Currently, I am studying the message format of OpenFlow, I am just curious about the relationship between the default value in length field of a message header and the buffer size of the struct of this message allocated in the compile time, are those two in the same value?
I have tried to find many resources for this, but no answer, I hope someone would guide me, thanks very much!
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I am receiving the data in sample.payload but i am not able to deserialize the data.
Data format:
b'\x00\x01\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x01\x00\x00\x00\x00\x00\x00\x00\x07\x00\x00\x00Demo20\x00\x00\x04\x00\x00\x00gap\x00\x00\x00\x80?\x00\x00\x00#\x00\x00\x80?\x00\x00\x00#{\x14\x8e?\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\xd3\x02\x96I\x07\x00\x00\x00tripid\x00\x00'
b'\x00\x01\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x01\x00\x00\x00\x00\x00\x00\x00\x07\x00\x00\x00Demo20\x00\x00\t\x00\x00\x00position\x00\x00\x00\x00\xa5\xbdJAH\x90\x9eB\x00\x00\x00#\xd3\x02\x96I\x07\x00\x00\x00tripid\x00\x00'
Getting a struct error.
struct.error: unpack_from requires a buffer of at least 1065353260 bytes for unpacking 1 bytes at offset 1065353259 (actual buffer size is 92)
where am I going wrong?
Assuming you're using pycdr, as done in this example, you should check if the Python class you defined for the expected deserialized type correspond exactly to the ROS2 message definition.
In this same example, the Log class corresponds exactly to the ROS2 Log message (ordering of fields is important).
I need to transfer data over a serial port. In order to ensure integrity of the data, I want a small envelope protocol around each protobuf message. I thought about the following:
message type (1 byte)
message size (2 bytes)
protobuf message (N bytes)
(checksum; optional)
The message type will mostly be a mapping between messages defined in proto files. However, if a message gets corrupted or some bytes are lost, the message size will not be correct and all subsequent bytes cannot be interpreted anymore. One way to solve this would be the introduction of limiters between messages, but for that I need to choose something that is not used by protobuf. Is there a byte sequence that is never used by any protobuf message?
I also thought about a different way. If the master finds out that packages are corrupted, it should reset the communication to a clean start. For that I want the master to send a RESTART command to the slave. The slave should answer with an ACK and then start sending complete messages again. All bytes received between RESTART and ACK are to be discarded by the master. I want to encode ACK and RESTART as special messages. But with that approach I face the same problem: I need to find byte sequences for ACK and RESTART that are not used by any protobuf messages.
Maybe I am also taking the wrong approach - feel free to suggest other approaches to deal with lost bytes.
Is there a byte sequence that is never used by any protobuf message?
No; it is a binary serializer and can contain arbitrary binary payloads (especially in the bytes type). You cannot use sentinel values. Length prefix is fine (your "message size" header), and a checksum may be a pragmatic option. Alternatively, you could impose an artificial sentinel to follow each message (maybe a guid chosen per-connection as part of the initial handshake), and use that to double-check that everything looks correct.
One way to help recover packet synchronization after a rare problem is to use synchronization words in the beginning of the message, and use the checksum to check for valid messages.
This means that you put a constant value, e.g. 0x12345678, before your message type field. Then if a message fails checksum check, you can recover by finding the next 0x12345678 in your data.
Even though that value could sometimes occur in the middle of the message, it doesn't matter much. The checksum check will very probably catch that there isn't a real message at that position, and you can search forwards until you find the next marker.
Does anyone know what 2 parameters in the fetchMaxBytes represent?
If its represented as 1024*1024, does that mean that the consumer will fetch 1024 messages of each 1Kb? Or will it jest fetch 1Mb of messages?
I was not able to find any relevant information from the documentation except this: "The maximum bytes to include in the message set for this partition. This helps bound the size of the response."
I need this parameter to get messages one by one rather than getting a couple of messages in a single shot.
I am not familiar with node.js but I assume fetchMaxBytes corresponds to replicate.fetch.max.bytes. For this case, the value is the maximum buffer size (in bytes, ie, 1024*1024 = 1MB) for fetching messages. A buffer can contain multiple messages of arbitrary size. It basically means, wait for fetching not longer as "until a buffer got filled up".
Is there a maximum amount of data that can be read in a namelist read using Intel Visual Fortran?
I'm interested to know total, but more specifically for an individual field.
I can't seem to find anything on it anywhere, but it seems to be crashing at around a 2,500 character array. I'm trying to put it into a 3,000 character array.
The data is on one line too if that could be an issue.
For Intel Fortran this is documented under the topic "Compiler Limits". The maximum size of a character value read in during list-directed and NAMELIST I/O is 2048. The variable itself may be longer.
I just have this issue with reading from a network stream in C#. Since am more of a Java developer I came across this issue.
In java I have this option of knowing the length of the received packet using the following code
int length = dataiInputStream.read(rcvPacket);
eventhough the size of the byte array rcvPacket assigned is larger than the amount of elements contained in it. which will allow me to read only the required length of elements so that i do not have elements in the byte array containing zeros.
While i was trying to use a similar thing on C# which was
long len = networkStream.length;
but the docucmentation says that this property is not supported. is there a workaround for this?
Thank you
The Java code doesn't show you the length of a packet. It shows you the amount of data read within that read call, which could have come from multiple packets.
NetworkStream.Read returns the same information, except using 0 to indicate the end of the stream instead of -1.