Identifying and Loading Plug-ins

Plug-ins must implement two exported functions for a host to identify the plug-ins and to initiate the boot strapping of communication between the two. These functions are...

  • int OfxGetNumberOfPlugins(void) - identifies how many plug-ins are implemented in the file,
  • OfxPlugin *OfxGetPlugin(int N) - returns a structure identifying the Nth plug-in in the file.


#include "ofxCore.h"
OfxExport int OfxGetNumberOfPlugins(void); 

This function returns the number of plug-ins contained in the binary. It is the very first function called by the host after the binary has been loaded.

This function must be exported by the binary that contains OFX plug-ins. The OfxExport defined in ofxCore.h is used as an export declaration for those compilers that require it.


#include "ofxCore.h"
OfxExport OfxPlugin *OfxGetPlugin(int nth); 

This function returns a pointer to the nth OfxPlugin struct in the binary. nth is between 0 and the number returned by OfxGetNumberOfPlugins , minus one. The returned structure defines the plug-in to the host.


  • OfxGetNumberOfPlugins has been previously called,
  • nth is between 0 and the value returned by OfxGetNumberOfPlugins , minus one.


  • The returned pointer and pointers in the struct point to valid data whilst the binary is loaded,
  • The returned pointer and pointers in the struct do not need to be freed in any way by the host.

The OfxPlugin Struct

This structure is returned by a plugin to identify itself to the host.

typedef struct OfxPlugin {
   const char        *pluginApi;
   int                apiVersion;
   const char        *pluginIdentifier; 
   unsigned int       pluginVersionMajor;
   unsigned int       pluginVersionMinor;
   void               (*setHost)(OfxHost *host);
   OfxPluginEntryPoint *mainEntry;
} OfxPlugin;

This C string tells the host what API the plug-in implements.


This integer tells the host which version of its API the plug-in implements.


This is the globally unique name for the plug-in.


Major version of this plug-in, this gets incremented whenever software is changed and breaks backwards compatibility.


Minor version of this plug-in, this gets incremented when software is changed, but does not break backwards compatibility.


Function used to set the host pointer (see below) which allows the plug-in to fetch suites associated with the API it implements.


The plug-in function that takes messages from the host telling it to do things.

Interpreting the OfxPlugin Struct

When a host gets a pointer back from OfxGetPlugin, it examines the string pluginApi . This identifies what kind of plug-in it is. Currently there is only one publicly specified API that uses the OFX mechanism, this is "OfxImageEffectPluginAPI" , which is the image effect API being discussed by this book. More APIs may be created at a future date, for example "OfxImageImportPluginAPI". Knowing the type of plug-in, the host then knows what suites and host handles are required for that plug-in and what functions the plug-in itself will have. The host passes a OfxHost structure appropriate to that plug-in via its setHost function. This allows for the same basic architecture to support different plug-in types trivially.

OFX explicitly versions plug-in APIs. By examining the apiVersion , the host knows exactly what set of functions the plug-in is going to supply and what version of what suites it will need to provide. This also allows plug-ins to implement several versions of themselves in the same binary, so it can take advantages of new features in a V2 API, but present a V1 plug-in to older hosts that only support V1.

If a host does not support the given plug-in type, or it does not support the given version it should simply ignore that plug-in.

A plug-in needs to uniquely identify itself to a host. This is the job of pluginIdentifier . This null terminated ASCII C string should be unique among all plug-ins, it is not necessarily meant to convey a sensible name to an end user. The recommended format is the reverse domain name format of the developer, for example "", followed by the developer's unique name for the plug-in. e.g. "".

A plug-in (as opposed to the API it implements) is versioned with two separate integers in the OfxPlugin struct. They serve two separate functions and are,

  • pluginVersionMajor flags the functionality contained within a plug-in. Incrementing this number means that you have broken backwards compatibility of the plug-in. More specifically, this means a setup from an earlier version, when loaded into this version, will not yield the same result.
  • pluginVersionMinor flags the release of a plug-in that does not break backwards compatibility, but otherwise enhances that plug-in. For example, increment this when you have fixed a bug or made it faster.

If a host encounters multiple versions of the same plug-in it should,

  • when creating a brand new instance, always use the version of a plug-in with the greatest major and minor version numbers,
  • when loading a setup, always use the plug-in with the major version that matches the setup, but has the greatest minor number.

As a more concrete example of versioning: the plug-in identified by "org.wibble:Fred" is initially released as 1.0, However a few months later, figure out how to make it faster and release it as 1.1. A year later, Fred can now do automatically what a user once needed to set up five parameters to do, thus making it much simpler to use. However this breaks backwards compatibility as the effect can no longer produce the same output as before, so then release this as v2.0.

A user's host might now have three versions of the Fred plug-in on it, v1.0, v1.1 and v2.0. When creating a new instance of the plug-in, the host should always use v2.0. When loading an old project which has a setup from a v1.x plug-in, it should always use the latest, in this case being v1.1.

Note that plug-ins can change the set of parameters between minor version releases. If a plug-in does so, it should do so in a backwards compatible manner, such that the default value of any new parameter would yield the same results as previously. See the chapter below about parameters.