[nanomsg] Re: When is it safe to use NN_MSG?
- From: Eduardo Morras <emorrasg@xxxxxxxx>
- To: nanomsg@xxxxxxxxxxxxx
- Date: Wed, 11 Dec 2013 11:57:35 +0100
On Wed, 11 Dec 2013 03:27:55 +0100
Stefan de Konink <stefan@xxxxxxxxx> wrote:
> I'm currently using a construction where an iov contains a uint32_t and a
> struct. It make sense that I have to manually set iov_len with the
> appropriate sizes of these objects.
>
> When receiving the object back, I should receive a string-like object
> (instead of the struct). I was expecting I could use a lazy method let
> nanomsg do my allocation like in the test.
>
> https://github.com/nanomsg/nanomsg/blob/master/tests/msg.c#L81
>
> For some reason I cannot get the following to work, either response side
> remains 0x0 or it plainly hangs.
>
> Sender>>
> void *msg = nn_allocmsg(result_length, 0);
> strncpy(msg, result_buf, result_length);
> iov[0].iov_base = &sd;
> iov[0].iov_len = sizeof(uint32_t);
> iov[1].iov_base = &msg;
> iov[1].iov_len = NN_MSG;
> bzero (&hdr, sizeof (hdr));
> hdr.msg_iov = iov;
> hdr.msg_iovlen = 2;
> nn_sendmsg(sock, &hdr, 0);
>
> <<Receiver
> char *response = NULL;
> uint32_t sd;
> struct nn_msghdr hdr;
> struct nn_iovec iov [2];
> iov [0]iov_base = &sd;
> iov [0].iov_len = sizeof(uint32_t);
> iov [1].iov_base = &response;
> iov [1].iov_len = NN_MSG;
> bzero(&hdr, sizeof (hdr));
> hdr.msg_iov = iov;
> hdr.msg_iovlen = 2;
> nn_recvmsg (broker_socket, &hdr, 0);
>
>
> When I change the iov[1] parts on both sides to preallocated buffers, with a
> fixed size, I do get messages around. Could someone give a hint?
First, I don't know. I'm begining to develop with nanomsg. But, from
documentation, http://nanomsg.org/v0.2/nn_sendmsg.3.html , after the successful
nn_sendmsg(sock, &hdr, 0);
"Alternatively, to send a buffer allocated by nn_allocmsg(3) function set
iov_base to point to the pointer to the buffer and iov_len to NN_MSG constant.
In this case a successful call to nn_send will deallocate the buffer. Trying to
deallocate it afterwards will result in undefined behaviour. Also, scatter
array in nn_msghdr structure can contain only one element in this case."
So internal iov data becomes corrupted, perhaps because msg is deallocated and
NN_MSG is modified.
>
> Stefan
HTH
--- ---
Eduardo Morras <emorrasg@xxxxxxxx>
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