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# Postcard | ||
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[![Documentation](https://docs.rs/postcard/badge.svg)](https://docs.rs/postcard) | ||
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Postcard is a `#![no_std]` focused serializer and deserializer for Serde. | ||
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Postcard aims to be convenient for developers in constrained environments, while | ||
allowing for flexibility to customize behavior as needed. | ||
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## Design Goals | ||
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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 | ||
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## Format Stability | ||
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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>. | ||
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Work towards the Postcard Specification and portions of the Postcard 1.0 Release | ||
were sponsored by Mozilla Corporation. | ||
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## Variable Length Data | ||
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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`. | ||
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For more information, see the [Varint] chapter of the wire specification. | ||
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[Varint]: https://postcard.jamesmunns.com/wire-format.html#varint-encoded-integers | ||
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## Example - Serialization/Deserialization | ||
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Postcard can serialize and deserialize messages similar to other `serde` formats. | ||
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Using the default `heapless` feature to serialize to a `heapless::Vec<u8>`: | ||
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```rust | ||
use core::ops::Deref; | ||
use serde::{Serialize, Deserialize}; | ||
use postcard::{from_bytes, to_vec}; | ||
use heapless::Vec; | ||
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#[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(); | ||
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assert_eq!( | ||
&[0x04, 0x01, 0x10, 0x02, 0x20, 0x05, b'h', b'E', b'l', b'L', b'o',], | ||
output.deref() | ||
); | ||
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let out: RefStruct = from_bytes(output.deref()).unwrap(); | ||
assert_eq!( | ||
out, | ||
RefStruct { | ||
bytes: &bytes, | ||
str_s: message, | ||
} | ||
); | ||
``` | ||
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Or the optional `alloc` feature to serialize to an `alloc::vec::Vec<u8>`: | ||
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```rust | ||
use core::ops::Deref; | ||
use serde::{Serialize, Deserialize}; | ||
use postcard::{from_bytes, to_allocvec}; | ||
extern crate alloc; | ||
use alloc::vec::Vec; | ||
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#[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(); | ||
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assert_eq!( | ||
&[0x04, 0x01, 0x10, 0x02, 0x20, 0x05, b'h', b'E', b'l', b'L', b'o',], | ||
output.deref() | ||
); | ||
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let out: RefStruct = from_bytes(output.deref()).unwrap(); | ||
assert_eq!( | ||
out, | ||
RefStruct { | ||
bytes: &bytes, | ||
str_s: message, | ||
} | ||
); | ||
``` | ||
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## Flavors | ||
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`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. | ||
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See the documentation of the `ser_flavors` or `de_flavors` modules for more information on usage. | ||
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## Setup - `Cargo.toml` | ||
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Don't forget to add [the `no-std` subset](https://serde.rs/no-std.html) of `serde` along with `postcard` to the `[dependencies]` section of your `Cargo.toml`! | ||
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```toml | ||
[dependencies] | ||
postcard = "1.0.0" | ||
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# 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 } | ||
``` | ||
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## License | ||
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Licensed under either of | ||
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- Apache License, Version 2.0 ([LICENSE-APACHE](LICENSE-APACHE) or | ||
<http://www.apache.org/licenses/LICENSE-2.0>) | ||
- MIT license ([LICENSE-MIT](LICENSE-MIT) or <http://opensource.org/licenses/MIT>) | ||
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at your option. | ||
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### Contribution | ||
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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. |
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