Page History

A bit on endianness

Endianness refers to the ordering of bytes in memory that make up a larger chunk of memory.  There's plenty of information on there on wikipedia for examplemore in depth reading out there, but here's a quick overview.  Systems are usually really simple summary.  Most computer systems are either big-endian or little-endian.  Stealing  Here's a table copied from wikipedia's page on endianness (http://en.wikipedia.org/wiki/Endianness) that explains the difference:

...

The systems we are working on are all little endian though, so the number 4097 would be stored as 0x01:0x10 in memory if we saved it as a short.  Saving it as an int would add some 0s for padding in there lie so: 0x01:0x10:0x00:0x00.  

...

Serializing our data

Now say we want to send a uint16 (unsigned short) through acomms along with a bunch of other data.  Well  We'll probably be constructing a long array or of bytes and copying our data into it for transmission, then picking it back out at the other end.  Sticking with our simple uint16, let's look at how we can reconstruct the number given an array of bytes.  

...

unsigned char data[] = {0x01, 0x10};


// method 1 - bitwise operations
unsigned short result1 = (data[0]<<8) + data[1];
cout << result1 << endl;


// method 2 - memcpy
unsigned short result2;
memcpy(&result2, &data[0],2);
cout << result2 << endl;

...

Neither one of these solutions is very elegant though, especially if we start to deal with longer arrays or larger numbers.  One solution is to use structs to send your data.  A struct can be defined that holds all the data you want to send and then memcpy can copy the whole thing to or from an array in one go.  

#include <iostream>
#include <string.h>
#include <vector>
#include <math.h>

using namespace std;

// our structure for sending data
struct MY_DATA {
	unsigned char packet_id;
	int latitude;
	int longitude;
	unsigned short heading;
};

int main() {
	// fill a struct with data
	MY_DATA data_to_send;
	data_to_send.packet_id = 0x01;
	data_to_send.latitude = (int) (42.358456 * pow(10,7));
	data_to_send.longitude = (int) (-71.087589 * pow(10,7));
	data_to_send.heading = 185;

	// construct our array/vector and fill it with data
	vector<unsigned char> packet (sizeof(data_to_send), 0);
	memcpy(&packet[0], &data_to_send.packet_id, sizeof(data_to_send));

	// to get our data out, do the reverse
	MY_DATA data_received;
	memcpy(&data_received.packet_id, &packet[0], sizeof(data_to_send));

	// print out the data
	cout << "packet id: 0x" << hex << (int) data_received.packet_id << dec << endl;
	cout << "lat: " << data_received.latitude / pow(10,7) << endl;
	cout << "lon: " << data_received.longitude / pow(10,7) << endl;
	cout << "heading: " << data_received.heading << endl;

	return 0;
}

Posting to MOOS

See Binary acomms data in MOOS for information on getting your serialized data into or out of a MOOS message.