I have been working on C-based Native Client module for Google Chrome. Many of the module functions that are called by the NaCl system have a parameter of PP_Instance which uniquely identifies the module instance.
My question: Is there any way to associate user data with this instance handle?
The C API specifies that it is an opaque handle. It provides no functions for linking user data to the handle. Right now, I have to use a bunch of global variables within the module to share state among the functions. It doesn’t feel like the right solution. I’m not sure if more than one instance will ever share the process space but I’m not making any assumptions here.
I suppose I could implement some sort of look up table to map instances to unique contexts that happen to live in the global scope. But that also seems like it should be unnecessary for a C-based API. The C++ API avoids this by virtue of its classes.
PP_Instance should be used as a key to lookup state / object associated with the plugin instance. More than one plugin instance may be instantiated in a module as per the API, when, for example, multiple embed tags are present in the containing frame. Currently the NaCl implementation of Pepper does not do this — instead, multiple processes each containing a single module each instantiating a single pepper plugin instance is created. However, this is an implementation detail (or maybe bug?) that is subject to change, and it would be better to defensively program and be able to handle multiple DidCreate events.
Of course, if your NaCl module is guaranteed to never be used by anyone else and you know you won’t ever have two embeds of the same module, then it might be okay to assume singleton instance and use global state, but doing things the “right” way isn’t that hard, so why not?
See native-client-discuss thread for more discussion on this topic.