How can I track send progress using libusb_bulk_transfer with libusb? - data-transfer

Given an application using libusb calling 'libusb_bulk_transfer' how can I track progress of how much has been sent over a data buffer to the end device?


Where data can be buffered on rfcomm connection?

i'm trying to pass data between two bluetooth devices, when both connected to two different computers.
After having hci device in each of the computers, i'm using rfcomm to pass information between the two.
I'm trying to pass 10MB of random data just to check the ability of the system.
On the beginning everything seems to work fine. After a few seconds, it looks like there is a delay between the sender and the receiver, when sometimes data stopps to arrive, and then a "massive" amount of data is suddenly arriving the receiver.
Exactly like some buffer is keeping all the data. As long i keep sending data
the delay is getting longer.
I'm trying to figure out where in the chain such buffer can be, or how to
solve this buffering.
Many Thanks :)

av_read_frame reads frames from cache

I want to detect an object with my camera. For performance reason, i like to keep the connection to my camera alive and read new images on demand.
The function to read images calls av_read_frame till the frame is complete and then does some calculation.
My problem now is, that the frames "chain-up". If i stop frequently asking for new frames, i get old-images and not the current, because they're not yet readed (even i don't need them). If possible, i don't want to read the images with an additional thread because i don't want to waste resources on my RaspberryPi. Any ideas how to disable this "cache" or other ideas?

Advice: trying to recognize when a device is not connected

I have some hard time trying to find a method to restart my state machine. In other words some part of what I ve got:
I have a module that when is powered up it stays for a debounce time of 0.5 s and then it goes in a state machine: first it send a string#anotherstring# then he start a timer of some period and when timer elapsed, it converts an analog signal/read a data (SPI,I2C) and sends that data followed by another #. The state machine goes back and start again the timer and send again the data ...
On another chip. I receive info from that module. So here is a state machine that complete the first string, second string, and then cumulates values in a buffer, again and again.
In some moment some external device ask for data, moment when the chip make some computation and sends it.
SO far so good. Every single part of this is working exept the part when the module is disconnected. Ok you may say no problem no data is send. Yes this is true, but what happens if the module is connected back. Until now to test my work I have reseted the chip disconect and connect the module. By doing this the chip is on the first state and the module goes from first state, everything is ok.
My qestion is how to determine when the device is disconected from the chip to restart the chip stat machine and to wait for the string#anotherstring# combination(first state).
Another question is how to determine if the communication is broken and not the power down. When putting back the comunication the data should be again send,preferably both modules to go from init state.
What I have in mind is to send some ack to the module from the chip. But I do not know exactly how. Basically I want this: when the module is disconected its state machine obviously start over and the chip state I want again to goes back to initial state.
if the comunication of the module is unplugged some how both statemachines to start over.
I do not know if I am clear with this. but please if there are questions ask. I will come with edits if I found something.
OTHER INFO: The module and the chip are some microcontrolers, the comunicaiton is UART.
Let me sketch a basic scheme you can use on the receiving side:
On your receiving side, you'll want to time-out if no valid/complete message is received within a reasonable time frame. This way, you'll detect when the module goes offline for whatever reason at any point in time.
The state machine that receives and processes the messages will also be reset in this case. This means you'll have a timer which, for example, is started when data is received and stopped when a message was correctly and fully received. If the timer times out, any message currently being received is declared invalid and discarded, and the receiver goes back to the start, looking for the next message.
Then, you'd have to implement in the receiver the code to detect when a message starts and/or ends. So if the module always starts by sending string#anotherstring# then the receiver will wait until it sees string# for example; anything else received is ignored by the receiver. Only after the expected prefix was detected the rest of the message receiving is done.
During the whole process, the receiver's messsage timeout timer is active and if any part of the message is not received in time the receiver assumes transmission problems and goes back to waiting for the start of the next message.

x number of threads sending data to Server for displaying output on GUI

I have developed a single server/multiple client TCP Application.
The client consists of x number of threads each thread doing processing on its own data and then sending the data over TCP socket to the Server for displaying.
The Server is basically a GUI having a window. Server receves data from the client and displays it.
Now, the problem is that since there are 40 threads inside the client and each thread wants to send data, how can I achieve this using one connected socket?
My Suggestion:
My approach was to create a data structure inside each of the 40 threads in which data to be sent will be maintained. A separate Send Thread with one connected socket on client side is then created. This thread will read data from data structure of first thread, send it over the socket and then read the data from second thread and so on.
but I am not sure how would this be implemented as I am new to all this? :( What if a thread is writing to data structure and the Send Thread tries to read the data at the same time. I am familiar with mutex, critical section etc but that sounds too complex for my simple application.
Any other suggestions/comments other than my own suggestion are welcome.
If you think my own approach is correct then please help me solving my confusions that I mentioned above.
Thanks a lot in advance :)
Can I put I timer on Send Thread and after a specific time the Send Thread suspends thread#1(so that it can access its data structure without any synchronization issues), reads data from its data structure, sends it over the tcp Socket, and resumes Thread#1 back, then it suspends Thread#2, reads data from its data structure, sends it over the tcp Socket, and resumes Thread#2 back and so on.
A common approach is to have one thread dedicated to sending the data. The other threads post their data into a shared container (list, deque, etc) and signal the sender thread that data is available. The sender then wakes up and processes whatever data is available.
The gist of it is as follows:
HANDLE data_available_event; // manual reset event; set when queue has data, clear when queue is empty
CRITICAL_SECTION cs; // protect access to data queue
std::deque<std::string> data_to_send;
std::string data = generate_data()
SetEvent(data_available_event); // signal sender thread that data is available
std::string data = data_to_send.front();
ResetEvent(data_available_event); // queue is empty; reset event and wait until more data is available
This is of course assuming the data can be sent in any order. I use strings only for illustrative purposes; you probably want some kind of custom object that knows how to serialize the data it holds.
Suspending thread#1 so you can access its data strcuture does not avoid synchronization issues. When you suspend it thread#1 could be in the midst of an update to the data, so the socket thread gets part of old data, part of new. That is data corruption.
You need a shared data structure such as a FIFO queue. The worker threads add to the queue, the socket thread removes the oldest item from the queue. All access to this shared queue must be protected with a critical section unless you implement a lock-free queue. (A circular buffer.)
Depending on your application needs, if you implement this queue you might not need the socket thread at all. Just do the dequeueing in the display thread.
There are a couple of ways to achieving it; Luke's idea suffers from race conditions that will still create data corruption
You avoid that by using UDP instead of TCP as the transport protocol. It'd be especially a good choice if you don't mind missing an occasional packet (which is okay for displaying rapidly changing data); it's fantastic for ensuring real-time updates on data where exact history doesn't matter (missing a point in a relatively smooth curve while plotting graphs is okay);
If the data packets are are small and sort of represent a stream then UDP is a great choice. Its benefit increases if you have multiple senders on different systems all displaying on a single screen.

synchronisation issue within callback functions

In a continuous loop barcodes are scanned and if scan is valid text data and voice data are read out/played, which happens in differnt context. Text playback happens first, then its callback function gets executed, from there voice playback happens, and then its callback function. The issue is seen when multiple scanning is done, sometimes there is synchronisation issues. While text playback of 1st barcode is happening, if another scan is done, then voice data of 2nd barcode gets played(skipping text playback of 2nd). This behaviour is observed only sometimes, so it is kind of difficult to debug.
Any ideas how to provide synchronisation within callbacks?
Thanks in advance for any help.
Your question isn't very clear so lets assume you currently have 3 threads
Reads the barcode from hardware
Displays the code?
Audio playback of the code as text contained in the barcode
The thread reading the barcode is a producer of data and should push a work unit onto a consumer stack and should notify the stack that there is a work unit. Run a cursory google search for producer-consumer queue.
Threads 2 & 3 appear to be synchronous and should be combined into a single thread?