprocessor definition in your project file.
Also, the plug-in's factory info now does not define the Unicode flag anymore, so a Unicode host sees the compiled plug-in as non-Unicode. Also, when reverting to single byte strings the plug-in's implementation also has to be changed to behave correctly.

Note
When undefining Unicode, the class IDs also revert to the old ones.