Linux虚拟串口修改终端属性无变化？不同波特率仍通信的原因

Your Setup Recap #

You created virtual serial ports using:

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

Which assigned /dev/pts/1 and /dev/pts/2. Then you configured each side with conflicting serial settings:

• Thread 1 (connected to /dev/pts/1): Set baud rate to B115200
• Thread 2 (connected to /dev/pts/2): Set baud rate to B9600

When you sent data via write() on one side and read it via read() on the other, communication worked flawlessly—even when modifying other serial attributes. You’re wondering why this doesn’t fail like it would on real hardware, and if virtual ports skip simulating real serial behavior.

The Short Answer #

socat doesn’t emulate physical serial port electrical behavior. It creates a raw byte-stream pipe between two pseudo-terminals (PTYs), not a true simulation of UART hardware. All serial configuration parameters (baud rate, parity, stop bits, flow control) are ignored by the underlying pipe—data is passed directly from one PTY to the other as raw bytes, regardless of how you configure the port settings.

The Longer Explanation #

Let’s break down the key differences between real serial ports and socat’s virtual pairs:

1. Physical Serial Ports vs. Socat PTYs

   • On real hardware, baud rate defines the clock speed for transmitting/receiving bits. Mismatched rates cause clock synchronization errors, leading to garbled data or complete communication failure.
   • Socat’s virtual ports are just wrapper interfaces around a bidirectional byte pipe. When you open() and configure /dev/pts/1, you’re setting parameters for the PTY’s terminal line discipline—but the data sent through write() is immediately pushed to the connected PTY (/dev/pts/2) without any processing related to serial hardware rules.

2. What Your termios Settings Actually Do
   The serial configuration you applied affects how the PTY interacts with your application, not the data transfer between the two ports:

   • CLOCAL and CREAD enable the port for reading/writing, but this is a terminal setting, not a hardware requirement.
   • CRTSCTS would enable hardware flow control on a real port, but on socat PTYs, it’s ignored because there’s no physical hardware to signal.
   • Baud rate settings don’t impact the pipe’s byte transfer speed at all.

3. How to Test Real Serial Behavior
   If you want to replicate scenarios where mismatched baud rates cause failures, use a tool that emulates physical UART characteristics:

   • tty0tty: A kernel module that creates virtual serial port pairs with partial UART emulation, including baud rate checking.
   • Physical serial ports connected with a null-modem cable: This is the most accurate way to replicate real-world serial communication issues.

Quick Verification Test #

Try sending a longer string (e.g., Hello, this is a test of virtual serial port behavior!) via your setup. You’ll notice every byte arrives exactly as sent, no matter the baud rate. On real hardware with mismatched rates, you’d see corrupted characters or missing data.

内容的提问来源于stack exchange，提问作者hillel