# Amazon IoT Device Defender library
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**Note**  <a name="out-of-date-message"></a>
The content on this page may not be up-to-date. Please refer to the [FreeRTOS.org library page](https://www.freertos.org/Documentation/03-Libraries/01-Library-overview/01-All-libraries) for the latest update.

## Introduction
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You can use the Amazon IoT Device Defender library to send security metrics from your IoT devices to Amazon IoT Device Defender. You can use Amazon IoT Device Defender to continuously monitor these security metrics from devices for deviations from what you have defined as appropriate behavior for each device. If something doesn't look right, Amazon IoT Device Defender sends out an alert so that you can take action to fix the issue. Interactions with Amazon IoT Device Defender use [MQTT](https://freertos.org/mqtt/index.html), a lightweight publish-subscribe protocol. This library provides an API to compose and recognize the MQTT topic strings used by Amazon IoT Device Defender.

 For more information, see [Amazon IoT Device Defender](https://docs.amazonaws.cn/iot/latest/developerguide/device-defender.html) in the *Amazon IoT Developer Guide*.

The library is written in C and designed to be compliant with [ISO C90](https://en.wikipedia.org/wiki/ANSI_C#C90) and [MISRA C:2012](https://misra.org.uk/product/misra-c2012-third-edition-first-revision/). The library has no dependencies on any additional libraries other than the standard C library. It also doesn't have any platform dependencies, such as threading or synchronization. It can be used with any MQTT library and any [JSON](https://freertos.org/json/json-terminology.html) or [CBOR](https://cbor.io/) library. The library has [proofs](https://www.cprover.org/cbmc/) showing safe memory use and no heap allocation, making it suitable for IoT microcontrollers, but also fully portable to other platforms.

The Amazon IoT Device Defender library can be freely used and is distributed under the [MIT open source license](https://freertos.org/a00114.html).


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<table>
<thead>
  <tr><th colspan="3">Code Size of Amazon IoT Device Defender (example generated with GCC for ARM Cortex-M)</th></tr>
  <tr><th>File</th><th>With -O1 Optimization</th><th>With -Os Optimization</th></tr>
</thead>
<tbody>
  <tr><td>defender.c</td><td>1.1K</td><td>0.6K</td></tr>
  <tr><td>Total estimates</td><td>1.1K</td><td>0.6K</td></tr>
</tbody>
</table>