Monday, August 31, 2009

Runnable object differ from a func ptr in a C thread creation API?

(C# Delegates???)
Q: how is a Runnable object different from a function pointer in a C thread creation API?

Let's start from the simple case of a fieldless Command object. This command object could modify a global object but the command object itself has no field. Such a Runnable object is a replicable wrapper over a func ptr. In C, you pass the same func ptr into one (or more) thread ctor; in java you clone the same Runnable object and feed it to a new Thread. In both cases, the actual address of the run() method is a single fixed address.

Now let's add a bit of complexity. I feel a Runnable can access a lot of objects in "creator-scope" -- Since a Runnable object is created at run time often inside some methods, that creator method has a small number of objects in its scope.

Assuming a 32bit platform,

Q: are these variables (more precisely objects AND their 32bit references) living the stack of the run() method or as hidden fields of the Runnable object?
%%A: no difference. Since java compiler guarantees that these variables are "stable", compiler can therefore create implicit fields in the Runnable object

Does a func ptr in C also have access to objects in creator-scope? I think so. Here's an example taken from my blog on void ptr --

  thread (void (FVVP*)(void *),   void* vptr) // creates and starts a new thread. FVVP means a functor returning Void, accepting a Void Pointer

Above is a thread api with 2 arguments -- a functor and a void ptr. The functor points to a function accepting a void ptr, so the 2nd argument (creator scope) feeds into the functor.

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