[zeromq-dev] Extending zmq_msg_t API

Thomas Rodgers rodgert at twrodgers.com
Wed Jul 9 23:53:16 CEST 2014


Right.  This is my trepidation about surfacing the refcount.  The
"sharedness" is indicated by the flags field on the msg type, and that, I'm
reasonably sure, is not altered once it is set.


On Wed, Jul 9, 2014 at 4:05 PM, KIU Shueng Chuan <nixchuan at gmail.com> wrote:

> Couldn't the refcount change after you have obtained its value?
> E.g.
> Make a copy
> Send the 1st
> Read the refcount (2)
> Background io releases 1st copy
>  On 9 Jul 2014 18:21, "Thomas Rodgers" <rodgert at twrodgers.com> wrote:
>
>> zmq_msg_get() could be extended to give either the refcount or an
>> indicator on whether a message was share; based on other refcounted designs
>> I'm hesitant to promote surfacing the actual count.  Similarly,
>> zmq_msg_set() could allow 'unsharing' by adding a ZMQ_SHARED property
>> #define and setting it's value to 0 (no effect on non-shared messages).
>>
>> So the only API surface area change is an additional message property.
>>  This seems the cleanest to me.
>>
>> On Wednesday, July 9, 2014, Goswin von Brederlow <goswin-v-b at web.de>
>> wrote:
>>
>>> On Tue, Jul 08, 2014 at 10:42:41AM -0500, Thomas Rodgers wrote:
>>> > tl;dr; Is there any objection to adding some sort of accessor to the
>>> API to
>>> > determine if a given zmq_msg_t is_shared()?
>>> >
>>> > Background/Rationale:
>>> >
>>> > Something I encountered while writing a "high level" C++ wrapper for
>>> > zmq_msg_t and it's API is the following set of behaviors -
>>> >
>>> > zmq_msg_init(&msg_vsm, 20);
>>> >
>>> > Results in a type_vsm message, the body of which is held entirely
>>> within
>>> > the space allocated to zmq_msg_t
>>> >
>>> > zmq_msg_init(&msg_lmsg, 1024);
>>> >
>>> > Results in a type_lmsg message, the body is held as a reference to a
>>> block
>>> > of size bytes.
>>> >
>>> > memcpy(zmq_msg_data(&msg_vsm), "VSM", 3);
>>> > memcpy(zmq_msg_data(&msg_lmsg), "LMSG", 4);
>>> >
>>> > So far so good.  Now copy -
>>> >
>>> > zmq_msg_copy(&msg_vsm2, &msg_vsm);
>>> > zmq_msg_copy(&msg_lmsg2, &msg_lmsg);
>>> >
>>> > Now change contents -
>>> >
>>> > memcpy(zmq_msg_data(&msg_vsm2), "vsm", 3);
>>> > memcpy(zmq_msg_data(&msg_lmsg2), "lmsg", 4);
>>> >
>>> > assert(memcmp(&msg_vsm, &msg_vsm2, 3) != 0); // ok
>>> > assert(memcmp(&msg_lmsg, &msg_lmsg2, 4) != 0); // fail
>>> >
>>> > This happens by design (lmsg's are refcounted on copy, not deep
>>> copied).
>>> > But it results in a situation where a zmq_msg_t is sometimes a Value
>>> and
>>> > sometimes a Reference.  This could lead to astonishment for the unwary.
>>> >
>>> > >From the perspective of a wrapper (particularly one that takes a
>>> strong
>>> > stand on value semantics and local reasoning), this behavior is
>>> ungood.  So
>>> > my options are deep copy always or implement copy-on-write.
>>> >
>>> > For efficiency I prefer the latter approach in the case of type_lmsg
>>> > messages.  I have implemented the copy-on-write logic through a
>>> horrible
>>> > brittle hack that examines the last byte of zmq_msg_t.  I would prefer
>>> a
>>> > less brittle solution.
>>>
>>> "lmsg's are refcounted on copy" Can't you access the refcount?
>>> Or is that the API call you want to add?
>>>
>>> Maybe instead of is_shared() an unshare() call would be more usefull,
>>> which would copy the message payload if it is shared. Or both?
>>>
>>> MfG
>>>         Goswin
>>> _______________________________________________
>>> zeromq-dev mailing list
>>> zeromq-dev at lists.zeromq.org
>>> http://lists.zeromq.org/mailman/listinfo/zeromq-dev
>>>
>>
>> _______________________________________________
>> zeromq-dev mailing list
>> zeromq-dev at lists.zeromq.org
>> http://lists.zeromq.org/mailman/listinfo/zeromq-dev
>>
>>
> _______________________________________________
> zeromq-dev mailing list
> zeromq-dev at lists.zeromq.org
> http://lists.zeromq.org/mailman/listinfo/zeromq-dev
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.zeromq.org/pipermail/zeromq-dev/attachments/20140709/893ff231/attachment.htm>


More information about the zeromq-dev mailing list