Warning

This document is not updated for ESP32C5 yet, so some of the content may not be correct.

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# ESP-MQTT

[中文]

## Overview

ESP-MQTT is an implementation of MQTT protocol client, which is a lightweight publish/subscribe messaging protocol. Now ESP-MQTT supports MQTT v5.0.

## Features

> - Support MQTT over TCP, SSL with Mbed TLS, MQTT over WebSocket, and MQTT over WebSocket Secure

> - Easy to setup with URI

> - Multiple instances (multiple clients in one application)

> - Support subscribing, publishing, authentication, last will messages, keep alive pings, and all 3 Quality of Service (QoS) levels (it should be a fully functional client)

## Application Examples

> - protocols/mqtt/tcp: MQTT over TCP, default port 1883

> - protocols/mqtt/ssl: MQTT over TLS, default port 8883

> - protocols/mqtt/ssl_ds: MQTT over TLS using digital signature peripheral for authentication, default port 8883

> - protocols/mqtt/ssl_mutual_auth: MQTT over TLS using certificates for authentication, default port 8883

> - protocols/mqtt/ssl_psk: MQTT over TLS using pre-shared keys for authentication, default port 8883

> - protocols/mqtt/ws: MQTT over WebSocket, default port 80

> - protocols/mqtt/wss: MQTT over WebSocket Secure, default port 443

> - protocols/mqtt5: Uses ESP-MQTT library to connect to broker with MQTT v5.0

## MQTT Message Retransmission

A new MQTT message is created by calling `esp_mqtt_client_publish` or its non blocking counterpart `esp_mqtt_client_enqueue`.

Messages with QoS 0 is sent only once. QoS 1 and 2 have different behaviors since the protocol requires extra steps to complete the process.

The ESP-MQTT library opts to always retransmit unacknowledged QoS 1 and 2 publish messages to avoid losses in faulty connections, even though the MQTT specification requires the re-transmission only on reconnect with Clean Session flag been set to 0 (set `disable_clean_session` to true for this behavior).

QoS 1 and 2 messages that may need retransmission are always enqueued, but first transmission try occurs immediately if `esp_mqtt_client_publish` is used. A transmission retry for unacknowledged messages will occur after `message_retransmit_timeout`. After CONFIG_MQTT_OUTBOX_EXPIRED_TIMEOUT_MS messages will expire and be deleted. If CONFIG_MQTT_REPORT_DELETED_MESSAGES is set, an event will be sent to notify the user.

## Configuration

The configuration is made by setting fields in `esp_mqtt_client_config_t` struct. The configuration struct has the following sub structs to configure different aspects of the client operation.

> - `esp_mqtt_client_config_t::broker_t` - Allow to set address and security verification.

> - `esp_mqtt_client_config_t::credentials_t` - Client credentials for authentication.

> - `esp_mqtt_client_config_t::session_t` - Configuration for MQTT session aspects.

> - `esp_mqtt_client_config_t::network_t` - Networking related configuration.

> - `esp_mqtt_client_config_t::task_t` - Allow to configure FreeRTOS task.

> - `esp_mqtt_client_config_t::buffer_t` - Buffer size for input and output.

In the following sections, the most common aspects are detailed.

### Broker

#### Address

Broker address can be set by usage of `address` struct. The configuration can be made by usage of `uri` field or the combination of `hostname`, `transport` and `port`. Optionally, `path` could be set, this field is useful in WebSocket connections.

The `uri` field is used in the format `scheme://hostname:port/path`.

- Currently support `mqtt`, `mqtts`, `ws`, `wss` schemes

- MQTT over TCP samples:

  - `mqtt://mqtt.eclipseprojects.io`: MQTT over TCP, default port 1883

  - `mqtt://mqtt.eclipseprojects.io:1884`: MQTT over TCP, port 1884

  - `mqtt://username:password@mqtt.eclipseprojects.io:1884`: MQTT over TCP, port 1884, with username and password

- MQTT over SSL samples:

  - `mqtts://mqtt.eclipseprojects.io`: MQTT over SSL, port 8883

  - `mqtts://mqtt.eclipseprojects.io:8884`: MQTT over SSL, port 8884

- MQTT over WebSocket samples:

  - `ws://mqtt.eclipseprojects.io:80/mqtt`

- MQTT over WebSocket Secure samples:

  - `wss://mqtt.eclipseprojects.io:443/mqtt`

- Minimal configurations:

    const esp_mqtt_client_config_t mqtt_cfg = {
        .broker.address.uri = "mqtt://mqtt.eclipseprojects.io",
    };
    esp_mqtt_client_handle_t client = esp_mqtt_client_init(&mqtt_cfg);
    esp_mqtt_client_register_event(client, ESP_EVENT_ANY_ID, mqtt_event_handler, client);
    esp_mqtt_client_start(client);

Note

By default MQTT client uses event loop library to post related MQTT events (connected, subscribed, published, etc.).

#### Verification

For secure connections with TLS used, and to guarantee Broker's identity, the `verification` struct must be set. The broker certificate may be set in PEM or DER format. To select DER, the equivalent `certificate_len` field must be set. Otherwise, a null-terminated string in PEM format should be provided to `certificate` field.

- Get certificate from server, example: `mqtt.eclipseprojects.io`

        openssl s_client -showcerts -connect mqtt.eclipseprojects.io:8883 < /dev/null \
        2> /dev/null | openssl x509 -outform PEM > mqtt_eclipse_org.pem

- Check the sample application: protocols/mqtt/ssl

- Configuration:

    const esp_mqtt_client_config_t mqtt_cfg = {
        .broker = {
          .address.uri = "mqtts://mqtt.eclipseprojects.io:8883",
          .verification.certificate = (const char *)mqtt_eclipse_org_pem_start,
        },
    };

For details about other fields, please check the API Reference and TLS Server Verification.

### Client Credentials

All client related credentials are under the `credentials` field.

> - `username`: pointer to the username used for connecting to the broker, can also be set by URI

> - `client_id`: pointer to the client ID, defaults to `ESP32_%CHIPID%` where `%CHIPID%` are the last 3 bytes of MAC address in hex format

#### Authentication

It is possible to set authentication parameters through the `authentication` field. The client supports the following authentication methods:

> - `password`: use a password by setting

> - `certificate` and `key`: mutual authentication with TLS, and both can be provided in PEM or DER format

> - `use_secure_element`: use secure element (ATECC608A) interfaced to ESP32

> - `ds_data`: use Digital Signature Peripheral available in some Espressif devices

### Session

For MQTT session-related configurations, `session` fields should be used.

#### Last Will and Testament

MQTT allows for a last will and testament (LWT) message to notify other clients when a client ungracefully disconnects. This is configured by the following fields in the `last_will` struct.

> - `topic`: pointer to the LWT message topic

> - `msg`: pointer to the LWT message

> - `msg_len`: length of the LWT message, required if `msg` is not null-terminated

> - `qos`: quality of service for the LWT message

> - `retain`: specifies the retain flag of the LWT message

### Change Settings in Project Configuration Menu

The settings for MQTT can be found using `idf.py menuconfig`, under `Component config` > `ESP-MQTT Configuration`.

The following settings are available:

- CONFIG_MQTT_PROTOCOL_311: enable 3.1.1 version of MQTT protocol

- CONFIG_MQTT_TRANSPORT_SSL and CONFIG_MQTT_TRANSPORT_WEBSOCKET: enable specific MQTT transport layer, such as SSL, WEBSOCKET, and WEBSOCKET_SECURE

- CONFIG_MQTT_CUSTOM_OUTBOX: disable default implementation of mqtt_outbox, so a specific implementation can be supplied

## Events

The following events may be posted by the MQTT client:

- `MQTT_EVENT_BEFORE_CONNECT`: The client is initialized and about to start connecting to the broker.

- `MQTT_EVENT_CONNECTED`: The client has successfully established a connection to the broker. The client is now ready to send and receive data.

- `MQTT_EVENT_DISCONNECTED`: The client has aborted the connection due to being unable to read or write data, e.g., because the server is unavailable.

- `MQTT_EVENT_SUBSCRIBED`: The broker has acknowledged the client's subscribe request. The event data contains the message ID of the subscribe message.

- `MQTT_EVENT_UNSUBSCRIBED`: The broker has acknowledged the client's unsubscribe request. The event data contains the message ID of the unsubscribe message.

- `MQTT_EVENT_PUBLISHED`: The broker has acknowledged the client's publish message. This is only posted for QoS level 1 and 2, as level 0 does not use acknowledgements. The event data contains the message ID of the publish message.

- `MQTT_EVENT_DATA`: The client has received a publish message. The event data contains: message ID, name of the topic it was published to, received data and its length. For data that exceeds the internal buffer, multiple `MQTT_EVENT_DATA` events are posted and `current_data_offset` and `total_data_len` from event data updated to keep track of the fragmented message.

- `MQTT_EVENT_ERROR`: The client has encountered an error. The field `error_handle` in the event data contains `error_type` that can be used to identify the error. The type of error determines which parts of the `error_handle` struct is filled.

## API Reference

### Header File

- components/mqtt/esp-mqtt/include/mqtt_client.h

- This header file can be included with:

  >     #include "mqtt_client.h"

- This header file is a part of the API provided by the `mqtt` component. To declare that your component depends on `mqtt`, add the following to your CMakeLists.txt:

  >     REQUIRES mqtt

  > or

  >     PRIV_REQUIRES mqtt

### Functions

esp_mqtt_client_handle_t esp_mqtt_client_init(const esp_mqtt_client_config_t *config)

  Creates MQTT client handle based on the configuration.

  Parameters:

    config -- MQTT configuration structure

  Returns:

    mqtt_client_handle if successfully created, NULL on error

esp_err_t esp_mqtt_client_set_uri(esp_mqtt_client_handle_t client, const char *uri)

  Sets MQTT connection URI. This API is usually used to overrides the URI configured in esp_mqtt_client_init.

  Parameters:

    - client -- MQTT client handle

    - uri --

  Returns:

    ESP_FAIL if URI parse error, ESP_OK on success

esp_err_t esp_mqtt_client_start(esp_mqtt_client_handle_t client)

  Starts MQTT client with already created client handle.

  Parameters:

    client -- MQTT client handle

  Returns:

    ESP_OK on success ESP_ERR_INVALID_ARG on wrong initialization ESP_FAIL on other error

esp_err_t esp_mqtt_client_reconnect(esp_mqtt_client_handle_t client)

  This api is typically used to force reconnection upon a specific event.

  Parameters:

    client -- MQTT client handle

  Returns:

    ESP_OK on success ESP_ERR_INVALID_ARG on wrong initialization ESP_FAIL if client is in invalid state

esp_err_t esp_mqtt_client_disconnect(esp_mqtt_client_handle_t client)

  This api is typically used to force disconnection from the broker.

  Parameters:

    client -- MQTT client handle

  Returns:

    ESP_OK on success ESP_ERR_INVALID_ARG on wrong initialization

esp_err_t esp_mqtt_client_stop(esp_mqtt_client_handle_t client)

  Stops MQTT client tasks.

  - Notes:

  - Cannot be called from the MQTT event handler

  Parameters:

    client -- MQTT client handle

  Returns:

    ESP_OK on success ESP_ERR_INVALID_ARG on wrong initialization ESP_FAIL if client is in invalid state

int esp_mqtt_client_subscribe_single(esp_mqtt_client_handle_t client, const char *topic, int qos)

  Subscribe the client to defined topic with defined qos.

  Notes:

  - Client must be connected to send subscribe message

  - This API is could be executed from a user task or from a MQTT event callback i.e. internal MQTT task (API is protected by internal mutex, so it might block if a longer data receive operation is in progress.

  - `esp_mqtt_client_subscribe` could be used to call this function.

  Parameters:

    - client -- MQTT client handle

    - topic -- topic filter to subscribe

    - qos -- Max qos level of the subscription

  Returns:

    message_id of the subscribe message on success -1 on failure -2 in case of full outbox.

int esp_mqtt_client_subscribe_multiple(esp_mqtt_client_handle_t client, const esp_mqtt_topic_t *topic_list, int size)

  Subscribe the client to a list of defined topics with defined qos.

  Notes:

  - Client must be connected to send subscribe message

  - This API is could be executed from a user task or from a MQTT event callback i.e. internal MQTT task (API is protected by internal mutex, so it might block if a longer data receive operation is in progress.

  - `esp_mqtt_client_subscribe` could be used to call this function.

  Parameters:

    - client -- MQTT client handle

    - topic_list -- List of topics to subscribe

    - size -- size of topic_list

  Returns:

    message_id of the subscribe message on success -1 on failure -2 in case of full outbox.

int esp_mqtt_client_unsubscribe(esp_mqtt_client_handle_t client, const char *topic)

  Unsubscribe the client from defined topic.

  Notes:

  - Client must be connected to send unsubscribe message

  - It is thread safe, please refer to `esp_mqtt_client_subscribe_single` for details

  Parameters:

    - client -- MQTT client handle

    - topic --

  Returns:

    message_id of the subscribe message on success -1 on failure

int esp_mqtt_client_publish(esp_mqtt_client_handle_t client, const char *topic, const char *data, int len, int qos, int retain)

  Client to send a publish message to the broker.

  Notes:

  - This API might block for several seconds, either due to network timeout (10s) or if publishing payloads longer than internal buffer (due to message fragmentation)

  - Client doesn't have to be connected for this API to work, enqueueing the messages with qos>1 (returning -1 for all the qos=0 messages if disconnected). If MQTT_SKIP_PUBLISH_IF_DISCONNECTED is enabled, this API will not attempt to publish when the client is not connected and will always return -1.

  - It is thread safe, please refer to `esp_mqtt_client_subscribe` for details

  Parameters:

    - client -- MQTT client handle

    - topic -- topic string

    - data -- payload string (set to NULL, sending empty payload message)

    - len -- data length, if set to 0, length is calculated from payload string

    - qos -- QoS of publish message

    - retain -- retain flag

  Returns:

    message_id of the publish message (for QoS 0 message_id will always be zero) on success. -1 on failure, -2 in case of full outbox.

int esp_mqtt_client_enqueue(esp_mqtt_client_handle_t client, const char *topic, const char *data, int len, int qos, int retain, bool store)

  Enqueue a message to the outbox, to be sent later. Typically used for messages with qos>0, but could be also used for qos=0 messages if store=true.

  This API generates and stores the publish message into the internal outbox and the actual sending to the network is performed in the mqtt-task context (in contrast to the esp_mqtt_client_publish() which sends the publish message immediately in the user task's context). Thus, it could be used as a non blocking version of esp_mqtt_client_publish().

  Parameters:

    - client -- MQTT client handle

    - topic -- topic string

    - data -- payload string (set to NULL, sending empty payload message)

    - len -- data length, if set to 0, length is calculated from payload string

    - qos -- QoS of publish message

    - retain -- retain flag

    - store -- if true, all messages are enqueued; otherwise only QoS 1 and QoS 2 are enqueued

  Returns:

    message_id if queued successfully, -1 on failure, -2 in case of full outbox.

esp_err_t esp_mqtt_client_destroy(esp_mqtt_client_handle_t client)

  Destroys the client handle.

  Notes:

  - Cannot be called from the MQTT event handler

  Parameters:

    client -- MQTT client handle

  Returns:

    ESP_OK ESP_ERR_INVALID_ARG on wrong initialization

esp_err_t esp_mqtt_set_config(esp_mqtt_client_handle_t client, const esp_mqtt_client_config_t *config)

  Set configuration structure, typically used when updating the config (i.e. on "before_connect" event.

  Notes:

  - When calling this function make sure to have all the intendend configurations set, otherwise default values are set.

  Parameters:

    - client -- MQTT client handle

    - config -- MQTT configuration structure

  Returns:

    ESP_ERR_NO_MEM if failed to allocate ESP_ERR_INVALID_ARG if conflicts on transport configuration. ESP_OK on success

esp_err_t esp_mqtt_client_register_event(esp_mqtt_client_handle_t client, esp_mqtt_event_id_t event, esp_event_handler_t event_handler, void *event_handler_arg)

  Registers MQTT event.

  Parameters:

    - client -- MQTT client handle

    - event -- event type

    - event_handler -- handler callback

    - event_handler_arg -- handlers context

  Returns:

    ESP_ERR_NO_MEM if failed to allocate ESP_ERR_INVALID_ARG on wrong initialization ESP_OK on success

esp_err_t esp_mqtt_client_unregister_event(esp_mqtt_client_handle_t client, esp_mqtt_event_id_t event, esp_event_handler_t event_handler)

  Unregisters mqtt event.

  Parameters:

    - client -- mqtt client handle

    - event -- event ID

    - event_handler -- handler to unregister

  Returns:

    ESP_ERR_NO_MEM if failed to allocate ESP_ERR_INVALID_ARG on invalid event ID ESP_OK on success

int esp_mqtt_client_get_outbox_size(esp_mqtt_client_handle_t client)

  Get outbox size.

  Parameters:

    client -- MQTT client handle

  Returns:

    outbox size 0 on wrong initialization

esp_err_t esp_mqtt_dispatch_custom_event(esp_mqtt_client_handle_t client, esp_mqtt_event_t *event)

  Dispatch user event to the mqtt internal event loop.

  Parameters:

    - client -- MQTT client handle

    - event -- MQTT event handle structure

  Returns:

    ESP_OK on success ESP_ERR_TIMEOUT if the event couldn't be queued (ref also CONFIG_MQTT_EVENT_QUEUE_SIZE)

### Structures

struct esp_mqtt_error_codes

  MQTT error code structure to be passed as a contextual information into ERROR event

  Important: This structure extends `esp_tls_last_error` error structure and is backward compatible with it (so might be down-casted and treated as `esp_tls_last_error` error, but recommended to update applications if used this way previously)

  Use this structure directly checking error_type first and then appropriate error code depending on the source of the error:

  | error_type | related member variables | note | | MQTT_ERROR_TYPE_TCP_TRANSPORT | esp_tls_last_esp_err, esp_tls_stack_err, esp_tls_cert_verify_flags, sock_errno | Error reported from tcp_transport/esp-tls | | MQTT_ERROR_TYPE_CONNECTION_REFUSED | connect_return_code | Internal error reported from MQTT broker on connection |

  Public Members

  esp_err_t esp_tls_last_esp_err

    last esp_err code reported from esp-tls component

  int esp_tls_stack_err

    tls specific error code reported from underlying tls stack

  int esp_tls_cert_verify_flags

    tls flags reported from underlying tls stack during certificate verification

  esp_mqtt_error_type_t error_type

    error type referring to the source of the error

  esp_mqtt_connect_return_code_t connect_return_code

    connection refused error code reported from MQTT* broker on connection

  int esp_transport_sock_errno

    errno from the underlying socket

struct esp_mqtt_event_t

  MQTT event configuration structure

  Public Members

  esp_mqtt_event_id_t event_id

    MQTT event type

  esp_mqtt_client_handle_t client

    MQTT client handle for this event

  char *data

    Data associated with this event

  int data_len

    Length of the data for this event

  int total_data_len

    Total length of the data (longer data are supplied with multiple events)

  int current_data_offset

    Actual offset for the data associated with this event

  char *topic

    Topic associated with this event

  int topic_len

    Length of the topic for this event associated with this event

  int msg_id

    MQTT messaged id of message

  int session_present

    MQTT session_present flag for connection event

  esp_mqtt_error_codes_t *error_handle

    esp-mqtt error handle including esp-tls errors as well as internal MQTT errors

  bool retain

    Retained flag of the message associated with this event

  int qos

    QoS of the messages associated with this event

  bool dup

    dup flag of the message associated with this event

  esp_mqtt_protocol_ver_t protocol_ver

    MQTT protocol version used for connection, defaults to value from menuconfig

struct esp_mqtt_client_config_t

  MQTT client configuration structure

  - Default values can be set via menuconfig

  - All certificates and key data could be passed in PEM or DER format. PEM format must have a terminating NULL character and the related len field set to 0. DER format requires a related len field set to the correct length.

  Public Members

  struct esp_mqtt_client_config_t::broker_t broker

    Broker address and security verification

  struct esp_mqtt_client_config_t::credentials_t credentials

    User credentials for broker

  struct esp_mqtt_client_config_t::session_t session

    MQTT session configuration.

  struct esp_mqtt_client_config_t::network_t network

    Network configuration

  struct esp_mqtt_client_config_t::task_t task

    FreeRTOS task configuration.

  struct esp_mqtt_client_config_t::buffer_t buffer

    Buffer size configuration.

  struct esp_mqtt_client_config_t::outbox_config_t outbox

    Outbox configuration.

  struct broker_t

    Broker related configuration

    Public Members

    struct esp_mqtt_client_config_t::broker_t::address_t address

      Broker address configuration

    struct esp_mqtt_client_config_t::broker_t::verification_t verification

      Security verification of the broker

    struct address_t

      Broker address

      - uri have precedence over other fields

      - If uri isn't set at least hostname, transport and port should.

      Public Members

      const char *uri

        Complete MQTT broker URI

      const char *hostname

        Hostname, to set ipv4 pass it as string)

      esp_mqtt_transport_t transport

        Selects transport

      const char *path

        Path in the URI

      uint32_t port

        MQTT server port

    struct verification_t

      Broker identity verification

      If fields are not set broker's identity isn't verified. it's recommended to set the options in this struct for security reasons.

      Public Members

      bool use_global_ca_store

        Use a global ca_store, look esp-tls documentation for details.

      esp_err_t (*crt_bundle_attach)(void *conf)

        Pointer to ESP x509 Certificate Bundle attach function for the usage of certificate bundles. Client only attach the bundle, the clean up must be done by the user.

      const char *certificate

        Certificate data, default is NULL. It's not copied nor freed by the client, user needs to clean up.

      size_t certificate_len

        Length of the buffer pointed to by certificate.

      const struct psk_key_hint *psk_hint_key

        Pointer to PSK struct defined in esp_tls.h to enable PSK authentication (as alternative to certificate verification). PSK is enabled only if there are no other ways to verify broker. It's not copied nor freed by the client, user needs to clean up.

      bool skip_cert_common_name_check

        Skip any validation of server certificate CN field, this reduces the security of TLS and makes the MQTT client susceptible to MITM attacks

      const char **alpn_protos

        NULL-terminated list of supported application protocols to be used for ALPN.

      const char *common_name

        Pointer to the string containing server certificate common name. If non-NULL, server certificate CN must match this name, If NULL, server certificate CN must match hostname. This is ignored if skip_cert_common_name_check=true. It's not copied nor freed by the client, user needs to clean up.

  struct buffer_t

    Client buffer size configuration

    Client have two buffers for input and output respectivelly.

    Public Members

    int size

      size of MQTT send/receive buffer

    int out_size

      size of MQTT output buffer. If not defined, defaults to the size defined by `buffer_size`

  struct credentials_t

    Client related credentials for authentication.

    Public Members

    const char *username

      MQTT username

    const char *client_id

      Set MQTT client identifier. Ignored if set_null_client_id == true If NULL set the default client id. Default client id is `ESP32_CHIPID%` where `CHIPID%` are last 3 bytes of MAC address in hex format

    bool set_null_client_id

      Selects a NULL client id

    struct esp_mqtt_client_config_t::credentials_t::authentication_t authentication

      Client authentication

    struct authentication_t

      Client authentication

      Fields related to client authentication by broker

      For mutual authentication using TLS, user could select certificate and key, secure element or digital signature peripheral if available.

      Public Members

      const char *password

        MQTT password

      const char *certificate

        Certificate for ssl mutual authentication, not required if mutual authentication is not needed. Must be provided with `key`. It's not copied nor freed by the client, user needs to clean up.

      size_t certificate_len

        Length of the buffer pointed to by certificate.

      const char *key

        Private key for SSL mutual authentication, not required if mutual authentication is not needed. If it is not NULL, also `certificate` has to be provided. It's not copied nor freed by the client, user needs to clean up.

      size_t key_len

        Length of the buffer pointed to by key.

      const char *key_password

        Client key decryption password, not PEM nor DER, if provided `key_password_len` must be correctly set.

      int key_password_len

        Length of the password pointed to by `key_password`

      bool use_secure_element

        Enable secure element, available in ESP32-ROOM-32SE, for SSL connection

      void *ds_data

        Carrier of handle for digital signature parameters, digital signature peripheral is available in some Espressif devices. It's not copied nor freed by the client, user needs to clean up.

  struct network_t

    Network related configuration

    Public Members

    int reconnect_timeout_ms

      Reconnect to the broker after this value in miliseconds if auto reconnect is not disabled (defaults to 10s)

    int timeout_ms

      Abort network operation if it is not completed after this value, in milliseconds (defaults to 10s).

    int refresh_connection_after_ms

      Refresh connection after this value (in milliseconds)

    bool disable_auto_reconnect

      Client will reconnect to server (when errors/disconnect). Set `disable_auto_reconnect=true` to disable

    esp_transport_handle_t transport

      Custom transport handle to use. Warning: The transport should be valid during the client lifetime and is destroyed when esp_mqtt_client_destroy is called.

    struct ifreq *if_name

      The name of interface for data to go through. Use the default interface without setting

  struct outbox_config_t

    Client outbox configuration options.

    Public Members

    uint64_t limit

      Size limit for the outbox in bytes.

  struct session_t

    MQTT Session related configuration

    Public Members

    struct esp_mqtt_client_config_t::session_t::last_will_t last_will

      Last will configuration

    bool disable_clean_session

      MQTT clean session, default clean_session is true

    int keepalive

      MQTT keepalive, default is 120 seconds When configuring this value, keep in mind that the client attempts to communicate with the broker at half the interval that is actually set. This conservative approach allows for more attempts before the broker's timeout occurs

    bool disable_keepalive

      Set `disable_keepalive=true` to turn off keep-alive mechanism, keepalive is active by default. Note: setting the config value `keepalive` to `0` doesn't disable keepalive feature, but uses a default keepalive period

    esp_mqtt_protocol_ver_t protocol_ver

      MQTT protocol version used for connection.

    int message_retransmit_timeout

      timeout for retransmitting of failed packet

    struct last_will_t

      Last Will and Testament message configuration.

      Public Members

      const char *topic

        LWT (Last Will and Testament) message topic

      const char *msg

        LWT message, may be NULL terminated

      int msg_len

        LWT message length, if msg isn't NULL terminated must have the correct length

      int qos

        LWT message QoS

      int retain

        LWT retained message flag

  struct task_t

    Client task configuration

    Public Members

    int priority

      MQTT task priority

    int stack_size

      MQTT task stack size

struct topic_t

  Topic definition struct

  Public Members

  const char *filter

    Topic filter to subscribe

  int qos

    Max QoS level of the subscription

### Macros

MQTT_ERROR_TYPE_ESP_TLS

  MQTT_ERROR_TYPE_TCP_TRANSPORT error type hold all sorts of transport layer errors, including ESP-TLS error, but in the past only the errors from MQTT_ERROR_TYPE_ESP_TLS layer were reported, so the ESP-TLS error type is re-defined here for backward compatibility

esp_mqtt_client_subscribe(client_handle, topic_type, qos_or_size)

  Convenience macro to select subscribe function to use.

  Notes:

  - Usage of `esp_mqtt_client_subscribe_single` is the same as previous esp_mqtt_client_subscribe, refer to it for details.

  Parameters:

    - client_handle -- MQTT client handle

    - topic_type -- Needs to be char* for single subscription or `esp_mqtt_topic_t` for multiple topics

    - qos_or_size -- It's either a qos when subscribing to a single topic or the size of the subscription array when subscribing to multiple topics.

  Returns:

    message_id of the subscribe message on success -1 on failure -2 in case of full outbox.

### Type Definitions

typedef struct esp_mqtt_client *esp_mqtt_client_handle_t

typedef enum esp_mqtt_event_id_t esp_mqtt_event_id_t

  MQTT event types.

  User event handler receives context data in `esp_mqtt_event_t` structure with

  - client - MQTT client handle

  - various other data depending on event type

typedef enum esp_mqtt_connect_return_code_t esp_mqtt_connect_return_code_t

  MQTT connection error codes propagated via ERROR event

typedef enum esp_mqtt_error_type_t esp_mqtt_error_type_t

  MQTT connection error codes propagated via ERROR event

typedef enum esp_mqtt_transport_t esp_mqtt_transport_t

typedef enum esp_mqtt_protocol_ver_t esp_mqtt_protocol_ver_t

  MQTT protocol version used for connection

typedef struct esp_mqtt_error_codes esp_mqtt_error_codes_t

  MQTT error code structure to be passed as a contextual information into ERROR event

  Important: This structure extends `esp_tls_last_error` error structure and is backward compatible with it (so might be down-casted and treated as `esp_tls_last_error` error, but recommended to update applications if used this way previously)

  Use this structure directly checking error_type first and then appropriate error code depending on the source of the error:

  | error_type | related member variables | note | | MQTT_ERROR_TYPE_TCP_TRANSPORT | esp_tls_last_esp_err, esp_tls_stack_err, esp_tls_cert_verify_flags, sock_errno | Error reported from tcp_transport/esp-tls | | MQTT_ERROR_TYPE_CONNECTION_REFUSED | connect_return_code | Internal error reported from MQTT broker on connection |

typedef struct esp_mqtt_event_t esp_mqtt_event_t

  MQTT event configuration structure

typedef esp_mqtt_event_t *esp_mqtt_event_handle_t

typedef struct esp_mqtt_client_config_t esp_mqtt_client_config_t

  MQTT client configuration structure

  - Default values can be set via menuconfig

  - All certificates and key data could be passed in PEM or DER format. PEM format must have a terminating NULL character and the related len field set to 0. DER format requires a related len field set to the correct length.

typedef struct topic_t esp_mqtt_topic_t

  Topic definition struct

### Enumerations

enum esp_mqtt_event_id_t

  MQTT event types.

  User event handler receives context data in `esp_mqtt_event_t` structure with

  - client - MQTT client handle

  - various other data depending on event type

  Values:

  enumerator MQTT_EVENT_ANY
  enumerator MQTT_EVENT_ERROR

    on error event, additional context: connection return code, error handle from esp_tls (if supported)

  enumerator MQTT_EVENT_CONNECTED

    connected event, additional context: session_present flag

  enumerator MQTT_EVENT_DISCONNECTED

    disconnected event

  enumerator MQTT_EVENT_SUBSCRIBED

    subscribed event, additional context:

    - msg_id message id

    - error_handle `error_type` in case subscribing failed

    - data pointer to broker response, check for errors.

    - data_len length of the data for this event

  enumerator MQTT_EVENT_UNSUBSCRIBED

    unsubscribed event, additional context: msg_id

  enumerator MQTT_EVENT_PUBLISHED

    published event, additional context: msg_id

  enumerator MQTT_EVENT_DATA

    data event, additional context:

    - msg_id message id

    - topic pointer to the received topic

    - topic_len length of the topic

    - data pointer to the received data

    - data_len length of the data for this event

    - current_data_offset offset of the current data for this event

    - total_data_len total length of the data received

    - retain retain flag of the message

    - qos QoS level of the message

    - dup dup flag of the message Note: Multiple MQTT_EVENT_DATA could be fired for one message, if it is longer than internal buffer. In that case only first event contains topic pointer and length, other contain data only with current data length and current data offset updating.

  enumerator MQTT_EVENT_BEFORE_CONNECT

    The event occurs before connecting

  enumerator MQTT_EVENT_DELETED

    Notification on delete of one message from the internal outbox, if the message couldn't have been sent and acknowledged before expiring defined in OUTBOX_EXPIRED_TIMEOUT_MS. (events are not posted upon deletion of successfully acknowledged messages)

    - This event id is posted only if MQTT_REPORT_DELETED_MESSAGES==1

    - Additional context: msg_id (id of the deleted message).

  enumerator MQTT_USER_EVENT

    Custom event used to queue tasks into mqtt event handler All fields from the esp_mqtt_event_t type could be used to pass an additional context data to the handler.

enum esp_mqtt_connect_return_code_t

  MQTT connection error codes propagated via ERROR event

  Values:

  enumerator MQTT_CONNECTION_ACCEPTED

    Connection accepted

  enumerator MQTT_CONNECTION_REFUSE_PROTOCOL

    MQTT connection refused reason: Wrong protocol

  enumerator MQTT_CONNECTION_REFUSE_ID_REJECTED

    MQTT connection refused reason: ID rejected

  enumerator MQTT_CONNECTION_REFUSE_SERVER_UNAVAILABLE

    MQTT connection refused reason: Server unavailable

  enumerator MQTT_CONNECTION_REFUSE_BAD_USERNAME

    MQTT connection refused reason: Wrong user

  enumerator MQTT_CONNECTION_REFUSE_NOT_AUTHORIZED

    MQTT connection refused reason: Wrong username or password

enum esp_mqtt_error_type_t

  MQTT connection error codes propagated via ERROR event

  Values:

  enumerator MQTT_ERROR_TYPE_NONE
  enumerator MQTT_ERROR_TYPE_TCP_TRANSPORT
  enumerator MQTT_ERROR_TYPE_CONNECTION_REFUSED
  enumerator MQTT_ERROR_TYPE_SUBSCRIBE_FAILED

enum esp_mqtt_transport_t

  Values:

  enumerator MQTT_TRANSPORT_UNKNOWN
  enumerator MQTT_TRANSPORT_OVER_TCP

    MQTT over TCP, using scheme: `MQTT`

  enumerator MQTT_TRANSPORT_OVER_SSL

    MQTT over SSL, using scheme: `MQTTS`

  enumerator MQTT_TRANSPORT_OVER_WS

    MQTT over Websocket, using scheme:: `ws`

  enumerator MQTT_TRANSPORT_OVER_WSS

    MQTT over Websocket Secure, using scheme: `wss`

enum esp_mqtt_protocol_ver_t

  MQTT protocol version used for connection

  Values:

  enumerator MQTT_PROTOCOL_UNDEFINED
  enumerator MQTT_PROTOCOL_V_3_1
  enumerator MQTT_PROTOCOL_V_3_1_1
  enumerator MQTT_PROTOCOL_V_5

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