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From Alexander Broekhuis <a.broekh...@gmail.com>
Subject Re: [Native-OSGi] OSGi API: Allocated memory ownership
Date Mon, 04 Jun 2012 10:00:47 GMT

> I'm not a C - expert, but I think the only viable solution is to
> accurately document the lifetime and ownership of pointers.

Well I think this is not a C specific problem, the same applies to C++ I
guess. Someone has to free allocated memory.

> For memory owned by the framework, any memory management technique which
> guarantees the stated lifetime will do (e.g. APR memory pools).

Since this is "internal" to the framework this can be anything the
framework implementation uses. For Celix this is indeed APR.

Memory which is to be owned by the caller (user) could probably be
> allocated and deleted by using special functions (hooks) provided by each
> bundle which use implementation specific routines for memory management.
> Otherwise we just state that the returned memory belongs to the caller.

This is the more interesting part of the problem.

How would such hooks look like, and doesn't this making bundles awkward?

A generic header for this could look like this:
  int mem_alloc(void *handle, size_t size, void **block);
  int mem_calloc(void *handle, size_t num, size_t size, void **block);
  int mem_realloc(void *handle, void *block, size_t size, void **newblock);
  int mem_free(void *handle, void *block);

These function are basically the "normal" C memory functions, does C++ have
different/additional ones?
I've added a *handle to all of them to be able to pass some "instance" to
it. For example, APR would need a memory pool, which can be a member of the
instance type/struct.

How does the framework get the handle? One additional method could work:
int mem_createHandle(void **handle);

For APR:
mem_alloc -> use apr_palloc(handle->pool, size)
mem_calloc -> use apr_pcalloc(handle->pool, size)
mem_realloc -> no alternative in APR, implement a solution like [1]
mem_free -> not implemented, the user has to clear the pool at some time

Looking at it like this, I just realize this solution has a huge drawback
when used in combination with APR. For different calls different pools are
needed. Using this model, there is only one pool which is used for every
allocation. Basically this means the user is not in control of the lifetime
of memory.
The only solution I can think of is to have the user supply the pool on
which the memory has to be allocated, but we don't want this on our API.

A possibility would be to supply the handle (instead of the pool) which is
used by the memory hooks. For example:

int bundle_getEntry(bundle, handle, char *name, char **entry)

Instead of letting the framework use handle, handle is simply passed on the
the memory hooks and used in a way the caller wants. This also eliminates
the need for the createHandle function in the header file. The user can
create one when needed. In case of APR, this handle probably can directly
be the pool.

I hope this still makes sense...

[1]: http://marc.info/?l=apr-dev&m=109065999530290&w=2

Met vriendelijke groet,

Alexander Broekhuis

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