[zeromq-dev] Fix for alignment issue of vsm_data inside zmq_msg_t message type.

[Rick Flower]

I stumbled across this issue this morning when testing out some
new message handling code using push/pulls.  I was sending some
small C++ classes from one thread to anther via the inproc backend.
What I found was that the receiver would get a BUS error when trying
to see what the packets had in them by examination of the packet
header and calling methods on the class.  In toying around I was
able to reproduce this issue on the sending end by calling data()
function of the message and casting it to my custom type (which
BTW has NO virtual functions -- just plain ol C++ data types
and some methods -- all inlined -- no statics even).

In this case my packet was only 12 bytes in size so it gets the
built in buffer -- no allocation required.  I'm using
zmq_msg_init_size.  So, the underlying message as a whole is
properly aligned, but NOT the vsm_data portion once you throw
two char's in front of it as you can see in the code below
taken from the 2.1.11 stable build I'm using :

typedef struct
{
     void *content;
     unsigned char flags;
     unsigned char vsm_size;
     unsigned char vsm_data [ZMQ_MAX_VSM_SIZE];
} zmq_msg_t;

Now, once you move vsm_data above flags so it's right after
content, my problem disappears since the alignment issue was
caused by flags & vsm_size.

The scenario I used to get this is more or less as follows :

1) call zmq_msg_init_size() with a buffer size of <30 to
    get you the built-in-buffer (vsm_data from above) -- no
    allocation required.
2) Create a custom C++ class (my_cool_class) with a few
    data items and getters/setters for those items
3) call zmq_msg_data() and cast the pointer to a my_cool_class
    pointer type.  Now call my_cool_class_obj->my_cool_method()
    and watch it crash.

You don't even need to send the buffer to see this.

Any issues you can see?  I've already swapped the order in
my local version of the code and rebuilt everything and it
works great!