[zeromq-dev] czmq container serialization

Brian Knox bknox at digitalocean.com
Tue Jan 13 12:22:22 CET 2015

This is not something I would personally have any need for.  I'm not
certain that it is a general enough problem that it needs solving in czmq
itself, but maybe others feel differently.


On Mon, Jan 12, 2015 at 7:58 PM, Alex Cornejo <acornejo at gmail.com> wrote:

> Hi all,
> As part of an application I am developing I need to serialize/unserialize a
> zlistx_t and a zhash_t into/from messages.
> In my particular use case, the elements stored in these containers
> belong to a custom czmq-style C class with its own
> my_class_new/my_class_destroy.
> When I first read the zhashx API I initially thought I could use
> zhashx_pack/zhashx_unpack (at least for the zhashx), but I soon realized
> this doesn't (and can't) work for custom types.
> The solution I adopted required the following changes:
> Implemented the following functions for the custom class:
>     // return the size of self in bytes
>     size_t my_class_size(my_class_t *self);
>     // serialize the contents of self into buf.
>     // the space used may not exceed my_class_size(self)
>     void my_class_store(my_class_t *self, void *buf);
>     // unserialize the contents of buf into a new my_class_t instance.
>     my_class_t *myclass_load(void *buf);
>     The behavior of myclass_store and myclass_load is such that the
> following
>     is a valid implementation of myclass_dup:
>     my_class_t *myclass_dup(myclass_t *self) {
>         void *buf = zmalloc(my_class_size(self));
>         myclass_store(self, buf);
>         myclass_t *other = myclass_load(buf);
>         free(buf);
>         return other;
>     }
> I also implemented the following methods for the container functions:
>     typedef size_t (size_func_t)(void *);
>     typedef void (store_func_t)(void *, void *);
>     typedef void *(load_func_t)(void *);
>     size_t zlistx_size(zlistx_t *self, size_func_t size_func);
>     void zlistx_store(zlistx_t *self, void *buf, size_func_t size_func,
> store_func_t store_func);
>     zlistx_t *zlistx_load(void *buf, size_func_t size_func, load_func_t
> load_func);
>     size_t zhashx_size(zhashx_t *self, size_func_t size_func);
>     void zhashx_store(zhashx_t *self, void *buf, size_func_t size_func,
> store_func_t store_func);
>     zhashx_t *zhashx_load(void *buf, size_func_t size_func, load_func_t
> load_func);
> You will notice the load and store function signatures for zlistx and
> zhashx are
> different than for my_class, this is to accomodate for the fact that
> these are containers and therefore must also be able to load and store
> their elements.
> Using the above methods its quite trivial to serialze and unserialize
> lists and hashes into zframes for sending messages or into binary files
> for long term storage. The same container serailization methods can be used
> regardless of the type of elements they store, as long as they implement
> the required load/store/size API for these elements.
> Although the above works, I wanted to ask experienced czqm developers if
> there is a more elegant way to implement container
> serialization/unserialization in czmq.
> Also, I would be surprised to find that I am the first czmq user that
> needs to serialize custom containers into messages. Is there a reason
> something like this is not already included in czmq's core? Should I
> polish and document this and submit a pull-request, or does this seem
> like a very uncommon use of containers in the czmq world?
> Alex
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> zeromq-dev at lists.zeromq.org
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