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Linux SP Knowledge Base

Usage of API

Enumeration and Opening of cameras from multiple processes


Calling of xiGetNumberDevices and/or xiOpenDevice returns unexpected results when running from multiple processes.
For example - multiple applications are started in one time and each calls the xiGetNumberDevices.


  • Possibility 1:
    • Use only one application in one time. It can run multiple threads if needed.
  • Possibility 2
    • Use one process for enumeration (using xiGetNumberDevices and xiOpenDevice) and other processes for other tasks.
    • Ensure that only one process in time can call xiGetNumberDevices and xiOpenDevice.

Application Development

xiQ/MQ camera is not accessible after Application Crash


API returns zero on call xiGetNumberDevices after camera was opened by application that crashed before while acquisition was active.


  • Possibility 1:
    • Disconnect and connect the camera
    • Start the application again

  • Possibility 2:
    • reload xhci_hcd driver, either using rmmod/modprobe or /sys/bus/pci/drivers/xhci_hcd/(un)bind interface (needs root privileges);

  • Possibility 3:
    • reset the USB device by execute shell command
      echo 1 > /sys/bus/usb/devices/4-2/remove
      # 4-2 is correct bus-device address for camera which can be looked up via lsusb for example

Trace Output Settings

In Linux, trace output is visible in error output (stderr).
For redirecting error output to a file use 2> like the following:

./xiSample 2> stderr.txt

For canceling error output (removing it) simply redirect it to the null like the following:
./xiSample 2> /dev/null

Cancelling error output can also be done by setting the xiAPI parameter XI_PRM_DEBUG_LEVEL to the value XI_DL_DISABLED:

Changing Real-Time Clock


API is using synchronization primitives, such as WaitForSingleObject. They are based on Linux/macOS on real time clock. API implements multiple mechanisms to overcome disadvantages of these implementations. However there are still some imperfections in behavior:


  • API versions V4.17.42 or newer:
    • If cameras delivers data periodically (e.g. one image every second) then you should notice no issue even if real-time clock is changed.
    • If cameras delivers data sporadically, your application depends on xiGetImage timeouts and real-time clock is changed to past, the timeout will increase accordingly.
      • Example: When camera does not deliver any image and time is changed by 1 seconds to past, the call of xiGetImage(h,image,4) take 5 seconds.
      • Workaround: change real-time in smaller increments than used in xiGetImage calls.
    • Changing of real-time clock to future does not affect the acquisition.
  • API versions V4.17.41 or older:
    • Changing of real-time clock (e.g. by command date -s) affects acquisition of cameras. Workaround: If your system needs to change real-time, perform this before opening of first camera (xiOpenDevice) or after close of last camera (xiCloseDevice).