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gary-rowe / hid4java

A cross-platform Java Native Access (JNA) wrapper for the libusb/hidapi library. Works out of the box on Windows/Mac/Linux.
MIT License
229 stars 71 forks source link

hid write not sending correct number of bytes #47

Closed bphagan closed 8 years ago

bphagan commented 8 years ago

I just started working on a project to communicate with a hid device. Since the Java app needs to run on Windows and Linux(maybe Mac down the road), I thought hid4java would be a good fit. I was able to get everything working and I can connect to my device. The issue I have and can't figure out is: When I send a 16 byte packet plus 1 byte report ID set to 0x01, the return bytes written length is 1025. I can't seem to understand why I am getting 1025 vs 17. This is causing my hid read to be too big and after the initial few bytes, wrong data. I am running on Windows 7 x64.

To be more specific, I put a print statement right after this call in HidApi: int res = hidApiLibrary.hid_write(device.ptr(), report, report.buffer.length); res is equal to 1025 after the call. report.buffer.length was equal to 17 right before the call.

Any idea why I may be getting a different return value?

Thanks, Brett

gary-rowe commented 8 years ago

It might be an artifact of an off-by-one error. Can you try using the develop branch of the library, and try setting HidApi.dropReportIdZero = true; before opening a device on Windows 7 only. There is a bug which could cause a report ID to be included in the packet that gets sent that could be confusing the device into returning different values.

bphagan commented 8 years ago

To clarify, I am using Report ID = 0x01 which the device is expecting. The part that is not behaving is the return values from hid write and hid read. I have length set to 17 which includes the ReportID. The value returned from the hid write is 1025. When I perform a 2 hid reads, I get the data expected in both for the command sent in write. So, it appears I have a working system, but was thrown off by the write return value.

Thanks.

gary-rowe commented 8 years ago

Did you find out what the problem was? I couldn't offer up any more help since I didn't have a means to replicate your environment.