Crate postcard

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Postcard

Postcard is a #![no_std] focused serializer and deserializer for Serde.

Postcard aims to be convenient for developers in constrained environments, while allowing for flexibility to customize behavior as needed.

Design Goals

  1. Design primarily for #![no_std] usage, in embedded or other constrained contexts
  2. Support a maximal set of serde features, so postcard can be used as a drop in replacement
  3. Avoid special differences in code between communication code written for a microcontroller or a desktop/server PC
  4. Be resource efficient - memory usage, code size, developer time, and CPU time; in that order
  5. Allow library users to customize the serialization and deserialization behavior to fit their bespoke needs

Format Stability

As of v1.0.0, postcard has a documented and stable wire format. More information about this wire format can be found in the spec/ folder of the Postcard repository, or viewed online at https://postcard.jamesmunns.com.

Work towards the Postcard Specification and portions of the Postcard 1.0 Release were sponsored by Mozilla Corporation.

Variable Length Data

All signed and unsigned integers larger than eight bits are encoded using a Varint. This includes the length of array slices, as well as the discriminant of enums.

For more information, see the Varint chapter of the wire specification.

Example - Serialization/Deserialization

Postcard can serialize and deserialize messages similar to other serde formats.

Using the default heapless feature to serialize to a heapless::Vec<u8>:

use core::ops::Deref;
use serde::{Serialize, Deserialize};
use postcard::{from_bytes, to_vec};
use heapless::Vec;

#[derive(Serialize, Deserialize, Debug, Eq, PartialEq)]
struct RefStruct<'a> {
    bytes: &'a [u8],
    str_s: &'a str,
}
let message = "hElLo";
let bytes = [0x01, 0x10, 0x02, 0x20];
let output: Vec<u8, 11> = to_vec(&RefStruct {
    bytes: &bytes,
    str_s: message,
}).unwrap();

assert_eq!(
    &[0x04, 0x01, 0x10, 0x02, 0x20, 0x05, b'h', b'E', b'l', b'L', b'o',],
    output.deref()
);

let out: RefStruct = from_bytes(output.deref()).unwrap();
assert_eq!(
    out,
    RefStruct {
        bytes: &bytes,
        str_s: message,
    }
);

Or the optional alloc feature to serialize to an alloc::vec::Vec<u8>:

use core::ops::Deref;
use serde::{Serialize, Deserialize};
use postcard::{from_bytes, to_allocvec};
extern crate alloc;
use alloc::vec::Vec;

#[derive(Serialize, Deserialize, Debug, Eq, PartialEq)]
struct RefStruct<'a> {
    bytes: &'a [u8],
    str_s: &'a str,
}
let message = "hElLo";
let bytes = [0x01, 0x10, 0x02, 0x20];
let output: Vec<u8> = to_allocvec(&RefStruct {
    bytes: &bytes,
    str_s: message,
}).unwrap();

assert_eq!(
    &[0x04, 0x01, 0x10, 0x02, 0x20, 0x05, b'h', b'E', b'l', b'L', b'o',],
    output.deref()
);

let out: RefStruct = from_bytes(output.deref()).unwrap();
assert_eq!(
    out,
    RefStruct {
        bytes: &bytes,
        str_s: message,
    }
);

Flavors

postcard supports a system called Flavors, which are used to modify the way postcard serializes or processes serialized data. These flavors act as “plugins” or “middlewares” during the serialization or deserialization process, and can be combined to obtain complex protocol formats.

See the documentation of the ser_flavors or de_flavors modules for more information on usage.

Setup - Cargo.toml

Don’t forget to add the no-std subset of serde along with postcard to the [dependencies] section of your Cargo.toml!

[dependencies]
postcard = "1.0.0"

serde = { version = "1.0.*", default-features = false }

License

Licensed under either of

at your option.

Contribution

Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.

Modules

accumulator

An accumulator used to collect chunked COBS data and deserialize it.

de_flavors

Deserialization Flavors

experimental

Experimental Postcard Features

ser_flavors

Serialization Flavors

Structs

Deserializer

A serde compatible deserializer, generic over “Flavors” of deserializing plugins.

Serializer

A serde compatible serializer, generic over “Flavors” of serializing plugins.

Enums

Error

This is the error type used by Postcard

Functions

from_bytes

Deserialize a message of type T from a byte slice. The unused portion (if any) of the byte slice is not returned.

from_bytes_cobs

Deserialize a message of type T from a cobs-encoded byte slice. The unused portion (if any) of the byte slice is not returned.

serialize_with_flavor

serialize_with_flavor() has three generic parameters, T, F, O.

take_from_bytes

Deserialize a message of type T from a byte slice. The unused portion (if any) of the byte slice is returned for further usage

take_from_bytes_cobs

Deserialize a message of type T from a cobs-encoded byte slice. The unused portion (if any) of the byte slice is returned for further usage

to_allocvec

Serialize a T to an alloc::vec::Vec<u8>. Requires the alloc feature.

to_allocvec_cobs

Serialize and COBS encode a T to an alloc::vec::Vec<u8>. Requires the alloc feature.

to_slice

Serialize a T to the given slice, with the resulting slice containing data in a serialized format.

to_slice_cobs

Serialize a T to the given slice, with the resulting slice containing data in a serialized then COBS encoded format. The terminating sentinel 0x00 byte is included in the output buffer.

to_stdvec

Serialize a T to an alloc::vec::Vec<u8>. Requires the alloc feature.

to_stdvec_cobs

Serialize and COBS encode a T to an alloc::vec::Vec<u8>. Requires the alloc feature.

to_vec

Serialize a T to a heapless::Vec<u8>, with the Vec containing data in a serialized format. Requires the (default) heapless feature.

to_vec_cobs

Serialize a T to a heapless::Vec<u8>, with the Vec containing data in a serialized then COBS encoded format. The terminating sentinel 0x00 byte is included in the output Vec. Requires the (default) heapless feature.

Type Definitions

Result

This is the Result type used by Postcard.