Re: [Libusb-devel] Bulk transfers and packet size

[Tim R. <ti...@pr...>]

Jef Driesen wrote:
> The second question is related to reading some data where the total amount of 
> data is not known in advance. What is the best way to do this? Allocating a 
> fixed buffer and keep reading until I get fewer bytes back than requested? Does 
> this work similar compared to normal I/O (e.g. read returning 0 on EOF)? Or is 
> it more similar to serial or socket I/O where you have to specify a timeout to 
> avoid blocking forever if no more data arrives?

A single USB "transfer", which can be larger than a packet, is
terminated when the device sends a packet that is smaller than the max
packet size.  So, if you send down a transfer of 30 packets (15,360
bytes), the transfer will continue as long as the device sends back full
512-byte packets.  When the device is out of data, it should send back a
short packet.  At that point, the transfer is complete and will be
returned to the application, regardless of how much data had been
transferred.

-- 
Tim Roberts, ti...@pr...
Providenza & Boekelheide, Inc.