Trouble when reading data into buffer using mqqueue

Question

I'm using mqueue to communicate between threads and I'm having a problem passing objects.

The mq_send and mq_receive take a char* as a parameter for the object.

I use them the following way.

foo* foo = new foo();
foo->set_id(3);
mq_send(myQueue, (char*)foo, 1024, 1);

then

char* buffer;
while(true)
{
    ssize_t bytes_read;
    bytes_read = mq_receive(myQueue, buffer, 1024, NULL);
    foo* foo = (foo*) buffer;
    foo->get_id(); //equals 3
    //Send the object to another queue
    mq_send(myOtherQueue, buffer, 1024, 1);
}

So far so good.

The problem is here

    char* buffer;
    while(true)
    {
        ssize_t bytes_read;
        bytes_read = mq_receive(myOtherQueue, buffer, 1024, NULL);
        foo* foo = (foo*) buffer;
        foo->get_id(); //equals garbage 323234234
    }

The second time I cast the buffer, I get garbage. I read about static_cast and dynamic_cast and I can't find what's the problem.

What is wrong?

score 4 · Accepted Answer · answered Dec 05 '16 at 16:30

4

I see two issues here. First, is your Foo TriviallyCopyable?

Second,

char* buffer;
bytes_read = mq_receive(myOtherQueue, buffer, 1024, NULL);

I do not see any allocation for buffer.

answered Dec 05 '16 at 16:30

SergeyA

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Thank you @SergeyA. I don't understand your second point. mq_receive sets the buffer variable – Marc Dec 05 '16 at 16:36
@Marc, no, it copies data to the buffer provided. But your buffer is not allocated. – SergeyA Dec 05 '16 at 16:37
Is it not done in mq_receive based on the lenght (1024)? – Marc Dec 05 '16 at 16:42
Basically, all I want is share an address to an object between multiple threads – Marc Dec 05 '16 at 16:44
No, it is not done. It expects an allocated buffer (as every other similar function). Also, using mq in intra-process communication is a terrible waste of efficiency. It is am IPC mechanism. – SergeyA Dec 05 '16 at 16:48
I'm new to C++. Could you give me an example and what would you use instead of mq_queue? – Marc Dec 05 '16 at 16:51
You need to start looking for thread queues. There is plenty of material out there. – SergeyA Dec 05 '16 at 16:52
Thank you very much! I just don't understand why it works for the first cast but not the second one – Marc Dec 05 '16 at 16:53
1

@Marc It was already explained - you didn't allocate any memory for your `buffer`, and didn't even initialize it - it will point to an indeterminate location in memory, hence writing data to it is Undefined behavior (it may _appear_ to work, or it may not). – Algirdas Preidžius Dec 05 '16 at 16:58
@AlgirdasPreidžius No buffer points to foo*, it's not undefined – Marc Dec 05 '16 at 18:02
@Marc No, it doesn't. Please Re-read the answer, and all of the comments again. Since I don't plan to repeating myself over, and over again. – Algirdas Preidžius Dec 05 '16 at 18:03
You are right. It works when I do char buffer[1024]. I just don't understand why it was working in the first thread. – Marc Dec 05 '16 at 18:09

Trouble when reading data into buffer using mqqueue

1 Answers1