MQTT-C (C23)

A small, portable MQTT 3.1.1 client library implemented in strict C23, with examples and tests.

Highlights

• Strict C23 codebase (nullptr, auto, constexpr) with -Wall -Wextra -Wpedantic -Werror.
• Single public header: include/mqtt/mqtt.h.
• Platform abstraction in include/mqtt/mqtt_pal.h, with an override hook.
• Examples for TCP, reconnects, and TLS (OpenSSL).
• Thread-safe API suitable for single-threaded and multi-threaded clients.

Requirements

• A C23-capable compiler (-std=c23 or -std=c2x).
• POSIX sockets and pthreads for the default PAL and examples.
• OpenSSL and pkg-config for the TLS example.
• Optional: just or Zig for build convenience.

Build With just

just
just test
just build-examples

Debug with sanitizers:

DEBUG=1 just test

Outputs land in bin/.

Build With Zig

zig build
zig build test
zig build examples

Enable sanitizers in Debug builds:

zig build -Doptimize=Debug -Dsanitize=true

Outputs land in zig-out/bin/.

Quick Usage Sketch

#include "mqtt/mqtt.h"

int main() {
  struct mqtt_client client;
  uint8_t sendbuf[2'048];
  uint8_t recvbuf[1'024];
  mqtt_pal_socket_handle sockfd = /* open socket */;

  mqtt_init(&client, sockfd, sendbuf, sizeof(sendbuf), recvbuf, sizeof(recvbuf),
            nullptr);
  mqtt_connect(&client, nullptr, nullptr, nullptr, 0, nullptr, nullptr,
               MQTT_CONNECT_CLEAN_SESSION, 400);

  while (true) {
    mqtt_sync(&client);
  }
}

See examples/ for complete programs using POSIX or OpenSSL sockets.

Tests

• just test or zig build test runs testing/tests.c.
• Tests and examples default to test.mosquitto.org and require network access.

Platform Abstraction

• The default PAL lives in include/mqtt/mqtt_pal.h and src/mqtt_pal.c.
• To supply your own PAL, define MQTTC_PAL_FILE to a header that provides the required types, macros, and function declarations:

cc -DMQTTC_PAL_FILE=my_mqtt_pal.h ...

Directory Layout

• include/mqtt/mqtt.h public API.
• src/mqtt.c core client.
• src/mqtt_pal.c platform helpers.
• examples/ example clients.
• testing/tests.c unit and protocol tests.

License MIT. See LICENSE.