#io-error #read-write #io #io-read #embedded-io #read #write

no-std genio

A type safe, low level replacement for std::io. Supports no_std for embedded development, just disable cargo feature std. Because of limitations of std::io::Error type, genio provides Read and Write traits that allow implementors to choose their own type. This type can be better at expressing what kinds of error can happen.

3 unstable releases

Uses old Rust 2015

0.2.1 Dec 23, 2019
0.2.0 Feb 22, 2019
0.1.0 Jan 28, 2017

#7 in #embedded-io

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Generic IO

A type safe, low level replacement for std::io.

Supports no_std for embedded development, just disable cargo feature std.


The IO routines you can find in standard library are very useful. However, because they use io::Error as the only error type they suffer these problems:

  • It's impossible to express infallible operations (e.g. in-memory operations) in types.
  • If you know the operation can't fail, you still have to call unwrap(). If you did mistake you get runtime error.
  • Compiler has to insert check (compare and branch), which slows down the code. Of course, it can be eliminated by inlining and optimization but it's not sure thing.
  • io::Error may allocate, which makes not just slower but prevents usage on bare metal.
  • io::Error is very broad and in some contexts it can have nonsense values (e.g. full disk when doing read()). Perfect program should not ignore these values but it's difficult to decide what to do about them.
  • You can reasonably decide what to do with error only using ErrorKind. That way some information may be lost.
  • The io::Error propagates to other interfaces and contaminates them. For example protobuf parser "could" return it even in case it's reading memory. Tokio-core uses io::Error everywhere.
  • Sometimes it is hard to tell from signature what exactly can fail and why. E.g. do you know why tokio_core::reactor::Core::new() may fail and what to do with the error?

The aim of genio is to enable better exploitation of Rust's type system to solve these problems. It steals many ideas from std::io with one important difference: Read and Write traits can define their own errors. There's also bunch of tools for handling errors as well as glue for std::io.

Since everything that impls genio::{Read, Write} can trivially impl std::io::{Read, Write}, it's better to write algorithms just for genio and let users wrap them in glue.


This crate is certainly not finished project and it needs more work to perfectly work with std::io. It'd also be beneficial to implement additional algorithms, wrappers and tools. Finally, other crates should start using genio. I invite everyone to help with this crate, to be as good as it can be.

I'm open to every PR you can imagine.


This crate is considered unstable, although mandatory types and methods in Read and Write traits are unlikely to change. Currently, there are only traits for synchronous streams. Please, if your type is not synchronous stream, don't implement these traits, but help designing other traits.


  • Synchronous streams
  • Asynchronous streams
  • Synchronous message streams with known message size (usually in the header of message)
  • Synchronous message streams with unknown size (usually using some delimiter)
  • Asynchronous message streams with known message size (usually in the header of message)
  • Asynchronous message streams with unknown size (usually using some delimiter)
  • Sound impls for primitive types
  • Sound impls for std::io types
  • Integrate with partial-io

And of course, appropriate combinators for all of them.

Differences between genio and std::io

There are other differences than just associated error types. Most importantly, genio aims to implement thinner wrappers and layers. For example, it doesn't handle EINTR automatically, but requires you to handle it yourself - for example, using Restarting wrapper. This gives you better control of what's going on in the code and also simplifies the implementation of the wrappers.

Some operations use slightly different types. read_exact may return UnexpectedEnd in addition to lower-level error type. Chain combines two error types. read_to_end enables to use any type which can be extended from reader. Flushing can return different error than write(). If there's nothing to flush, it can return Void type.

In addition, read() may hint that there are not enough bytes (usable to skip reading in error cases) and Write may be hinted how many bytes will be written. (This is unstable yet.)

Blanket impls

There are intentionally no blanket impls of genio traits for std::io or vice versa. This is to enable people to implement both traits. If you use std::io traits in your crates, you can add support for genio without fear. Please, do not impl genio traits with associated types being io::Error. If you don't have time to write full-featured impls, stick to glue wrappers. Don't waste opportunity to implement genio the best way!


I highly appreciate all the hard work Rust developers did to create std::io. The genio crate isn't meant to bash them, judge them, etc. They likely didn't have enough time to implement something like this or feared it'd be too complicated. The aim of this crate is to improve the world by providing tool for people who want more generic IO.