Hi Frank,
did you stumble over this issue? I assume your thought didn't
spring from thin air! Or maybe it did...
It wouldn't be hard to make this arrangement. If we are using
malloc to allocate the payload area, which I think we are, then this
might have some alignment guarantee, although this is dependent on C
library and OS. Gcc doc says:
"The address of a block returned by malloc or realloc
in
GNU systems is always a multiple of eight (or sixteen on 64-bit
systems)"
I read the Linux man page for malloc first, which didn't say
anything about alignment but did remind me of one of my favourite
Linux man page quotes:
"By default, Linux follows an optimistic memory allocation
strategy. This means that when malloc() returns non-NULL there is
no guarantee that the memory really is available."
LOL - and I mean it.
Ian
On 01/23/2015 03:07 PM, Frank Pagliughi
wrote:
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
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--
Ian Craggs
icraggs@xxxxxxxxxx IBM United Kingdom
Paho Project Lead; Committer on Mosquitto
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