Fake serial communication under Linux

Question

I have an application where I want to simulate the connection between a device and a \"modem\". The device will be connected to a serial port and will talk to the software m

Answer 1

You are on the right track with pseudo-terminals. To do this, your mock software device needs to first open a pseudo-terminal master - this is the file descriptor it will read from and write to, when it is talking to the serial software that you're testing. It then needs to grant access to and unlock the pseudo-terminal slave, and obtain the name of the slave device. It should then print out the name of the slave device somewhere, so that you can tell the other software to open that as it's serial port (ie. that software will be opening a name like /dev/pts/0 instead of /dev/ttyS1).

The simulator software then just reads and writes from the master side of the pseudoterminal. In C, it would look like this:

#define _XOPEN_SOURCE
#include <stdlib.h>
#include <stdio.h>
#include <fcntl.h>

int main(int argc, char *argv[])
{
    int pt;

    pt = open("/dev/ptmx", O_RDWR | O_NOCTTY);
    if (pt < 0)
    {
        perror("open /dev/ptmx");
        return 1;
    }

    grantpt(pt);
    unlockpt(pt);

    fprintf(stderr, "Slave device: %s\n", ptsname(pt));

    /* Now start pretending to be a modem, reading and writing "pt" */
    /* ... */
    return 0;
}

Hopefully that is easy enough to convert to Python.

Answer 2

It's probably best to use pyserial to communicate with the serial port, and you can just create a mock version of the serial.Serial class which implements read, readline, write and any other methods you need.

Answer 3

Here's pythonic version of pts-emulated (caf's) serial communication:

from serial import Serial

driver = MyDriver()  # what I want to test
peer = serial.Serial()
driver.port.fd, peer.fd = posix.openpty()
driver.port._reconfigurePort()
peer.setTimeout(timeout=0.1)
peer._reconfigurePort()
driver.start()

# peer.write("something")
# driver.get_data_from_serial()

It has some advantages over mocking Serial, namely that Serial code is used and some serial port artefacts are exercised.

If you want to test opening of serial ports, you could swap master and slave around and use os.ttyname(salve_fd) as serial port name. I can't vouch for side-effects of swapping master and slave around though. Most notable is that you can close and reopen slave, but fi you close master slave dies too.

This works like a charm if your test code runs within same process. I didn't iron out the kinks with multiple/separate processes yet.