Code:

static uchar buffer[64];
static uchar currentPosition, bytesRemaining;
 
usbMsgLen_t usbFunctionSetup(uchar setupData[8])
{
    usbRequest_t *rq = (void *)setupData;   // cast to structured data for parsing
    switch(rq->bRequest){
    case VENDOR_RQ_WRITE_BUFFER:
        currentPosition = 0;                // initialize position index
        bytesRemaining = rq->wLength.word;  // store the amount of data requested
        if(bytesRemaining > sizeof(buffer)) // limit to buffer size
            bytesRemaining = sizeof(buffer);
        return USB_NO_MSG;        // tell driver to use usbFunctionWrite()
    }
    return 0;                               // ignore all unknown requests
}
 
uchar usbFunctionWrite(uchar *data, uchar len)
{
    uchar i;
    if(len > bytesRemaining)                // if this is the last incomplete chunk
        len = bytesRemaining;               // limit to the amount we can store
    bytesRemaining -= len;
    for(i = 0; i < len; i++)
        buffer[currentPosition++] = data[i];
    return bytesRemaining == 0;             // return 1 if we have all data
}