[paho-dev] Payload buffer alignment in C/C++ clients

[Frank Pagliughi <fpagliughi@xxxxxxxxxxxxxx>]

Hey All,

Here's a strange one.  In the C/C+ clients, can we (or should we) make an alignment guarantee on the incoming payload buffers for an application?

I know that the payloads are supposed to just be binary byte blobs, so a byte alignment is all that is to be expected. But with a modest alignment guarantee, a subscriber can make use of the payload buffer directly rather than incurring a copy penalty on each incoming buffer.

For example, say that an app is publishing a native array of 32-bit int's:

int32_t arr[N];
...
MQTTClient_publish(cli, topicStr, (int) (N*sizeof(int32_t)), arr, ...);

Assuming that the receiver has verified that it is using the same byte ordering, it could do this:

int msg_arrvd(void *context, char *topicName, int topicLen, MQTTClient_message *msg)
{
    size_t i, n = msg->payloadlen / sizeof(int32_t)
    const int* arr = (const int*) msg->payload;

    for(i=0; i<n; ++i)
        do_something_with_int(*arr++);

    // ...
    return 1;
}

This will work on many platforms, including a PC, but if the incoming payload buffer does not have at least 4-byte alignment, it would cause a severe performance penalty, and some CPU's like an ARM will generate alignment faults.

So maybe the libraries could make some guarantee about alignment for the payload buffers? Aligned to the native word size? Or the native "long" size? Or to a 4, 8, 16, 256 byte boundary, etc?

Just a thought.

Thanks,
Frank