un-nest curl

curl-8.15.0/docs/internals/MQTT.md

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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