Question

I have a very odd issue trying to run this quite simple C program which is using zmq and msgpack.

There is no problem with server.c, however in clinet.c:39 there
is this msgpack_pack_int (&mpkg, i); and the value of i seems
to be picked up as 0 and doesn’t change on each iteration. I
have tried a bunch of different things (e.g. making a pointer to
i and using that, also tried to split it into a function etc)
and nothing seems to help. I can see that msgpack_pack_int() is
a macro, but why it would introduce such a behaviour and what can
I do to overcome it? Is there a flag that could change the behaviour
of this kind of macro (as I see it expands to an inline function)…
I have tried -Werror -Wall, with gcc and clang, and nothing
comes up in warning either ;(*

I tried debugging it and i increments as expected.

I even tried this, and it would do the same thing anyway:

void pack (msgpack_packer *p, msgpack_sbuffer *b) {

   static volatile int i = 0;

   printf("\ni=%d\n", i);
   msgpack_packer_init (p, b, msgpack_sbuffer_write);
   msgpack_pack_array (p, 2);
   msgpack_pack_int (p, i++);
   msgpack_pack_str (p, "/i/am/a/clinet/");

}

I have even tried something which was supposed to be different, but no luck here either –

int count (void) {
    static int i = 0;
    i += 1; return i;
}

can anyone see why would this happen?

Update 1: Also I have recompiled msgpack library itself without optimization flags,
and that didn’t change the behaviour either.

Update 2: Installed msgpack from the git repo and I get still have the same issue.

Answer 1

It turns out that on each iteration I was doing this:

 msgpack_packer_init (&mpkg, &sbuf, msgpack_sbuffer_write);

that needs to be done only once, and this should be there instead:

 msgpack_sbuffer_init (&sbuf);

or:

 msgpack_sbuffer_clear (&sbuf);

It was rather logical to put msg_pack* functions together and indeed
that was taken from the simple example and
the problem is really with the documentation, one extra comment would help!

Update: working version & version without memcpy.