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bug#22737: 25.1; Finalizer should be optional in dynamic modules


From: Jess Balint
Subject: bug#22737: 25.1; Finalizer should be optional in dynamic modules
Date: Fri, 26 Feb 2016 12:53:20 -0600

On Fri, Feb 26, 2016 at 12:36 PM, Eli Zaretskii <address@hidden> wrote:
> Date: Fri, 26 Feb 2016 10:17:26 -0600
> From: Jess Balint <address@hidden>
> Cc: address@hidden
>
> Situation #1 - globals:
>
> I have pointers to data that are global (not on the heap). I return these pointers from module functions so that
> they may be used as parameters to other module function calls. These pointers should *never* be freed. In
> this case I need to supply a no-op finalizer when creating the user pointer.
>
> Situation #2 - manual memory management:
>
> I have heap-allocated structures whose memory should not be managed by Emacs. I may return pointers to
> this data one or many times from module calls. The data should be freed only when explicitly requested. I may
> return many user pointers to the same heap-allocated structure. Even when all these are freed by Emacs, I
> still retain a pointer in my module which may be returned in a future module call. Again, I'm required to supply
> a no-op finalizer when creating these user pointers.

What will happen if such objects are exposed to Lisp, copied or
assigned to other Lisp variables, etc.?  Won't this cause all kinds of
trouble, like modifying one such object will magically modify several
others, which share its storage?

This is how C code works. If you return a pointer from a function, you may have to free that pointer yourself or you may not. You may get the same pointer back from multiple calls to the same function. If you use the pointer after it's been freed, it's your problem. You need to agree with the owner of the pointer how the memory is to be managed. With pointers, modifications to the underlying data are visible by all who have a pointer to the data. I wouldn't call this "magically modifying others".

Jess

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