lcm - Lightweight Communication and Marshalling

1.5.2 verified Mon Apr 27 auth: no python

lcm (Lightweight Communication and Marshalling) is a library for interprocess communication with real-time constraints, providing a low-latency publish/subscribe message passing model. Version 1.5.2 supports Python >=3.7. Release cadence is irregular, typically yearly.

pip install lcm

Common errors

error ModuleNotFoundError: No module named 'lcm' ↓

cause lcm not installed or installed in wrong environment.

fix

Run: pip install lcm

error AttributeError: module 'lcm' has no attribute 'LCM' ↓

cause Imported as 'import lcm' instead of 'from lcm import LCM'.

fix

Use: from lcm import LCM

error OSError: [Errno 19] No such device ↓

cause LCM tries to open multicast socket but network interface doesn't support multicast or is missing.

fix

Check network configuration, or use LCM with a directed UDP address. On Linux, ensure multicast is enabled: sudo apt-get install net-tools && route -n

Warnings

gotcha lcm requires a running LCM daemon (lcm-logger or similar) or multicast network. Without it, publish/subscribe silently fails. ↓

fix Ensure LCM daemon is running or network multicast is configured. Test with lcm-spy or a simple sender/receiver.

gotcha The LCM class is not thread-safe. Calling methods from multiple threads can cause undefined behavior. ↓

fix Use locking mechanisms or ensure all LCM calls happen from a single thread.

gotcha LCM messages are raw bytes; data serialization is left to the user. There's no built-in type marshalling beyond bytes. ↓

fix Use struct.pack/unpack, protobuf, or similar for structured data. Example: data = struct.pack('i', 42).

Imports

LCM
wrong
```
import lcm
```
correct
```
from lcm import LCM
```
Common mistake: using 'import lcm' gives you the module object, not the class. 'from lcm import LCM' is required to access the main class.
EventLog
```
from lcm import EventLog
```

Quickstart

Initialize LCM, subscribe to a channel, handle incoming messages, and publish a test message.

from lcm import LCM

lc = LCM()

def handler(channel, data):
    print(f"Received on {channel}: {data}")

lc.subscribe("EXAMPLE_CHANNEL", handler)
lc.handle()

lc.publish("EXAMPLE_CHANNEL", b"Hello, world!")