One purpose of this project is to show how to use mosquitto.
With the provided Dockerfile and sample codes, one can easily set up a working environment to start testing mosquitto and quickly prototyping solutions (Test the C API instead of using mosquitto_sub/_pub tools, ...).
However, the main purpose is to experiment security mechanisms which can allow to authenticate clients and/or secure communication between clients and the broker.
For more details, see:
- Introduction to MQTT Security Mechanisms
- MQTTS : Comment utiliser MQTT avec TLS ?
- mosquitto-tls man page
- OpenSSL Quick Reference Guide
docker build -t mosquitto-image ci/
docker run --privileged -it -u $(id -u) --rm -v $(pwd):/workdir mosquitto-image:latest
mkdir build && cd build
cmake .. -DCMAKE_INSTALL_PREFIX=./out && make && make install
| Which bash terminal? | Commands |
|---|---|
| #1 | mosquitto -v -c out/etc/<example_directory_name>/mosquitto.conf |
| #2 | out/etc/<example_directory_name>/subscriber <args> |
| #3 | out/etc/<example_directory_name>/publisher <args> |
A "semaphore + signal handler" is used to handle the way the process is stopped.
Why a semaphore?
- sem_post() is async-signal-safe meaning it can be safely called inside a signal handler
- mosquitto_loop_forever() is stopped when mosquitto_disconnect() is called. mosquitto_disconnect() seems not reentrant (internal use of malloc()/free() and printf-family functions, ...) so it is not safe to call it inside a handler
Other solutions exist. For example:
- Make the signal handler only update an atomic variable of type "volatile sig_atomic_t" then unblock the main program
- Use signalfd as an alternative to the use of a signal handler
Also note that
- Errno is always saved and restored by the signal handler to make sure it will not cause
any unexpected behaviour.
For example, the following scenario may occur:
- A function call sets errno value
- That function call gets interrupted by the signal handler
- Back to the function call which checks errno value
To improve readability and keep focused on the purpose of this project, no error checking is performed.