How to make your own VST host


Writing VST plugins is a lot of fun, but it’s even more fun to write your own host which uses the wide variety of plugins already out there to do something original and new. Making your own VST host is not a trivial task, but the trickiest part is figuring out how to load the plugins and connect them to your code’s callback functions. As the VST documentation is a bit sparse on the subject of hosting, this guide will assist you in setting up your own host.

This guide only covers loading the plugin and basic communication, and the language of choice here is C++. C# programmers should consider using the VST.NET framework, and I’m not sure what frameworks exist for other languages.

Also, it’s worth noting that Teragon Audio has developed an open-source VST host, MrsWatson. Feel free to look at the code and fork it for your own project! If you find yourself using a substantial portion of the MrsWatson source in your own code, please let me know so I can add a link to your project from the MrsWatson page.

Code conventions

In the course of your development, you will probably require logging, error handling, etc. To simplify the code in this tutorial, I have simply written “return -1” or “return NULL” statements, but you should consider expanding this to log some info or handle the error.

Also, this tutorial is written for both Windows and Mac OSX developers. As such, there is a lot of platform-specific code, which you will probably need to box with #ifdef/#endif statements in the preprocessor.

Setting up your build environment

You’ll need to first download and install the following tools:

  1. Steinberg’s VST SDK, which requires you to make a free Steinberg Developer account. This tutorial assumes you are working with the VST 2.4 SDK.
  2. Microsoft’s Visual C++ 2010 Express, if you wish to support Windows.
  3. Microsoft’s Platform SDK, again if you are developing on Windows.
  4. Xcode 4.x, if you are developing on Mac OS X.

Project configuration

Aside from your project files, you need only to add the VST SDK headers into your project’s include path. This includes the following files, which are located under the vstsdk2.4/pluginterfaces/vst2.x directory:

  • aeffect.h
  • aeffectx.h
  • vsfxstore.h

On both Windows and Mac OSX, you should probably configure your program to build as a 32-bit binary, simply because most VST plugins are not 64-bit compatible yet. On the Mac, this gets to be a bit hairy because Apple is working to deprecate Carbon, which is a 32-bit framework. If anyone out there has example code in C (not objective-C) to load a plugin from bundle without using Carbon, please let me know so I can update this article.

Loading the VST plugin

After your host performs its own internal initialization routines, it is time to load the VST plugin from source. This procedure varies a bit depending on the platform, but the algorithm is fundamentally the same: find the plugin, load the dynamic library into memory, acquire the plugin’s main address, and create a VST callback connection. These callbacks are defined function pointers which you should define in one of your project’s header files, and are as follows:

#include "aeffectx.h"

// C callbacks
extern "C" {
// Main host callback
  VstIntPtr VSTCALLBACK hostCallback(AEffect *effect, VstInt32 opcode,
    VstInt32 index, VstInt32 value, void *ptr, float opt);

// Plugin's entry point
typedef AEffect *(*vstPluginFuncPtr)(audioMasterCallback host);
// Plugin's dispatcher function
typedef VstIntPtr (*dispatcherFuncPtr)(AEffect *effect, VstInt32 opCode,
  VstInt32 index, VstInt32 value, void *ptr, float opt);
// Plugin's getParameter() method
typedef float (*getParameterFuncPtr)(AEffect *effect, VstInt32 index);
// Plugin's setParameter() method
typedef void (*setParameterFuncPtr)(AEffect *effect, VstInt32 index, float value);
// Plugin's processEvents() method
typedef VstInt32 (*processEventsFuncPtr)(VstEvents *events);
// Plugin's process() method
typedef void (*processFuncPtr)(AEffect *effect, float **inputs,
  float **outputs, VstInt32 sampleFrames);

On Windows, VST plugins are simply dynamically linked libraries (DLL’s). The code for opening a DLL library in Windows is fairly simple:

AEffect* loadPlugin() {
  AEffect *plugin = NULL;
  char *vstPath = "c:\\wherever\\the\\plugin\\is\\located.vst";

  modulePtr = LoadLibrary(vstPath);
  if(modulePtr == NULL) {
    printf("Failed trying to load VST from '%s', error %d\n",
      vstPath, GetLastError());
    return NULL;

  vstPluginFuncPtr mainEntryPoint =
    (vstPluginFuncPtr)GetProcAddress(modulePtr, "VSTPluginMain");
  // Instantiate the plugin
  plugin = mainEntryPoint(hostCallback);
  return plugin;

On Mac OSX, VST plugins are also dynamic libraries, but they are packaged as bundles. Your host can open these bundles through the Carbon API. On Mac OS9, VST plugins were packaged as CFM files, which has long since been deprecated, and it is highly unlikely that any modern VST host should need to support this format.

The procedure for opening a plugin under OSX is a bit more complex, but the code should be fairly straightforward. Keep in mind that although a VST plugin can be loaded from any location on disk, they are usually stored in either /Library/Audio/Plug-Ins/VST or $HOME/Library/Audio/Plug-Ins/VST.

Anyways, to load the VST plugin on Mac OSX, that will look something like this:

AEffect* loadPlugin() {
  AEffect *plugin = NULL;
  audioMasterCallback hostCallbackFuncPtr = hostCallback;
  char *pluginPath = "/wherever/the/plugin/is/located.vst";

  // Create a path to the bundle
  CFStringRef pluginPathStringRef = CFStringCreateWithCString(NULL,
    pluginPath, kCFStringEncodingASCII);
  CFURLRef bundleUrl = CFURLCreateWithFileSystemPath(kCFAllocatorDefault,
    pluginPathStringRef, kCFURLPOSIXPathStyle, true);
  if(bundleUrl == NULL) {
    printf("Couldn't make URL reference for plugin\n");
    return NULL;

  // Open the bundle
  CFBundleRef bundle;
  bundle = CFBundleCreate(kCFAllocatorDefault, bundleUrl);
  if(bundle == NULL) {
    printf("Couldn't create bundle reference\n");
    return NULL;

  vstPluginFuncPtr mainEntryPoint = NULL;
  mainEntryPoint = (vstPluginFuncPtr)CFBundleGetFunctionPointerForName(bundle,
  // VST plugins previous to the 2.4 SDK used main_macho for the entry point name
  if(mainEntryPoint == NULL) {
    mainEntryPoint = (vstPluginFuncPtr)CFBundleGetFunctionPointerForName(bundle,

  if(mainEntryPoint == NULL) {
    printf("Couldn't get a pointer to plugin's main()\n");
    return NULL;

  plugin = mainEntryPoint(hostCallback);
  if(plugin == NULL) {
    printf("Plugin's main() returns null\n");
    return NULL;

  // Clean up

  return plugin;

You need to keep the bundle pointer around until the host is ready to unload the plugin. At this point, you call CFBundleUnloadExecutable and then CFRelease on the bundle’s reference.

Setting up plugin callbacks

At this point, you should now have successfully loaded the plugin into memory, and you can now establish the plugin dispatcher callbacks:

int configurePluginCallbacks(AEffect *plugin) {
  // Check plugin's magic number
  // If incorrect, then the file either was not loaded properly, is not a
  // real VST plugin, or is otherwise corrupt.
  if(plugin->magic != kEffectMagic) {
    printf("Plugin's magic number is bad\n");
    return -1;

  // Create dispatcher handle
  dispatcherFuncPtr dispatcher = (dispatcherFuncPtr)(plugin->dispatcher);

  // Set up plugin callback functions
  plugin->getParameter = (getParameterFuncPtr)plugin->getParameter;
  plugin->processReplacing = (processFuncPtr)plugin->processReplacing;
  plugin->setParameter = (setParameterFuncPtr)plugin->setParameter;

  return plugin;

Plugin initialization

At this point, the plugin should be ready to go, so you can initialize it through the dispatcher handle created in the previous step:

void startPlugin(AEffect *plugin) {
  dispatcher(plugin, effOpen, 0, 0, NULL, 0.0f);

  // Set some default properties
  float sampleRate = 44100.0f;
  dispatcher(plugin, effSetSampleRate, 0, 0, NULL, sampleRate);
  int blocksize = 512;
  dispatcher(plugin, effSetBlockSize, 0, blocksize, NULL, 0.0f);


Suspending and resuming

Calling the plugin’s suspend and resume methods are a bit counter-intuitive, and are done like this:

void resumePlugin(AEffect *plugin) {
  dispatcher(plugin, effMainsChanged, 0, 1, NULL, 0.0f);

void suspendPlugin(AEffect *plugin) {
  dispatcher(plugin, effMainsChanged, 0, 0, NULL, 0.0f);

Plugin capabilities

The VST protocol uses “canDo” strings to define plugin capabilities, the most common of which are defined in audioeffectx.cpp in the PlugCanDos namespace near the top of the file. To ask a plugin if it supports one of these capabilities, make the following dispatcher call:

bool canPluginDo(char *canDoString) {
  return (dispatcher(plugin, effCanDo, 0, 0, (void*)canDoString, 0.0f) > 0);

Host capabilities

The plugin can also ask the host if it supports a given capability, which is done through the hostCallback() function defined above. The implementation of this file looks something like this:

extern "C" {
VstIntPtr VSTCALLBACK hostCallback(AEffect *effect, VstInt32 opcode, VstInt32 index,
  VstInt32 value, void *ptr, float opt) {
  switch(opcode) {
    case audioMasterVersion:
      return 2400;
    case audioMasterIdle:
      effect->dispatcher(effect, effEditIdle, 0, 0, 0, 0);
    // Handle other opcodes here... there will be lots of them
      printf("Plugin requested value of opcode %d\n", opcode);

The full list of opcodes is defined in aeffect.h (for the VST 1.x protocol) and aeffectx.h (for VST 2.x protocol). There are a lot of opcodes, and your application doesn’t need to support them all, but you will soon figure out which ones are the most important through trial and error. Depending on the nature of the opcall, you will either be required to return a given integer value, call a method in the plugin’s dispatcher, or fill the *ptr pointer with some type of data. The VST SDK header files have fairly good documentation specifying what you need to do depending on the opcode.

The MrsWatson source code also contains an example implementation of this function with the most common opcode cases.

Processing audio

In the VST SDK 2.4, processReplacing() became the new standard call. You may have to add in support to your host for the old style of process() plugins, though there aren’t so many plugins out there which still do this. To have the plugin process some audio:

void initializeIO() {
  // inputs and outputs are assumed to be float** and are declared elsewhere,
  // most likely the are fields owned by this class. numChannels and blocksize
  // are also fields, both should be size_t (or unsigned int, if you prefer).
  inputs = (float**)malloc(sizeof(float**) * numChannels);
  outputs = (float**)malloc(sizeof(float**) * numChannels);
  for(int channel = 0; channel < numChannels; channel++) {
    inputs[i] = (float*)malloc(sizeof(float*) * blocksize);
    outputs[i] = (float*)malloc(sizeof(float*) * blocksize);

void processAudio(AEffect *plugin, float **inputs, float **outputs,
  long numFrames) {
  // Always reset the output array before processing.
  silenceChannel(outputs, numChannels, numFrames);

  // Note: If you are processing an instrument, you should probably zero
  // out the input channels first to avoid any accidental noise. If you
  // are processing an effect, you should probably zero the values in the
  // output channels. See the silenceChannel() function below.
  // However, if you are reading input data from file (or elsewhere), this
  // step is not necessary.
  silenceChannel(inputs, numChannels, numFrames);

  plugin->processReplacing(plugin, inputs, outputs, numFrames);

void silenceChannel(float **channelData, int numChannels, long numFrames) {
  for(int channel = 0; channels < numChannels; ++channel) {
    for(long frame = 0; frame < numFrames; ++frame) {
      channelData[channel][frame] = 0.0f;

In the above code, there is an inputs and outputs array which should be initialized by your application as soon you have calculated the desired channel count and buffer size. You should not allocate the inputs and outputs arrays in the processAudio() function, as doing so may severely impact performance. Hence, the call to initializeIO() should be made as soon as possible and before the first call to processAudio(). You should also take care to properly initialize the data in both the inputs and outputs array to zero, or else you can get static or other random noise in the processed signal.

Sending MIDI messages

Processing MIDI events is very similar to processing audio:

void processMidi(AEffect *plugin, VstEvents *events) {
  dispatcher(plugin, effProcessEvents, 0, 0, events, 0.0f);

The above events array should be allocated and properly initialized by the host to contain the MIDI events which the plugin will receive. The VstEvent structure is defined in aeffectx.h, and there you will also find the respective VstEvent types, all of which are deprecated except for kVstMidiType and kVstSysExType.

Note that the plugin must support the receiveVstMidiEvent canDo in order to process MIDI.

Final Notes

At this point, you should have a basic working host capable of loading and communicating with a VST plugin. As you continue your development, take care to thoroughly read the VST SDK header files and other associated documentation, as they will provide you with further hints as to the correct implementation. Also, you should take time to create good logging facilities in your host, particularly in the hostCallback() method, as most plugin incompatibilities are usually triggered from some error there.