Oh God! Now I am totally confused!
Here's my loop:
for (...) {
int *n = new int;
*n = 4;
*n += 2;
zmq_socket.send(&n, sizeof(n));
}
Doesn't zmq_send(), which is called by socket_t::send(), make a copy of
whatever message that is passed in before returning?
So a different pointer gets passed in and copied in each iteration. What is
wrong with doing that?
int zmq_send (void *s_, const void *buf_, size_t len_, int flags_)
{
if (!s_ || !((zmq::socket_base_t*) s_)->check_tag ()) {
errno = ENOTSOCK;
return -1;
}
zmq_msg_t msg;
int rc = zmq_msg_init_size (&msg, len_);
if (rc != 0)
return -1;
memcpy (zmq_msg_data (&msg), buf_, len_); // isn't copying done here?
zmq::socket_base_t *s = (zmq::socket_base_t *) s_;
rc = s_sendmsg (s, &msg, flags_);
if (unlikely (rc < 0)) {
int err = errno;
int rc2 = zmq_msg_close (&msg);
errno_assert (rc2 == 0);
errno = err;
return -1;
}
// Note the optimisation here. We don't close the msg object as it is
// empty anyway. This may change when implementation of zmq_msg_t changes.
return rc;
}
________________________________
From: Pieter Hintjens <[email protected]>
To: Kah-Chan Low <[email protected]>
Sent: Thursday, November 29, 2012 8:58 AM
Subject: Re: [zeromq-dev] inproc: passing an object pointer between threads
causing datarace?? (3.2.2 stable release)
I think you are sending a value that is allocated on the stack, and which is
not valid by the time the asynchronous send completes. I suspect the pointer is
immediately reallocated in the next iteration. So, yes, it will often not have
the correct value.
Pieter
On Nov 29, 2012 1:18 PM, "Kah-Chan Low" <[email protected]> wrote:
Sorry, my last sentence should read " In this case,
>is it possible for the program to output any value other than '6' for '*n'?"
>
>
>Basically I declare n as an int*, and I allocate memory for *n, assign a value
>to *n, and send the pointer n.
>
>
>int *n = new int; *n = 4; *n += 2; zmq_socket.send(&n, sizeof(n));
>
>
>
>So the pointer value is valid when sending actually takes place.
>
>________________________________
> From: Pieter Hintjens <[email protected]>
>To: Kah-Chan Low <[email protected]>; ZeroMQ development list
><[email protected]>
>Sent: Wednesday, November 28, 2012 11:22 PM
>Subject: Re: [zeromq-dev] inproc: passing an object pointer between threads
>causing datarace?? (3.2.2 stable release)
>
>On Tue, Nov 27, 2012 at 10:23 PM, Kah-Chan Low <[email protected]> wrote:
>
>> 1. I don't understand the error message above: how does accessing the value
>> of variable n have anything to do with the internals of ZMQ?
>
>It doesn't, as far as I can see.
>
>> 2. Is it possible for the compiler to rearrange the machine instructions
>> such that the assignment of value of n (i.e. *n=4; *n += 2;) occurs after
>> the message has been sent and received on the other thread? In this case,
>> is it possible for the program to output any value other than '6' for 'n'?
>
>That's very doubtful. However are you not declaring n on the stack?
>Meaning, it will not be valid when the send actually happens.
>
>-Pieter
>
>
>_______________________________________________
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