XPLMSendMessageToPlugin
From X-Plane SDK
XPLMSendMessageToPlugin
XPLM_API void XPLMSendMessageToPlugin(
XPLMPluginID inPlugin,
int inMessage,
void * inParam);
This function sends a message to another plug-in or X-Plane. Pass XPLM_NO_PLUGIN_ID to broadcast to all plug-ins. Only enabled plug-ins with a message receive function receive the message. In order to receive messages, you need to create a function in your plugin called XPluginReceiveMessage. Look in http://www.xsquawkbox.net/xpsdk/docs/Overview.html.
Also look at AdvancedSDKExamples/SampleCode/PluginMessaging.cpp for a C example.
X-Plane SDK