[zeromq-dev] czmq container serialization

Alex Cornejo acornejo at gmail.com
Tue Jan 13 01:58:11 CET 2015

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

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
    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);
        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

    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

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?

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