# 9.1 msgpack.h — MessagePack codec

_Peios / Developing for Peios / SDK Reference / msgpack.h — Encoding_

> A general MessagePack codec whose reason for being is event payloads — its writer, reader and validator.

`<peios/msgpack.h>` is a small, self-contained [MessagePack](https://msgpack.org) codec. It exists because KMES event payloads *are* MessagePack: the kernel only *structurally validates* a payload on emit — it does not build or interpret it — so userspace owns the encode and decode. This codec is that path, and its validator's acceptance is deliberately **matched to the kernel's emit-time check**, so a payload this codec produces and validates is guaranteed to be accepted by [`peios_event_emit`](/peios/developing-for-peios/sdk-reference/sdk-events-api/event-h-events-kmes.md#emitting-events).

You can use it as a general MessagePack codec, but its reason for being is [events](/peios/developing-for-peios/sdk-reference/sdk-events-api/event-h-events-kmes.md).

It has three parts: a heap-backed **writer**, a stack-allocatable **reader**, and a **validator**.

## 9.1.1 See also

- **[`<peios/event.h>`](/peios/developing-for-peios/sdk-reference/sdk-events-api/event-h-events-kmes.md)** — the KMES events these payloads travel in.
- **[Library conventions](/peios/developing-for-peios/sdk-reference/sdk-conventions/library-conventions.md)** — the sticky-error builder model the writer follows.