Zero padding frames on Linux

[VictorBonjour]

Dear Benedek, dear Luke,

I am using your marvelous library to communicate with some devices on Linux (raspbian). While doing so I noticed that my device would only reply when the buffer was filled with Device.GetMaxOutputReportLength() bytes. In my case, this value is 32. So if I am writting only 8 bytes to the device, I would have to pad my frame with 24 extra bytes (zeros) so the buffer would be flushed.

I took a look at your code and this is the fix I propose. Commit is 87ad1d3 - RELEASE 2.1.0 (May 4, 2019) File is HidSharp\Platform\SysHidStream.cs, line 112 Replace byte[] packet = new byte[count]; with byte[] packet = new byte[maxOutputReportLength];

Please find patch file attached. ZeroPaddingLinux.patch.zip

This fixes the issue on my Linux system. Bug is not present on Windows.

Thank you for your work. I remain available in case of questions. Victor

[benedekkupper]

Hi Victor,

Thanks for bringing this issue to our attention. The library itself isn't our work, the base is the mirror of James F. Bellinger's library.

Before I jump into merging this (which would be better filed as a pull request than an issue), can you explain to me the HID device's reports layout? Does this device have multiple OUT reports? What's the size of the OUT report that you're trying to set? I hope you are aware that HID requires the full length of any report to be transferred, partial report updates aren't supported.

[VictorBonjour]

Hi Benedek,

I may be a bit confused here, please bear with me.

I'm using only one OUT report and its length is 32. I might have mistakenly imagined that I could send only 8 bytes and that HidSharp would zero-pad it to 32 bytes for me. If I read you well, this is not the expected behavior.

I'm a bit confused because on Windows, I can send my 8 bytes to HidSharp and it will send 32 bytes to my device regardless. Have a look at WinHidStream.Write(byte[] buffer, int offset, int count), line 164:

int minOut = Device.GetMaxOutputReportLength();
if (minOut <= 0) { throw new IOException("Can't write to this device."); }
if (_writeBuffer == null || _writeBuffer.Length < Math.Max(count, minOut)) { Array.Resize(ref _writeBuffer, Math.Max(count, minOut)); }
Array.Copy(buffer, offset, _writeBuffer, 0, count);

If the actual message length ("count", in my example this would be 8) is less than the report length ("minOut", in my example 32), then the write buffer is 32. Perhaps this is not a behavior I should rely on?

Essentially I'm confused because the behavior is different on Windows and Linux. Cheers Victor

PS: Sorry for not making a pull request, will do next time!

[benedekkupper]

Hi Victor,

Yes, it seems that the windows behavior is the faulty one, I will verify and correct it to only accept full size report buffers. Thank you for bringing this to our attention.