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<!--
Copyright (C) Daniel Stenberg, <daniel@haxx.se>, et al.
SPDX-License-Identifier: curl
-->
# MQTT in curl
## Usage
A plain "GET" subscribes to the topic and prints all published messages.
Doing a "POST" publishes the post data to the topic and exits.
### Subscribing
Command usage:
curl mqtt://host/topic
Example subscribe:
curl mqtt://host.home/bedroom/temp
This sends an MQTT SUBSCRIBE packet for the topic `bedroom/temp` and listen in
for incoming PUBLISH packets.
You can set the upkeep interval ms option to make curl send MQTT ping requests to the
server at an internal, to prevent the connection to get closed because of idleness.
You might then need to use the progress callback to cancel the operation.
### Publishing
Command usage:
curl -d payload mqtt://host/topic
Example publish:
curl -d 75 mqtt://host.home/bedroom/dimmer
This sends an MQTT PUBLISH packet to the topic `bedroom/dimmer` with the
payload `75`.
## What does curl deliver as a response to a subscribe
Whenever a PUBLISH packet is received, curl outputs two bytes topic length (MSB | LSB), the topic followed by the
payload.
## Caveats
Remaining limitations:
- Only QoS level 0 is implemented for publish
- No way to set retain flag for publish
- No TLS (mqtts) support
- Naive EAGAIN handling does not handle split messages
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