Method: LIBUSB::Device#max_alt_packet_size

Defined in:
lib/libusb/device.rb

#max_alt_packet_size(interface, alternate_setting, endpoint) ⇒ Fixnum

Calculate the maximum packet size which a specific endpoint is capable of sending or receiving in the duration of 1 microframe

Only the active configuration is examined. The calculation is based on the wMaxPacketSize field in the endpoint descriptor as described in section 9.6.6 in the USB 2.0 specifications.

If acting on an isochronous or interrupt endpoint, this function will multiply the value found in bits 0:10 by the number of transactions per microframe (determined by bits 11:12). Otherwise, this function just returns the numeric value found in bits 0:10. For USB 3.0 device, it will attempts to retrieve the Endpoint Companion Descriptor to return wBytesPerInterval.

This function is useful for setting up isochronous transfers, for example you might pass the return value from this function to IsochronousTransfer.packet_lengths= in order to set the length field of every isochronous packet in a transfer.

Available since libusb-1.0.27.

Parameters:

  • interface (Interface, Fixnum) —

    the interface or its bInterfaceNumber of the interface the endpoint belongs to

  • alternate_setting (Setting, Fixnum) —

    the alternate setting or its bAlternateSetting

  • endpoint (Endpoint, Fixnum) —

    (address of) the endpoint in question

Returns:

  • (Fixnum) —

    the maximum packet size which can be sent/received on this endpoint

See Also:


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# File 'lib/libusb/device.rb', line 194

def max_alt_packet_size(interface, alternate_setting, endpoint)
  interface = interface.bInterfaceNumber if interface.respond_to? :bInterfaceNumber
  alternate_setting = alternate_setting.bAlternateSetting if alternate_setting.respond_to? :bAlternateSetting
  endpoint = endpoint.bEndpointAddress if endpoint.respond_to? :bEndpointAddress
  res = Call.libusb_get_max_alt_packet_size(@pDev, interface, alternate_setting, endpoint)
  LIBUSB.raise_error res, "in libusb_get_max_alt_packet_size" unless res>=0
  res
end