MQTTAsync_message(3)

💖 MQTTAsync_message(3) 💖

okay bestie, welcome to the Library Functions Manual page for MQTTAsync_message — yes that's the section 3 vibe, we're in the C library pages now, no cap. pull up a chair, grab a strawberry 🍓, and let's talk structs.

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✨ NAME ✨

MQTTAsync_message — the struct that holds a message's payload + all its lil attributes 💅

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🌸 SYNOPSIS 🌸

#include <MQTTAsync.h>

🎀 Data Fields

char struct_id [4]
int struct_version
int payloadlen
void * payload
int qos
int retained
int dup
int msgid
MQTTProperties properties

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🦋 DETAILED DESCRIPTION 🦋

So this is a structure representing the payload and attributes of an MQTT message. literally the whole message bundle, glowing, iconic.

🚨 IMPORTANT: the message topic is NOT part of this structure. like, the topic is not here, bestie. it lives elsewhere. go look at MQTTAsync_publishMessage(), MQTTAsync_publish(), MQTTAsync_receive(), MQTTAsync_freeMessage() and MQTTAsync_messageArrived() for that. iykyk.

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👑 FIELD DOCUMENTATION 👑

💜 char struct_id[4]

The eyecatcher for this structure. must be MQTM. that's it, that's the vibe check. if it's not MQTM, something is Wrong™.

💜 int struct_version

The version number of this structure. Must be 0 or 1. 0 indicates no message properties. so if you want properties (see below 👀) you need version 1.

💜 int payloadlen

The length of the MQTT message payload in bytes. just the number, the count, the receipts on how long the payload is. 📏

💜 void* payload

A pointer to the payload of the MQTT message. it points. that's its job. 🫶

💜 int qos

The quality of service (QoS) assigned to the message. there are three levels of QoS and yes we're listing all of them:

• QoS0 — Fire and forget 🔥 the message may not be delivered. no promises. she said what she said.
• QoS1 — At least once ✨ the message will be delivered, but may be delivered more than once in some circumstances. (so, duplicates possible — noted, remember that for dup later 👀)
• QoS2 — Once and one only 💅 the message will be delivered exactly once. main character guarantee, periodt.

💜 int retained

The retained flag serves two purposes depending on whether the message it is associated with is being published or received. plot twist: one flag, two jobs.

retained = true — for messages being published, a true setting indicates that the MQTT server should retain a copy of the message. The message will then be transmitted to new subscribers to a topic that matches the message topic. For subscribers registering a new subscription, the flag being true indicates that the received message is not a new one, but one that has been retained by the MQTT server. 🧚

retained = false — for publishers, this indicates that this message should not be retained by the MQTT server. For subscribers, a false setting indicates this is a normal message, received as a result of it being published to the server. 🌈

💜 int dup

The dup flag indicates whether or not this message is a duplicate. It is only meaningful when receiving QoS1 messages. When true, the client application should take appropriate action to deal with the duplicate message.

⚠️ This is an output parameter only. don't write to it, bestie, it won't do anything for you. 🙈

💜 int msgid

The message identifier is reserved for internal use by the MQTT client and server. It is an output parameter only — writing to it will serve no purpose. It contains the MQTT message id of an incoming publish message. so like, it's giving "look but don't touch" energy. 💀

💜 MQTTProperties properties

The MQTT V5 properties associated with the message. fancy, modern, elevated. ⭐

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🌟 AUTHOR 🌟

Generated automatically by Doxygen for the Paho Asynchronous MQTT C Client Library from the source code. (doxygen did the typing, i just did the makeup 💄)

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and that's the whole page, babe 🎀 Paho Asynchronous MQTT C Client Library, dated Tue Jun 2 2026 00:00:00, page MQTTAsync_message(3) — you're all caught up, go forth and publish slay 💅✨🔥