# Advanced 3: Writing Your Own Heap

## Sample code

- Directory: `docs/os-on-arduino/code/adv3_heap`

```bash
uv run pio run -d adv3_heap -t upload
uv run pio device monitor -b 115200
```

Output starts when you send the single character `S`.

## Source code

The sketch for this chapter. Click a file name to expand. The code is mirrored automatically from the manuscript repository, so what you see here is always current.

:::{dropdown} `src/main.cpp` — 75 lines
:icon: code

[Open on GitHub](https://github.com/iory/learning-os-from-arduino/blob/main/docs/os-on-arduino/code/adv3_heap/src/main.cpp)

```{literalinclude} ../../../../code/adv3_heap/src/main.cpp
:language: cpp
:linenos:
```

:::

:::{dropdown} `include/os_heap.h` — 22 lines
:icon: code

[Open on GitHub](https://github.com/iory/learning-os-from-arduino/blob/main/docs/os-on-arduino/code/adv3_heap/include/os_heap.h)

```{literalinclude} ../../../../code/adv3_heap/include/os_heap.h
:language: cpp
:linenos:
```

:::

:::{dropdown} `src/os_heap.cpp` — 128 lines
:icon: code

[Open on GitHub](https://github.com/iory/learning-os-from-arduino/blob/main/docs/os-on-arduino/code/adv3_heap/src/os_heap.cpp)

```{literalinclude} ../../../../code/adv3_heap/src/os_heap.cpp
:language: cpp
:linenos:
```

:::


## On the board

```{figure} ../_static/adv3_heap.gif
:name: fig-adv3-heap
:width: 100%

Sending `S` prints all the dumps at once. The last part shows that an allocation fails when there is no contiguous block even though the total free space is enough, and the detection of a double `free`.
```

## Pitfalls

(Collected from reader questions — ask via
[GitHub Issues](https://github.com/iory/build-your-own-arduino-rtos/issues).)