Re: Passing USB file descriptor to Android NDK program

[Philippe Simons]

what about http://developer.android.com/guide/topics/connectivity/usb/index.html ?

or http://developer.android.com/tools/adk/index.html 

On Thu, Jun 6, 2013 at 3:50 PM, Tim Cooper <timothy.an...@gmail.com> wrote:

I am trying to port some software written in C to the Android platform.  This
software has a component that reads and writes from and to a connected USB
device.  What I am trying to do is open up a connection to the device in Java,
then pass the file descriptor for the USB device(s) to the JNI code.

Below is the (relevant) output of `lsof` for my app which shows I have two
descriptors for the USB device:


    com.tim    8861     u0_a66   35       ???                ???       ???        ??? /dev/bus/usb/001/002
    com.tim    8861     u0_a66   36       ???                ???       ???        ??? socket:[51170]
    com.tim    8861     u0_a66   37       ???                ???       ???        ??? socket:[51173]
    com.tim    8861     u0_a66   38       ???                ???       ???        ??? /dev/bus/usb/001/003


I've passed both descriptors (above as 35 and 38) to my native method, but when
I try and write to either of the file descriptors, `write()` returns `-1`, and
I get an `EINVAL` error.

Here is the body of my native method:

    char buff[1024] = {0};
    jsize len = (*env)->GetArrayLength(env, fds);
    jint *arr = (*env)->GetIntArrayElements(env, fds, 0);
    int i;
   
    char data[4] = {
        0x09,
        0x90,
        0x50,
        0x50,
    };

    for (i = 0; i < len; i++) {
        int wrote = write(arr[i], data, 4);

        int flags = fcntl(arr[i], F_GETFL);
        char *err = strerror(errno);
        sprintf(buff, "%sFD: %d  \n"
        "wrote: %d  \n"
        "(err: %d %s)  \n"
        "flags: %d  \n"
        "NBIO %d  \n"
        "readonly %d  \n"
        "writeonly %d  \n"
        "both %d  \n"
        "append %d  \n"
        "large file %d  \n\n", buff, arr[i], wrote, errno, err, flags, flags & O_NONBLOCK,
            flags & O_RDONLY, flags & O_WRONLY, flags & O_RDWR, flags & O_APPEND,
            flags & O_LARGEFILE);
    }
    return (*env)->NewStringUTF(env, buff);

The string that is returned when invoking that method is:

    FD: 35 
    wrote: -1 
    (err: 22 Invalid argument) 
    flags: 32770 
    NBIO 0 
    readonly 0 
    writeonly 0 
    both 2 
    append 0 
    large file 32768 

    FD: 38 
    wrote: -1 
    (err: 22 Invalid argument) 
    flags: 32770 
    NBIO 0 
    readonly 0 
    writeonly 0 
    both 2 
    append 0 
    large file 32768

Writing to the USB device does work through Java, so it appears to just be an
issue when trying to do it via native code.

Does anyone have any experience doing something like this? Any input would be
great.

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[Philippe Simons]

the EINVAL is probably because you are trying to write to a file you dont have the permission for.

[Tim Cooper]

The permissions don't transfer to native code after you've been granted permission from Java code?  That seems unlikely, but I don't know enough about the platform to make any assumptions about it. Any idea on a possible work around?

[Tim Cooper]

I was able to work around this by using ioctl() instead of write().  Details here:
http://stackoverflow.com/a/17046674/142162