Function XPLMEnablePlugin 

pub unsafe extern "C" fn XPLMEnablePlugin(
    inPluginID: XPLMPluginID,
) -> c_int

XPLMEnablePlugin

This routine enables a plug-in if it is not already enabled. It returns 1 if the plugin was enabled or successfully enables itself, 0 if it does not. Plugins may fail to enable (for example, if resources cannot be acquired) by returning 0 from their XPluginEnable callback.