Does v4l2 support multi-map?

Question

I'm trying to share frames(images) that I receive from a USB camera(logitech c270) between two processes so that I can avoid a memcpy. I'm using memory mapping streaming I/O method described here and I can successfully get frames from the camera after using v4l2_mmap. However, I have another process(for image processing) which has to use the image buffers after the dequeue and signal the first process to queue the buffer again.

Searching online, I could find that opening a video device multiple times is allowed, but when I try to map(tried both v4l2_mmap and just mmap) in the second process after a successful v4l2_open, I get an EINVAL error.

I found this pdf which talks about implementing multi-map in v4l2(Not official) and was wondering if this is implemented. I have also tried using User pointer streaming I/O method, the document of which explicitly states that a shared memory can be implemented in this method, but I get an EINVAL when I request for buffers(According to the documentation in linuxtv.org this means the camera doesn't support User pointer streaming I/O).

Note: I want to keep the code modular, hence two processes. If this is not possible, doing all the work in a single process(multiple threads & global frame buffer) is still possible. Using standard shared memory function calls is not possible as the two processes have to map to the video device file(/dev/video0) and I cannot have it under /dev/shm.

Answer 1

The main problem with multi-consumer mmap is that this needs to be implemented on the device driver side. That is: even if some devices might support multi-map, others might not.

So unless you can control the camera that is being used with your application, you will eventually come across one that does not, in which case your application would not work.

So in any case, your application should provide means to handle non multi-map devices.

Btw, you do not need multiple processes to keep your code modular. Multiple processes have their merits (e.g. privilige separation, crash resilience,...), but might also encourage code duplication...

Answer 2

This may not be relevant now.....

You don't need to use the full monty multi consumer thing to do this. I have used Python to hand off the processing of the mmap buffers to multiple processes (python multi-threading only allows 1 thread at a time to execute)

If you're running multi-threaded then worker threads can pick up the buffer and process it independently when triggered by the master thread

Since the code is obviously very pythonesq I won't post it here as it wouldn't make sense in other languages as it uses python multi-processing support.