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I need to test a serial port application on Linux, however, my test machine only has one serial port.

Is there a way to add a virtual serial port to Linux and test my application by emulating a device through a shell or script?

Note: I cannot remap the port, it hard coded on ttys2 and I need to test the application as it is written.

Kara
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JeffV
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10 Answers10

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Complementing the @slonik's answer.

You can test socat to create Virtual Serial Port doing the following procedure (tested on Ubuntu 12.04):

Open a terminal (let's call it Terminal 0) and execute it:

socat -d -d pty,raw,echo=0 pty,raw,echo=0

The code above returns:

2013/11/01 13:47:27 socat[2506] N PTY is /dev/pts/2
2013/11/01 13:47:27 socat[2506] N PTY is /dev/pts/3
2013/11/01 13:47:27 socat[2506] N starting data transfer loop with FDs [3,3] and [5,5]

Open another terminal and write (Terminal 1):

cat < /dev/pts/2

this command's port name can be changed according to the pc. it's depends on the previous output.

2013/11/01 13:47:27 socat[2506] N PTY is /dev/pts/**2**
2013/11/01 13:47:27 socat[2506] N PTY is /dev/pts/**3**
2013/11/01 13:47:27 socat[2506] N starting data transfer loop with FDs

you should use the number available on highlighted area.

Open another terminal and write (Terminal 2):

echo "Test" > /dev/pts/3

Now back to Terminal 1 and you'll see the string "Test".

nalakastar
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cantoni
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  • This worked better for me than slonik's answer, because it auto-assigns to virtual COM port files, and doesn't echo. – gbmhunter Jan 08 '14 at 04:17
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    If you want reproducible filename use `link=/path/to/link` after each device-declaration (after echo=0). It can thus be used in automated tests. (as slonik does in their answer) – Patrick B. Aug 09 '17 at 20:31
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    To create a pty that links to a real serial port: `socat -d -d pty,raw,echo=0 /dev/ttyUSB5,raw,echo=0`. – Penghe Geng Aug 31 '18 at 15:28
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    can i create a serial port with names like `/dev/ttyS0` instead of `/dev/pts/1` ? – mrid Apr 29 '20 at 10:35
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    I tried this but I can only open it once, (using c# SerialPort) eventhough a I dispose before trying to connect with a new instance it always crashes on the open – trampster Jan 24 '22 at 07:27
  • @trampster did you find any alternative for that issue at the end? – eglasius Mar 20 '23 at 11:13
86

You can use a pty ("pseudo-teletype", where a serial port is a "real teletype") for this. From one end, open /dev/ptyp5, and then attach your program to /dev/ttyp5; ttyp5 will act just like a serial port, but will send/receive everything it does via /dev/ptyp5.

If you really need it to talk to a file called /dev/ttys2, then simply move your old /dev/ttys2 out of the way and make a symlink from ptyp5 to ttys2.

Of course you can use some number other than ptyp5. Perhaps pick one with a high number to avoid duplicates, since all your login terminals will also be using ptys.

Wikipedia has more about ptys: http://en.wikipedia.org/wiki/Pseudo_terminal

Nikola K.
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apenwarr
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    On linux you can use the openpty / forkpty system calls. See [man page](http://www.manpagez.com/man/3/openpty/) – Matthew Smith Oct 27 '08 at 05:25
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    how to create a virtual serial port pair by using command line tool? – linjunhalida Jan 20 '10 at 02:53
  • use chroot to to fake device names without affecting the system – Dima Tisnek Apr 03 '12 at 07:08
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    note that many serial port parameters, e.g. baudrate, parity, hw flow control, character size (?) are not implemented in pty, it is thus impossible to test your application in presence of serial transmission errors. – Dima Tisnek Apr 03 '12 at 07:10
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    This is helpful, but it describes the "old style" BSD pseudo-terminals. The "new style" UNIX 98 pseudo-terminals operate a bit differently—see [`pts` man page](http://linux.die.net/man/4/pts) for details. – Craig McQueen Aug 21 '12 at 03:16
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    The advantage of using old-style ptys in this question is he really wanted one to be named /dev/ttys2. Old-style ptys can be renamed easily, new style ones not so much. Normal people should use new-style ptys as you say. – apenwarr Aug 22 '12 at 04:18
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    @MattSmith: do you actually have a working example for this with openpty? Just calling it is not enough to bind the one's output to the other's input, and vice versa... – László Papp Nov 16 '13 at 21:33
  • @LaszloPapp I got it working by following up the man pages for openpty and forkpty. I no longer have access to the source code as I have moved on from that company. – Matthew Smith Nov 19 '13 at 06:12
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    @MattSmith: as I already wrote, openpty and forkpty are not enough on their own to establish a loopback! It is kind of strange, you do not even know the theory anymore... Anyway, I do not recommend this to others because I spent one full day without figuring out how to do it. It was a time waste. – László Papp Nov 19 '13 at 06:20
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    @LaszloPapp I do apologise, I was lying the whole time – Matthew Smith Nov 20 '13 at 01:58
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    @DimaTisnek I think it's about time for that to change! Just googling 'linux virtual serial port' to make sure I'm not off my rocker and re-writing the wheel. I think I'm going to add a `CVSERIAL` flag to the termios `c_cflag` field (lots of bits currently unused) as a switch to turn it on and off (from the master side, of course) and when set, the slave and master will have all of the modem control ioctls, except that the master will do all RI, CD, etc. lines. I guess we'll see if I can get it past the maintainers then. – Daniel Santos Mar 24 '20 at 23:48
  • @DanielSantos sounds like a plan. Perhaps, depending on what you want to virtualise, such virtual serial "hardware" should be an own module... For example if you wanted to simulate baud rate mismatch. In any case, a flag sounds like a good way to start hacking! – Dima Tisnek Mar 25 '20 at 01:42
  • My pts terminal always dissapears on a first keypress- when connected with serial library from python. – jaromrax Mar 25 '21 at 17:39
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Use socat for this:

For example:

socat PTY,link=/dev/ttyS10 PTY,link=/dev/ttyS11
Ahsan
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slonik
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  • This worked well for me, tested with minicom! Seems as though the input to one terminal gets echoed to both (so it will reappear on the input terminal also). – gbmhunter Jan 08 '14 at 04:09
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    I don't have the same echo behavior ... minicom has a "local echo" feature... but when it is disabled, it works exactly as a real serial port would. thanks for the tip. – cptHammer Jun 09 '16 at 14:46
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    This doesn't appear to work without special permissions: ``` 2021/12/10 16:08:47 socat[90013] E unlink("/dev/ttyS10"): Permission denied ``` – Raleigh L. Dec 11 '21 at 00:09
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There is also tty0tty http://sourceforge.net/projects/tty0tty/ which is a real null modem emulator for linux.

It is a simple kernel module - a small source file. I don't know why it only got thumbs down on sourceforge, but it works well for me. The best thing about it is that is also emulates the hardware pins (RTC/CTS DSR/DTR). It even implements TIOCMGET/TIOCMSET and TIOCMIWAIT iotcl commands!

On a recent kernel you may get compilation errors. This is easy to fix. Just insert a few lines at the top of the module/tty0tty.c source (after the includes):

#ifndef init_MUTEX
#define init_MUTEX(x) sema_init((x),1)
#endif

When the module is loaded, it creates 4 pairs of serial ports. The devices are /dev/tnt0 to /dev/tnt7 where tnt0 is connected to tnt1, tnt2 is connected to tnt3, etc. You may need to fix the file permissions to be able to use the devices.

edit:

I guess I was a little quick with my enthusiasm. While the driver looks promising, it seems unstable. I don't know for sure but I think it crashed a machine in the office I was working on from home. I can't check until I'm back in the office on monday.

The second thing is that TIOCMIWAIT does not work. The code seems to be copied from some "tiny tty" example code. The handling of TIOCMIWAIT seems in place, but it never wakes up because the corresponding call to wake_up_interruptible() is missing.

edit:

The crash in the office really was the driver's fault. There was an initialization missing, and the completely untested TIOCMIWAIT code caused a crash of the machine.

I spent yesterday and today rewriting the driver. There were a lot of issues, but now it works well for me. There's still code missing for hardware flow control managed by the driver, but I don't need it because I'll be managing the pins myself using TIOCMGET/TIOCMSET/TIOCMIWAIT from user mode code.

If anyone is interested in my version of the code, send me a message and I'll send it to you.

HoldOffHunger
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Peter Remmers
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    I'd be interested to see your code. Can you contribute it back to the tty0tty project? However, I'd prefer to see people improve the pseudo-terminal code in the Linux kernel. E.g. add hardware handshaking support and TIOCMIWAIT. – Craig McQueen Mar 20 '13 at 09:47
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    "If anyone is interested in my version of the the code, send me a message and I'll send it to you." Yes, I'm interested! Can you point to it somewhere, e.g. on GitHub? – Craig McQueen Jul 01 '13 at 02:26
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    I uploaded the driver to: https://github.com/pitti98/nullmodem Sorry it took so long to reply. I'm not very active on stackoverflow and overlooked your comment! – Peter Remmers Nov 13 '13 at 20:57
  • No, I wrote it because I needed it and stopped once it was good enough to do what I wanted. Now that it's public, I hope it's useful for someone else, and maybe someone picks up where I left it. – Peter Remmers Nov 16 '13 at 21:16
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You may want to look at Tibbo VSPDL for creating a linux virtual serial port using a Kernel driver -- it seems pretty new, and is available for download right now (beta version). Not sure about the license at this point, or whether they want to make it available commercially only in the future.

There are other commercial alternatives, such as http://www.ttyredirector.com/.

In Open Source, Remserial (GPL) may also do what you want, using Unix PTY's. It transmits the serial data in "raw form" to a network socket; STTY-like setup of terminal parameters must be done when creating the port, changing them later like described in RFC 2217 does not seem to be supported. You should be able to run two remserial instances to create a virtual nullmodem like com0com, except that you'll need to set up port speed etc in advance.

Socat (also GPL) is like an extended variant of Remserial with many many more options, including a "PTY" method for redirecting the PTY to something else, which can be another instance of Socat. For Unit tets, socat is likely nicer than remserial because you can directly cat files into the PTY. See the PTY example on the manpage. A patch exists under "contrib" to provide RFC2217 support for negotiating serial line settings.

7

Using the links posted in the previous answers, I coded a little example in C++ using a Virtual Serial Port. I pushed the code into GitHub: https://github.com/cymait/virtual-serial-port-example .

The code is pretty self explanatory. First, you create the master process by running ./main master and it will print to stderr the device is using. After that, you invoke ./main slave device, where device is the device printed in the first command.

And that's it. You have a bidirectional link between the two process.

Using this example you can test you the application by sending all kind of data, and see if it works correctly.

Also, you can always symlink the device, so you don't need to re-compile the application you are testing.

Mauro Ciancio
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    while (read(fd, &inputbyte, 1) == 1) { ... } read is undefined in your code. write is undefined. close is undefined. – Mattis Asp Jan 04 '18 at 09:35
  • @MattisAsp has a point, you should at least include ``. Plus there's a bunch of should-not-ignore-warnings fixable stuffs but overall simple code as I like. – Yves Lhuillier Nov 10 '20 at 09:58
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$ socat -d -d pty,link=/tmp/vserial1,raw,echo=0 pty,link=/tmp/vserial2,raw,echo=0

Will generate symlinks of /tmp/vserial1 and /tmp/vserial2 for generated virtual serial ports in /dev/pts/*

Resource

Sadeesh Kalhara
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Would you be able to use a USB->RS232 adapter? I have a few, and they just use the FTDI driver. Then, you should be able to rename /dev/ttyUSB0 (or whatever gets created) as /dev/ttyS2 .

Howler
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5

I can think of three options:

Implement RFC 2217

RFC 2217 covers a com port to TCP/IP standard that allows a client on one system to emulate a serial port to the local programs, while transparently sending and receiving data and control signals to a server on another system which actually has the serial port. Here's a high-level overview.

What you would do is find or implement a client com port driver that would implement the client side of the system on your PC - appearing to be a real serial port but in reality shuttling everything to a server. You might be able to get this driver for free from Digi, Lantronix, etc in support of their real standalone serial port servers.

You would then implement the server side of the connection locally in another program - allowing the client to connect and issuing the data and control commands as needed.

It's probably non trivial, but the RFC is out there, and you might be able to find an open source project that implements one or both sides of the connection.

Modify the linux serial port driver

Alternately, the serial port driver source for Linux is readily available. Take that, gut the hardware control pieces, and have that one driver run two /dev/ttySx ports, as a simple loopback. Then connect your real program to the ttyS2 and your simulator to the other ttySx.

Use two USB<-->Serial cables in a loopback

But the easiest thing to do right now? Spend $40 on two serial port USB devices, wire them together (null modem) and actually have two real serial ports - one for the program you're testing, one for your simulator.

-Adam

Adam Davis
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    Actually the null modem USB UART cables seem like quite an elegant solution to me as it supports both local testing (get an USB hub if you are short on ports) and remote debugging. – Maxthon Chan Oct 20 '14 at 16:31
  • I haven't reviewed it's quality, but [ttynvt](https://gitlab.com/lars-thrane-as/ttynvt) implements RFC 2217 via Linux FUSE – Daniel Santos Mar 24 '20 at 23:55
2

Combining all other amazingly useful answers, I found the below command to be VERY useful for testing on different types of Linux distros where there's no guarantee you're going to get the same /dev/pts/#'s every time and/or you need to test multiple psuedo serial devices and connections at once.

parallel 'i="{1}"; socat -d -d pty,raw,echo=0,link=$HOME/pty{1} pty,raw,echo=0,link=$HOME/pty$(($i+1))' ::: $(seq 0 2 3;)

Breaking this down:

parallel runs the same command for each argument supplied to it. So for example if we run it with the --dryrun flag it gives us:

i="0"; socat -d -d pty,raw,echo=0,link=$HOME/pty0 pty,raw,echo=0,link=$HOME/pty$(($i+1))
i="2"; socat -d -d pty,raw,echo=0,link=$HOME/pty2 pty,raw,echo=0,link=$HOME/pty$(($i+1))

This is due to the $(seq x y z;) at the end, where x = start #, y = increment by, and z = end # (or # of devices you need to spawn)

parallel 'i="{1}"; echo "make psuedo_devices {1} $(($i+1))"' ::: $(seq 0 2 3;)

Outputs:

make psuedo_devices 0 1
make psuedo_devices 2 3

Gathering all this together the final above command symlinks the proper psuedo devices together regardless of whats in /dev/pts/ to whatever directory supplied to socat via the link flag.

pstree -c -a $PROC_ID gives:

perl /usr/bin/parallel i="{1}"; socat -d -d pty,raw,echo=0,link=$HOME/pty{1} pty,raw,echo=0,link=$HOME/pty$(($i+1)) ::: 0 2
  ├─bash -c i="0"; socat -d -d pty,raw,echo=0,link=$HOME/pty0 pty,raw,echo=0,link=$HOME/pty$(($i+1))
  │   └─socat -d -d pty,raw,echo=0,link=/home/user/pty0 pty,raw,echo=0,link=/home/user/pty1
  └─bash -c i="2"; socat -d -d pty,raw,echo=0,link=$HOME/pty2 pty,raw,echo=0,link=$HOME/pty$(($i+1))
      └─socat -d -d pty,raw,echo=0,link=/home/user/pty2 pty,raw,echo=0,link=/home/user/pty3

ls -l $HOME/pty* yield:

lrwxrwxrwx 1 user user 10 Sep  7 11:46 /home/user/pty0 -> /dev/pts/4
lrwxrwxrwx 1 user user 10 Sep  7 11:46 /home/user/pty1 -> /dev/pts/6
lrwxrwxrwx 1 user user 10 Sep  7 11:46 /home/user/pty2 -> /dev/pts/7
lrwxrwxrwx 1 user user 10 Sep  7 11:46 /home/user/pty3 -> /dev/pts/8

This was all because I was trying running tests against a platform where I needed to generated a lot of mach-serial connections and to test their input/output via containerization (Docker). Hopefully someone finds it useful.

Dan2theR
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