keyboard emulator device behavior on ubuntu - ruby

Question

I'm building a device driver of sorts that consumes data from a keyboard emulating device.
The device is a card swipe, so its behavior is as follows:
User walks up, swipes card
I get a string of characters (key codes, really, including modifier keys for capital letters)
I don't know how many characters I'm going to get
I don't know when I'm getting something
Since I don't know how many characters I'm going to get, blocking reads on the keyboard tty aren't useful - I'd end up blocking after the last character. What I'm doing is, in Ruby, using the IO module to perform async reads against the keyboard device, and using a timeout to determine that the end of data was reached. This works fine logically (even a user swiping his or her card fast will do so slower than the send rate between characters).
The issue is that sometimes, I lose data from the middle of the string. My hunch is that there's some sort of buffer overflow happening because I'm reading the data too slowly. Trying to confirm this, I inserted small waits in between each key process. Longer waits (20ms+) do exacerbate the problem. However, a wait of around 5ms actually makes it go away? The only explanation I can come up with is that the async read itself is expensive (because Ruby), and doing them without a rate limit is actually slower than doing them with a 5ms delay.
Does this sound rational? Are there other ideas on what this could be?
The ruby is actually JRuby 9000. The machine is Ubuntu LTS 16.
Edit: here's a snippet of the relevant code
private def read_swipe(buffer_size, card_reader_input, pause_between_reads, seconds_to_complete)
limit = Time.now + seconds_to_complete.seconds
swipe_data = ''
begin
start_time = Time.now
sleep pause_between_reads
batch = card_reader_input.read_nonblock(buffer_size)
swipe_data << batch
rescue IO::WaitReadable
IO.select([card_reader_input], nil, nil, 0.5)
retry unless limit < start_time
end while start_time < limit
swipe_data
end
where card_reader_input = File.new(event_handle, 'rb')

Answer 1

I am not sure about Ruby code but you can use linux sysfs to access the characters coming out of keyboard 'like' device, and if feasible you can call C code from ruby application. I had done this for barcode reader and following is the code:
static int init_barcode_com(char* bcr_portname)
{
int fd;
/* Open the file descriptor in non-blocking mode */
fd = open(bcr_portname, O_RDONLY | O_NOCTTY | O_NDELAY);
cout << "Barcode Reader FD: " << fd <<endl;
if (fd == -1)
{
cerr << "ERROR: Cannot open fd for barcode communication with error " << fd <<endl;
}
fcntl(fd, F_SETFL, 0);
/* Set up the control structure */
struct termios toptions;
/* Get currently set options for the tty */
tcgetattr(fd, &toptions);
/* Set custom options */
/* 9600 baud */
cfsetispeed(&toptions, B9600);
cfsetospeed(&toptions, B9600);
/* 8 bits, no parity, no stop bits */
toptions.c_cflag &= ~PARENB;
toptions.c_cflag &= ~CSTOPB;
toptions.c_cflag &= ~CSIZE;
toptions.c_cflag |= CS8;
/* no hardware flow control */
toptions.c_cflag &= ~CRTSCTS;
/* enable receiver, ignore status lines */
toptions.c_cflag |= CREAD | CLOCAL;
/* disable input/output flow control, disable restart chars */
toptions.c_iflag &= ~(IXON | IXOFF | IXANY);
/* disable canonical input, disable echo,
* disable visually erase chars,
* disable terminal-generated signals */
toptions.c_lflag &= ~(ICANON | ECHO | ECHOE | ISIG);
/* disable output processing */
toptions.c_oflag &= ~OPOST;
/* wait for n (in our case its 1) characters to come in before read returns */
/* WARNING! THIS CAUSES THE read() TO BLOCK UNTIL ALL */
/* CHARACTERS HAVE COME IN! */
toptions.c_cc[VMIN] = 0;
/* no minimum time to wait before read returns */
toptions.c_cc[VTIME] = 100;
/* commit the options */
tcsetattr(fd, TCSANOW, &toptions);
/* Wait for the Barcode to reset */
usleep(10*1000);
return fd;
}
static int read_from_barcode_reader(int fd, char* bcr_buf)
{
int i = 0, nbytes = 0;
char buf[1];
/* Flush anything already in the serial buffer */
tcflush(fd, TCIFLUSH);
while (1) {
nbytes = read(fd, buf, 1); // read a char at a time
if (nbytes == -1) {
return -1; // Couldn't read
}
if (nbytes == 0) {
return 0;
}
if (buf[0] == '\n' || buf[0] == '\r') {
return 0;
}
bcr_buf[i] = buf[0];
i++;
}
return 0;
}
Now that you do not know how many characters your going to get you can use VMIN and VTIME combination to address your concern. This document details various possibilities with VMIN and VTIME.