Module postcard::de_flavors

Deserialization Flavors

“Flavors” in postcard are used as modifiers to the serialization or deserialization process. Flavors typically modify one or both of the following:

1. The source medium of the deserialization, e.g. whether the data is serialized from a [u8] slice, or some other container
2. The format of the deserialization, such as if the original data is encoded in a COBS format, contains a CRC32 checksum appended to the message, etc.

Flavors are implemented using the Flavor trait, which acts as a “middleware” for retrieving the bytes before they are passed to serde for deserialization

Multiple flavors may be combined to obtain a desired combination of behavior and storage. When flavors are combined, it is expected that the storage flavor (such as Slice) is the innermost flavor.

Custom flavors may be defined by users of the postcard crate, however some commonly useful flavors have been provided in this module. If you think your custom flavor would be useful to others, PRs adding flavors are very welcome!

Usability

Flavors may not always be convenient to use directly, as they may expose some implementation details of how the inner workings of the flavor behaves. It is typical to provide a convenience method for using a flavor, to prevent the user from having to specify generic parameters, setting correct initialization values, or handling the output of the flavor correctly. See postcard::from_bytes() for an example of this.

When to use (multiple) flavors

Combining flavors are nice for convenience, as they perform potentially multiple steps of serialization at one time.

This can often be more memory efficient, as intermediate buffers are not typically required.

When NOT to use (multiple) flavors

The downside of passing deserialization through multiple steps is that it is typically slower than performing each step serially. Said simply, “cobs decoding while deserializing” is often slower than “cobs decode then deserialize”, due to the ability to handle longer “runs” of data in each stage. The downside is that if these stages can not be performed in-place on the buffer, you will need additional buffers for each stage.

Additionally, deserializating flavors can be more restrictive or difficult to work with than serialization flavors, as deserialization may require that the deserialized types borrow some portion of the original message.

Examples

Using a single flavor

In the first example, we use the Slice flavor, to retrieve the serialized output from a [u8] slice. No other modification is made to the serialization process.

use postcard::{
    de_flavors::Slice,
    Deserializer,
};
use serde::Deserialize;

#[derive(Deserialize, Debug, PartialEq)]
struct Tup(u8, u8, u8);

let msg = [0x04, 0x00, 0x04, 0x01, 0x02, 0x03];
let slice = Slice::new(&msg);
let mut deserializer = Deserializer::from_flavor(slice);
let t = Tup::deserialize(&mut deserializer).unwrap();
assert_eq!(t, Tup(4, 0, 4));
let remainder = deserializer.finalize().unwrap();
assert_eq!(remainder, &[1, 2, 3]);

Modules

• crcuse-crc
  This Cyclic Redundancy Check flavor applies the CRC crate’s Algorithm struct on the serialized data. The flavor will check the CRC assuming that it has been appended to the bytes.
• io
  Support for std::io or [embedded-io] traits

Structs

• Slice
  A simple Flavor representing the deserialization from a borrowed slice

Traits

• Flavor
  The deserialization Flavor trait