Crate rmp

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§The Rust MessagePack Library

RMP is a pure Rust MessagePack implementation of an efficient binary serialization format. This crate provides low-level core functionality, writers and readers for primitive values with direct mapping between binary MessagePack format.

Looking for Serde support?

This crate represents the very basic functionality needed to work with MessagePack format. Ideologically it is developed as a basis for building high-level abstractions.


To use rmp, first add this to your Cargo.toml:

rmp = "0.8"


  • Low-level API

    RMP is designed to be lightweight and straightforward. There are low-level APIs, which give you full control over the encoding/decoding process. no-std environments are supported.

  • Zero-copy value decoding

    RMP allows to decode bytes from a buffer in a zero-copy manner, without any heap allocations. easily and blazingly fast. Rust static checks guarantee that the data will be valid until buffer lives.

  • Clear error handling

    RMP’s error system guarantees that you never receive an error enum with unreachable variant.

  • Robust and tested

    This project is developed using TDD and CI, so any found bugs will be fixed without breaking existing functionality.


Currently there are two large modules: encode and decode. More detail you can find in the corresponding sections.

Formally every MessagePack message consists of some marker encapsulating a data type and the data itself. Sometimes there are no separate data chunk, for example for booleans. In these cases a marker contains the value. For example, the true value is encoded as 0xc3.

let mut buf = Vec::new();
rmp::encode::write_bool(&mut buf, true).unwrap();

assert_eq!([0xc3], buf[..]);

Sometimes a single value can be encoded in multiple ways. For example a value of 42 can be represented as: [0x2a], [0xcc, 0x2a], [0xcd, 0x00, 0x2a] and so on, and all of them are considered as valid representations. To allow fine-grained control over encoding such values the library provides direct mapping functions.

let mut bufs = vec![vec![]; 5];

rmp::encode::write_pfix(&mut bufs[0], 42).unwrap();
rmp::encode::write_u8(&mut bufs[1], 42).unwrap();
rmp::encode::write_u16(&mut bufs[2], 42).unwrap();
rmp::encode::write_u32(&mut bufs[3], 42).unwrap();
rmp::encode::write_u64(&mut bufs[4], 42).unwrap();

assert_eq!([0x2a], bufs[0][..]);
assert_eq!([0xcc, 0x2a], bufs[1][..]);
assert_eq!([0xcd, 0x00, 0x2a], bufs[2][..]);
assert_eq!([0xce, 0x00, 0x00, 0x00, 0x2a], bufs[3][..]);
assert_eq!([0xcf, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x2a], bufs[4][..]);

But they aren’t planned to be widely used. Instead we often need to encode bytes compactly to save space. In these cases RMP provides functions that guarantee that for encoding the most compact representation will be chosen.

let mut buf = Vec::new();

rmp::encode::write_sint(&mut buf, 300).unwrap();

assert_eq!([0xcd, 0x1, 0x2c], buf[..]);

On the other hand for deserialization it is not matter in which representation the value is encoded - RMP deals with all of them.

Sometimes you know the exact type representation and want to enforce the deserialization process to make it strongly type safe.

let buf = [0xcd, 0x1, 0x2c];

assert_eq!(300, rmp::decode::read_u16(&mut &buf[..]).unwrap());

However if you try to decode such bytearray as other integer type, for example u32, there will be type mismatch error.

let buf = [0xcd, 0x1, 0x2c];
rmp::decode::read_u32(&mut &buf[..]).err().unwrap();

But sometimes all you want is just to encode an integer that must fit in the specified type no matter how it was encoded. RMP provides such function to ease integration with other MessagePack libraries.

let buf = [0xcd, 0x1, 0x2c];

assert_eq!(300i16, rmp::decode::read_int(&mut &buf[..]).unwrap());
assert_eq!(300i32, rmp::decode::read_int(&mut &buf[..]).unwrap());
assert_eq!(300i64, rmp::decode::read_int(&mut &buf[..]).unwrap());
assert_eq!(300u16, rmp::decode::read_int(&mut &buf[..]).unwrap());
assert_eq!(300u32, rmp::decode::read_int(&mut &buf[..]).unwrap());
assert_eq!(300u64, rmp::decode::read_int(&mut &buf[..]).unwrap());


Almost all API are represented as pure functions, which accepts a generic Write or Read and the value to be encoded/decoded. For example let’s do a round trip for π number.

let pi = std::f64::consts::PI;
let mut buf = Vec::new();
rmp::encode::write_f64(&mut buf, pi).unwrap();

assert_eq!([0xcb, 0x40, 0x9, 0x21, 0xfb, 0x54, 0x44, 0x2d, 0x18], buf[..]);
assert_eq!(pi, rmp::decode::read_f64(&mut &buf[..]).unwrap());

License: MIT


  • Provides various functions and structs for MessagePack decoding.
  • Provides various functions and structs for MessagePack encoding.