Re: Module support: No environment in pointer release function

[Philipp Stephani <p.stephani2@gmail.com>]

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Elias Mårtenson <lokedhs@gmail.com> schrieb am Di., 7. Feb. 2017 um
04:16 Uhr:

> On 6 February 2017 at 13:33, Paul Eggert <eggert@cs.ucla.edu> wrote:
>
> Elias Mårtenson wrote:
>
> In
> addition to the pointer itself, the ‘emacs_env’ pointer is needed. An
> arbitrary ‘void *data’ pointer would be useful too.
>
>
> Can you package up these pointers into a single data structure, and pass a
> pointer to that structure? That's what's usually done in situations like
> this, and is why a single 'void *' should suffice.
>
>
> I could do this, but as far as I understood, the emacs_env pointer is not
> guaranteed to be immutable. If it isn't immutable, then I might just as
> well save it to a global variable and not have to pass it through every
> function that needs it.
>
> What are the guarantees with regards to the emacs_env value?
>

The big restriction is that its lifetime is limited to the lifetime
(storage duration) of the argument that is used to obtain the pointer. I.e.
in a function

emacs_value module_fun(emacs_env* env, ...) { ... }

the environment represented by env is only valud in the body of module_fun;
reusing env or *env once the body is finished is undefined behavior.
(I'm aware that this is not documented at all, and I've been meaning to
write some documentation for a while.)

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