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variable size array of uint8 as msg field has strange values

asked 2019-09-03 05:59:58 -0500

dimaluigi gravatar image

Hi everybody, i'm using MKR1300 arduino board and rosserial. I have this msg type:

uint32 id
uint8 lenght
uint8[] value
uint64 timestamp

These fields should contain information on CAN type packages. In the field value I am inserting the sequence of bytes in this way (arduino side):

void AddOneToROStail(int packetSize){

  int i = 0;

  if(countR == BUFFER_LENGHT){
    Serial.print("Overrun from CAN..losing packets");

  buff_toROS[headR].id = CAN.packetId();
  buff_toROS[headR].len = packetSize;
  buff_toROS[headR].timestamp = now();

  //get the raw bytes
  buff_toROS[headR].value = new unsigned char[packetSize];
  for(i=0; i<buff_toROS[headR].len; i++){
    buff_toROS[headR].value[i] =;

  headR = headR + 1;
  headR = headR%BUFFER_LENGHT; //make sure head [0 % BUFFER_LENGHT-1]
  countR = countR + 1; //new alement in the queue  

So when a package arrives I fill a struct and immediately after that I publish it on the ros topic. The problem is that despite being bytes if I try to do echo of the topic the values ​​become negative and this is problematic when converting from byte array to actual value reported by the sensor. I tried making (on the serial monitor of arduino) control prints but everything seems to be correct:

Packet ID:21 Packet length:2 Byte sequence: 134|1|
Packet ID:20 Packet length:2 Byte sequence: 134|3|
Packet ID:18 Packet length:2 Byte sequence: 240|0|
Packet ID:19 Packet length:2 Byte sequence: 134|1|
Packet ID:21 Packet length:2 Byte sequence: 134|1|
Packet ID:20 Packet length:2 Byte sequence: 137|3|
Packet ID:18 Packet length:2 Byte sequence: 240|0|

Than that when I run "rostopic echo /fromarduino":

id: 18
lenght: 2
value: [-16, 0]
timestamp: 62
id: 19
lenght: 2
value: [124, 1]
timestamp: 62
id: 21
lenght: 2
value: [124, 1]
timestamp: 62
id: 20
lenght: 2
value: [-112, 3]
timestamp: 62

Don't mind the timestamp that works correctly. I really hope some of you can help me. Thanks in advance.

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As you have an cortex m0 you may use the rosbridge

duck-development gravatar image duck-development  ( 2019-09-03 14:42:21 -0500 )edit

Afaik, arrays require special handling in rosserial. See rosserial/Overview/Limitations: Arrays.

I don't completely follow the code snippet you posted, so you may already be setting the length, but it's something to check.

Note: I'm not referring to the length field of your custom message. rosserial adds an additional <fieldname>_length field for every array that doesn't have a fixed size.

gvdhoorn gravatar image gvdhoorn  ( 2019-09-04 02:39:45 -0500 )edit

yes I've already done it. As I said it is strange that, after setting the array length and after filling the array (via pointer), just before publishing, if I try to print the contents of the array I get all unsigned int (so 0-255 which is what i want) while when echo using "rostopic echo /fromarduino" they become signed int (-128.127) .. This is the code piece for publish one to ros (part of the arduino sketch):

void PublishOneToROS(){ = buff_toROS[headR].id;
    ros_pkt.timestamp = buff_toROS[headR].timestamp;
    l = buff_toROS[headR].len;
    ros_pkt.lenght = l;

    //define lenght and assign pointer
    ros_pkt.value_length = l;
    ros_pkt.value = buff_toROS[headR].value;

    tailR = tailR + 1;
    tailR = tailR%BUFFER_LENGHT;
    countR = countR - 1;


dimaluigi gravatar image dimaluigi  ( 2019-09-04 05:48:05 -0500 )edit

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answered 2019-09-03 14:29:05 -0500

duck-development gravatar image

updated 2019-09-03 14:31:34 -0500

If you programm in c++ then there will bei no signum Error. I Think it is an ros topic echo isue. It will be negativ in echo at it get bigger then 128

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Yes I've just checked that: it goes fine until it reach 127, then it start to go backwards from -127 to 0 and it starts again. I can find a way to cast a signed byte tuple to an integer in python (to rapresent the actual value of the sensor) but still don't understand why this is appening..

dimaluigi gravatar image dimaluigi  ( 2019-09-03 14:48:23 -0500 )edit

If look here then you see it. It is an uin8[] is an str in python

duck-development gravatar image duck-development  ( 2019-09-03 14:56:33 -0500 )edit

This thing dosen't compute for me..when i try to print the data type of the uint8[] array in the python script it returns:

<type 'tuple'>

But assuming that everything can remain this way, can anyone suggest me a way to proper convert a signed int tuple (-128, 127) in just one single integer?

dimaluigi gravatar image dimaluigi  ( 2019-09-03 17:26:17 -0500 )edit

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Asked: 2019-09-03 05:59:58 -0500

Seen: 554 times

Last updated: Sep 03 '19