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From Rafael Schloming <...@alum.mit.edu>
Subject Re: question on Proton memory management
Date Fri, 28 Mar 2014 14:38:44 GMT
On Fri, Mar 28, 2014 at 4:20 AM, Fraser Adams <fraser.adams@blueyonder.co.uk
> wrote:

> I mentioned the other day about the lack of API documentation for things
> like message and codec/data
>

Yeah, I missed those in my documentation pass a few weeks ago.


>
> It'd be really useful to have documentation for at least the non-obvious
> calls.
>
> One thing I'm currently struggling with is understanding the
> ownership/responsibility for the memory of items that consume or return
> pn_bytes_t
>
> What I *think* is the case is that for say pn_data_get_binary if I were
> applying that to say the message body then it refers to memory that is
> *owned* by the message, so I could access the bytes pointed to by
> bytes.start whilst the message was in scope and I wouldn't need to do any
> explicit free of bytes.start, however if I wanted to retain that data I'd
> have to copy the data into my own buffer before I did say pn_messenger_get
> again into the same message instance.
>
> I guess in the case of data retrieval I'm asking if I don't generally need
> to explicitly free the underlying data from a pn_bytes_t because it's owned
> by the underlying message (or pn_data if I'm doing lower level things).
>

Your assumption is correct. When pn_bytes_t is returned it is giving you a
pointer to memory owned by the object you are accessing.


>
>
> So what about the reverse case? If I'm going to do pn_data_put_binary I
> clearly need to create a pn_bytes_t and the start pointer for that would
> likely be a block of memory that I've malloc'd - so at which point is the
> ownership of that data "transferred" so that I can free my client side
> buffer? I'm *guessing* that pn_data_put_binary copies the data from the
> byte array pointed to by the pn_bytes_t start somewhere into the underlying
> pn_data_t but that's only an assumption on my part because that behaviour
> seems to make logical sense to me, it's far from clear that this is what is
> actually going own.
>

Your assumption is again correct.


>
> Clearly understanding ownership of dynamically allocated memory is pretty
> important for application efficiency (I'd like to avoid unnecessary copies)
> and correctness (I definitely want to avoid leaks) and this sort of thing
> gets even more important to understand if one ends up passing or retrieving
> more complex data structures such as maps or lists that might contain
> binary elements.
>

The missing docs should be up soon.

--Rafael

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