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
- Design primarily for
#![no_std]
usage, in embedded or other constrained contexts - Support a maximal set of
serde
features, sopostcard
can be used as a drop in replacement - Avoid special differences in code between communication code written for a microcontroller or a desktop/server PC
- Be resource efficient - memory usage, code size, developer time, and CPU time; in that order
- 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.
Cargo.toml
Setup - 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"
# By default, `serde` has the `std` feature enabled, which makes it unsuitable for embedded targets
# disabling default-features fixes this
serde = { version = "1.0.*", default-features = false }
License
Licensed under either of
- Apache License, Version 2.0 (LICENSE-APACHE or http://www.apache.org/licenses/LICENSE-2.0)
- MIT license (LICENSE-MIT or http://opensource.org/licenses/MIT)
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.