This RFC proposes a significant redesign of the std::io and std::os modules in preparation for API stabilization. The specific problems addressed by the redesign are given in the Problems section below, and the key ideas of the design are given in Vision for IO.

Note about RFC structure

This RFC was originally posted as a single monolithic file, which made it difficult to discuss different parts separately.

It has now been split into a skeleton that covers (1) the problem statement, (2) the overall vision and organization, and (3) the std::os module.

Other parts of the RFC are marked with (stub) and will be filed as follow-up PRs against this RFC.

Table of contents


The io and os modules are the last large API surfaces of std that need to be stabilized. While the basic functionality offered in these modules is largely traditional, many problems with the APIs have emerged over time. The RFC discusses the most significant problems below.

This section only covers specific problems with the current library; see Vision for IO for a higher-level view. section.

Atomicity and the Reader/Writer traits

One of the most pressing – but also most subtle – problems with std::io is the lack of atomicity in its Reader and Writer traits.

For example, the Reader trait offers a read_to_end method:

fn read_to_end(&mut self) -> IoResult<Vec<u8>>

Executing this method may involve many calls to the underlying read method. And it is possible that the first several calls succeed, and then a call returns an Err – which, like TimedOut, could represent a transient problem. Unfortunately, given the above signature, there is no choice but to simply throw this data away.

The Writer trait suffers from a more fundamental problem, since its primary method, write, may actually involve several calls to the underlying system – and if a failure occurs, there is no indication of how much was written.

Existing blocking APIs all have to deal with this problem, and Rust can and should follow the existing tradition here. See Revising Reader and Writer for the proposed solution.


The std::io module supports “timeouts” on virtually all IO objects via a set_timeout method. In this design, every IO object (file, socket, etc.) has an optional timeout associated with it, and set_timeout mutates the associated timeout. All subsequent blocking operations are implicitly subject to this timeout.

This API choice suffers from two problems, one cosmetic and the other deeper:

  • The “timeout” is actually a deadline and should be named accordingly.

  • The stateful API has poor composability: when passing a mutable reference of an IO object to another function, it’s possible that the deadline has been changed. In other words, users of the API can easily interfere with each other by accident.

See Deadlines for the proposed solution.

Posix and libuv bias

The current io and os modules were originally designed when librustuv was providing IO support, and to some extent they reflect the capabilities and conventions of libuv – which in turn are loosely based on Posix.

As such, the modules are not always ideal from a cross-platform standpoint, both in terms of forcing Windows programmings into a Posix mold, and also of offering APIs that are not actually usable on all platforms.

The modules have historically also provided no platform-specific APIs.

Part of the goal of this RFC is to set out a clear and extensible story for both cross-platform and platform-specific APIs in std. See Design principles for the details.


Rust has followed the utf8 everywhere approach to its strings. However, at the borders to platform APIs, it is revealed that the world is not, in fact, UTF-8 (or even Unicode) everywhere.

Currently our story for platform APIs is that we either assume they can take or return Unicode strings (suitably encoded) or an uninterpreted byte sequence. Sadly, this approach does not actually cover all platform needs, and is also not highly ergonomic as presently implemented. (Consider os::getenv which introduces replacement characters (!) versus os::getenv_as_bytes which yields a Vec<u8>; neither is ideal.)

This topic was covered in some detail in the Path Reform RFC, but this RFC gives a more general account in String handling.


The stdio module provides access to readers/writers for stdin, stdout and stderr, which is essential functionality. However, it also provides a means of changing e.g. “stdout” – but there is no connection between these two! In particular, set_stdout affects only the writer that println! and friends use, while set_stderr affects panic!.

This module needs to be clarified. See The std::io facade and [Functionality moved elsewhere] for the detailed design.

Overly high-level abstractions

There are a few places where io provides high-level abstractions over system services without also providing more direct access to the service as-is. For example:

  • The Writer trait’s write method – a cornerstone of IO – actually corresponds to an unbounded number of invocations of writes to the underlying IO object. This RFC changes write to follow more standard, lower-level practice; see Revising Reader and Writer.

  • Objects like TcpStream are Clone, which involves a fair amount of supporting infrastructure. This RFC tackles the problems that Clone was trying to solve more directly; see Splitting streams and cancellation.

The motivation for going lower-level is described in Design principles below.

The error chaining pattern

The std::io module is somewhat unusual in that most of the functionality it proves are used through a few key traits (like Reader) and these traits are in turn “lifted” over IoResult:

impl<R: Reader> Reader for IoResult<R> { ... }

This lifting and others makes it possible to chain IO operations that might produce errors, without any explicit mention of error handling:

                      ^~~~~~~~~~~ can produce an error
      ^~~~ can produce an error

The result of such a chain is either Ok of the outcome, or Err of the first error.

While this pattern is highly ergonomic, it does not fit particularly well into our evolving error story (interoperation or try blocks), and it is the only module in std to follow this pattern.

Eventually, we would like to write


to take advantage of the FromError infrastructure, hook into error handling control flow, and to provide good chaining ergonomics throughout all Rust APIs – all while keeping this handling a bit more explicit via the ? operator. (See for the rough direction).

In the meantime, this RFC proposes to phase out the use of impls for IoResult. This will require use of try! for the time being.

(Note: this may put some additional pressure on at least landing the basic use of ? instead of today’s try! before 1.0 final.)

Detailed design

There’s a lot of material here, so the RFC starts with high-level goals, principles, and organization, and then works its way through the various modules involved.

Vision for IO

Rust’s IO story had undergone significant evolution, starting from a libuv-style pure green-threaded model to a dual green/native model and now to a pure native model. Given that history, it’s worthwhile to set out explicitly what is, and is not, in scope for std::io


For Rust 1.0, the aim is to:

  • Provide a blocking API based directly on the services provided by the native OS for native threads.

    These APIs should cover the basics (files, basic networking, basic process management, etc) and suffice to write servers following the classic Apache thread-per-connection model. They should impose essentially zero cost over the underlying OS services; the core APIs should map down to a single syscall unless more are needed for cross-platform compatibility.

  • Provide basic blocking abstractions and building blocks (various stream and buffer types and adapters) based on traditional blocking IO models but adapted to fit well within Rust.

  • Provide hooks for integrating with low-level and/or platform-specific APIs.

  • Ensure reasonable forwards-compatibility with future async IO models.

It is explicitly not a goal at this time to support asynchronous programming models or nonblocking IO, nor is it a goal for the blocking APIs to eventually be used in a nonblocking “mode” or style.

Rather, the hope is that the basic abstractions of files, paths, sockets, and so on will eventually be usable directly within an async IO programming model and/or with nonblocking APIs. This is the case for most existing languages, which offer multiple interoperating IO models.

The long term intent is certainly to support async IO in some form, but doing so will require new research and experimentation.

Design principles

Now that the scope has been clarified, it’s important to lay out some broad principles for the io and os modules. Many of these principles are already being followed to some extent, but this RFC makes them more explicit and applies them more uniformly.

What cross-platform means

Historically, Rust’s std has always been “cross-platform”, but as discussed in Posix and libuv bias this hasn’t always played out perfectly. The proposed policy is below. With this policies, the APIs should largely feel like part of “Rust” rather than part of any legacy, and they should enable truly portable code.

Except for an explicit opt-in (see Platform-specific opt-in below), all APIs in std should be cross-platform:

  • The APIs should only expose a service or a configuration if it is supported on all platforms, and if the semantics on those platforms is or can be made loosely equivalent. (The latter requires exercising some judgment). Platform-specific functionality can be handled separately (Platform-specific opt-in) and interoperate with normal std abstractions.

    This policy rules out functions like chown which have a clear meaning on Unix and no clear interpretation on Windows; the ownership and permissions models are very different.

  • The APIs should follow Rust’s conventions, including their naming, which should be platform-neutral.

    This policy rules out names like fstat that are the legacy of a particular platform family.

  • The APIs should never directly expose the representation of underlying platform types, even if they happen to coincide on the currently-supported platforms. Cross-platform types in std should be newtyped.

    This policy rules out exposing e.g. error numbers directly as an integer type.

The next subsection gives detail on what these APIs should look like in relation to system services.

Relation to the system-level APIs

How should Rust APIs map into system services? This question breaks down along several axes which are in tension with one another:

  • Guarantees. The APIs provided in the mainline io modules should be predominantly safe, aside from the occasional unsafe function. In particular, the representation should be sufficiently hidden that most use cases are safe by construction. Beyond memory safety, though, the APIs should strive to provide a clear multithreaded semantics (using the Send/Sync kinds), and should use Rust’s type system to rule out various kinds of bugs when it is reasonably ergonomic to do so (following the usual Rust conventions).

  • Ergonomics. The APIs should present a Rust view of things, making use of the trait system, newtypes, and so on to make system services fit well with the rest of Rust.

  • Abstraction/cost. On the other hand, the abstractions introduced in std must not induce significant costs over the system services – or at least, there must be a way to safely access the services directly without incurring this penalty. When useful abstractions would impose an extra cost, they must be pay-as-you-go.

Putting the above bullets together, the abstractions must be safe, and they should be as high-level as possible without imposing a tax.

  • Coverage. Finally, the std APIs should over time strive for full coverage of non-niche, cross-platform capabilities.

Platform-specific opt-in

Rust is a systems language, and as such it should expose seamless, no/low-cost access to system services. In many cases, however, this cannot be done in a cross-platform way, either because a given service is only available on some platforms, or because providing a cross-platform abstraction over it would be costly.

This RFC proposes platform-specific opt-in: submodules of os that are named by platform, and made available via #[cfg] switches. For example, os::unix can provide APIs only available on Unix systems, and os::linux can drill further down into Linux-only APIs. (You could even imagine subdividing by OS versions.) This is “opt-in” in the sense that, like the unsafe keyword, it is very easy to audit for potential platform-specificity: just search for os::anyplatform. Moreover, by separating out subsets like linux, it’s clear exactly how specific the platform dependency is.

The APIs in these submodules are intended to have the same flavor as other io APIs and should interoperate seamlessly with cross-platform types, but:

  • They should be named according to the underlying system services when there is a close correspondence.

  • They may reveal the underlying OS type if there is nothing to be gained by hiding it behind an abstraction.

For example, the os::unix module could provide a stat function that takes a standard Path and yields a custom struct. More interestingly, os::linux might include an epoll function that could operate directly on many io types (e.g. various socket types), without any explicit conversion to a file descriptor; that’s what “seamless” means.

Each of the platform modules will offer a custom prelude submodule, intended for glob import, that includes all of the extension traits applied to standard IO objects.

The precise design of these modules is in the very early stages and will likely remain #[unstable] for some time.

Proposed organization

The io module is currently the biggest in std, with an entire hierarchy nested underneath; it mixes general abstractions/tools with specific IO objects. The os module is currently a bit of a dumping ground for facilities that don’t fit into the io category.

This RFC proposes the revamp the organization by flattening out the hierarchy and clarifying the role of each module:

  env           environment manipulation
  fs            file system
  io            core io abstractions/adapters
    prelude     the io prelude
  net           networking
    unix        platform-specific APIs
    linux         ..
    windows       ..
  os_str        platform-sensitive string handling
  process       process management

In particular:

  • The contents of os will largely move to env, a new module for inspecting and updating the “environment” (including environment variables, CPU counts, arguments to main, and so on).

  • The io module will include things like Reader and BufferedWriter – cross-cutting abstractions that are needed throughout IO.

    The prelude submodule will export all of the traits and most of the types for IO-related APIs; a single glob import should suffice to set you up for working with IO. (Note: this goes hand-in-hand with removing the bits of io currently in the prelude, as recently proposed.)

  • The root os module is used purely to house the platform submodules discussed above.

  • The os_str module is part of the solution to the Unicode problem; see String handling below.

  • The process module over time will grow to include querying/manipulating already-running processes, not just spawning them.

Revising Reader and Writer

The Reader and Writer traits are the backbone of IO, representing the ability to (respectively) pull bytes from and push bytes to an IO object. The core operations provided by these traits follows a very long tradition for blocking IO, but they are still surprisingly subtle – and they need to be revised.

  • Atomicity and data loss. As discussed above, the Reader and Writer traits currently expose methods that involve multiple actual reads or writes, and data is lost when an error occurs after some (but not all) operations have completed.

    The proposed strategy for Reader operations is to (1) separate out various deserialization methods into a distinct framework, (2) never have the internal read implementations loop on errors, (3) cut down on the number of non-atomic read operations and (4) adjust the remaining operations to provide more flexibility when possible.

    For writers, the main change is to make write only perform a single underlying write (returning the number of bytes written on success), and provide a separate write_all method.

  • Parsing/serialization. The Reader and Writer traits currently provide a large number of default methods for (de)serialization of various integer types to bytes with a given endianness. Unfortunately, these operations pose atomicity problems as well (e.g., a read could fail after reading two of the bytes needed for a u32 value).

    Rather than complicate the signatures of these methods, the (de)serialization infrastructure is removed entirely – in favor of instead eventually introducing a much richer parsing/formatting/(de)serialization framework that works seamlessly with Reader and Writer.

    Such a framework is out of scope for this RFC, but the endian-sensitive functionality will be provided elsewhere (likely out of tree).

With those general points out of the way, let’s look at the details.


The updated Reader trait (and its extension) is as follows:

trait Read {
    fn read(&mut self, buf: &mut [u8]) -> Result<usize, Error>;

    fn read_to_end(&mut self, buf: &mut Vec<u8>) -> Result<(), Error> { ... }
    fn read_to_string(&self, buf: &mut String) -> Result<(), Error> { ... }

// extension trait needed for object safety
trait ReadExt: Read {
    fn bytes(&mut self) -> Bytes<Self> { ... }

    ... // more to come later in the RFC
impl<R: Read> ReadExt for R {}

Following the trait naming conventions, the trait is renamed to Read reflecting the clear primary method it provides.

The read method should not involve internal looping (even over errors like EINTR). It is intended to faithfully represent a single call to an underlying system API.

The read_to_end and read_to_string methods now take explicit buffers as input. This has multiple benefits:

  • Performance. When it is known that reading will involve some large number of bytes, the buffer can be preallocated in advance.

  • “Atomicity” concerns. For read_to_end, it’s possible to use this API to retain data collected so far even when a read fails in the middle. For read_to_string, this is not the case, because UTF-8 validity cannot be ensured in such cases; but if intermediate results are wanted, one can use read_to_end and convert to a String only at the end.

Convenience methods like these will retry on EINTR. This is partly under the assumption that in practice, EINTR will most often arise when interfacing with other code that changes a signal handler. Due to the global nature of these interactions, such a change can suddenly cause your own code to get an error irrelevant to it, and the code should probably just retry in those cases. In the case where you are using EINTR explicitly, read and write will be available to handle it (and you can always build your own abstractions on top).

Removed methods

The proposed Read trait is much slimmer than today’s Reader. The vast majority of removed methods are parsing/deserialization, which were discussed above.

The remaining methods (read_exact, read_at_least, push, push_at_least) were removed for various reasons:

  • read_exact, read_at_least: these are somewhat more obscure conveniences that are not particularly robust due to lack of atomicity.

  • push, push_at_least: these are special-cases for working with Vec, which this RFC proposes to replace with a more general mechanism described next.

To provide some of this functionality in a more composition way, extend Vec<T> with an unsafe method:

unsafe fn with_extra(&mut self, n: uint) -> &mut [T];

This method is equivalent to calling reserve(n) and then providing a slice to the memory starting just after len() entries. Using this method, clients of Read can easily recover the push method.


The Writer trait is cut down to even smaller size:

trait Write {
    fn write(&mut self, buf: &[u8]) -> Result<uint, Error>;
    fn flush(&mut self) -> Result<(), Error>;

    fn write_all(&mut self, buf: &[u8]) -> Result<(), Error> { .. }
    fn write_fmt(&mut self, fmt: &fmt::Arguments) -> Result<(), Error> { .. }

The biggest change here is to the semantics of write. Instead of repeatedly writing to the underlying IO object until all of buf is written, it attempts a single write and on success returns the number of bytes written. This follows the long tradition of blocking IO, and is a more fundamental building block than the looping write we currently have. Like read, it will propagate EINTR.

For convenience, write_all recovers the behavior of today’s write, looping until either the entire buffer is written or an error occurs. To meaningfully recover from an intermediate error and keep writing, code should work with write directly. Like the Read conveniences, EINTR results in a retry.

The write_fmt method, like write_all, will loop until its entire input is written or an error occurs.

The other methods include endian conversions (covered by serialization) and a few conveniences like write_str for other basic types. The latter, at least, is already uniformly (and extensibly) covered via the write! macro. The other helpers, as with Read, should migrate into a more general (de)serialization library.

String handling

The fundamental problem with Rust’s full embrace of UTF-8 strings is that not all strings taken or returned by system APIs are Unicode, let alone UTF-8 encoded.

In the past, std has assumed that all strings are either in some form of Unicode (Windows), or are simply u8 sequences (Unix). Unfortunately, this is wrong, and the situation is more subtle:

  • Unix platforms do indeed work with arbitrary u8 sequences (without interior nulls) and today’s platforms usually interpret them as UTF-8 when displayed.

  • Windows, however, works with arbitrary u16 sequences that are roughly interpreted at UTF-16, but may not actually be valid UTF-16 – an “encoding” often called UCS-2; see for a bit more detail.

What this means is that all of Rust’s platforms go beyond Unicode, but they do so in different and incompatible ways.

The current solution of providing both str and [u8] versions of APIs is therefore problematic for multiple reasons. For one, the [u8] versions are not actually cross-platform – even today, they panic on Windows when given non-UTF-8 data, a platform-specific behavior. But they are also incomplete, because on Windows you should be able to work directly with UCS-2 data.

Key observations

Fortunately, there is a solution that fits well with Rust’s UTF-8 strings and offers the possibility of platform-specific APIs.

Observation 1: it is possible to re-encode UCS-2 data in a way that is also compatible with UTF-8. This is the WTF-8 encoding format proposed by Simon Sapin. This encoding has some remarkable properties:

  • Valid UTF-8 data is valid WTF-8 data. When decoded to UCS-2, the result is exactly what would be produced by going straight from UTF-8 to UTF-16. In other words, making up some methods:

    my_ut8_data.to_wtf8().to_ucs2().as_u16_slice() == my_utf8_data.to_utf16().as_u16_slice()
  • Valid UTF-16 data re-encoded as WTF-8 produces the corresponding UTF-8 data:

    my_utf16_data.to_wtf8().as_bytes() == my_utf16_data.to_utf8().as_bytes()

These two properties mean that, when working with Unicode data, the WTF-8 encoding is highly compatible with both UTF-8 and UTF-16. In particular, the conversion from a Rust string to a WTF-8 string is a no-op, and the conversion in the other direction is just a validation.

Observation 2: all platforms can consume Unicode data (suitably re-encoded), and it’s also possible to validate the data they produce as Unicode and extract it.

Observation 3: the non-Unicode spaces on various platforms are deeply incompatible: there is no standard way to port non-Unicode data from one to another. Therefore, the only cross-platform APIs are those that work entirely with Unicode.

The design: os_str

The observations above lead to a somewhat radical new treatment of strings, first proposed in the Path Reform RFC. This RFC proposes to introduce new string and string slice types that (opaquely) represent platform-sensitive strings, housed in the std::os_str module.

The OsString type is analogous to String, and OsStr is analogous to str. Their backing implementation is platform-dependent, but they offer a cross-platform API:

pub mod os_str {
    /// Owned OS strings
    struct OsString {
        inner: imp::Buf
    /// Slices into OS strings
    struct OsStr {
        inner: imp::Slice

    // Platform-specific implementation details:
    mod imp {
        type Buf = Vec<u8>;
        type Slice = [u8];

    mod imp {
        type Buf = Wtf8Buf; // See
        type Slice = Wtf8;

    impl OsString {
        pub fn from_string(String) -> OsString;
        pub fn from_str(&str) -> OsString;
        pub fn as_slice(&self) -> &OsStr;
        pub fn into_string(Self) -> Result<String, OsString>;
        pub fn into_string_lossy(Self) -> String;

        // and ultimately other functionality typically found on vectors,
        // but CRUCIALLY NOT as_bytes

    impl Deref<OsStr> for OsString { ... }

    impl OsStr {
        pub fn from_str(value: &str) -> &OsStr;
        pub fn as_str(&self) -> Option<&str>;
        pub fn to_string_lossy(&self) -> CowString;

        // and ultimately other functionality typically found on slices,
        // but CRUCIALLY NOT as_bytes

    trait IntoOsString {
        fn into_os_str_buf(self) -> OsString;

    impl IntoOsString for OsString { ... }
    impl<'a> IntoOsString for &'a OsStr { ... }


These APIs make OS strings appear roughly as opaque vectors (you cannot see the byte representation directly), and can always be produced starting from Unicode data. They make it possible to collapse functions like getenv and getenv_as_bytes into a single function that produces an OS string, allowing the client to decide how (or whether) to extract Unicode data. It will be possible to do things like concatenate OS strings without ever going through Unicode.

It will also likely be possible to do things like search for Unicode substrings. The exact details of the API are left open and are likely to grow over time.

In addition to APIs like the above, there will also be platform-specific ways of viewing or constructing OS strings that reveals more about the space of possible values:

pub mod os {
    pub mod unix {
        trait OsStringExt {
            fn from_vec(Vec<u8>) -> Self;
            fn into_vec(Self) -> Vec<u8>;

        impl OsStringExt for os_str::OsString { ... }

        trait OsStrExt {
            fn as_byte_slice(&self) -> &[u8];
            fn from_byte_slice(&[u8]) -> &Self;

        impl OsStrExt for os_str::OsStr { ... }


    pub mod windows{
        // The following extension traits provide a UCS-2 view of OS strings

        trait OsStringExt {
            fn from_wide_slice(&[u16]) -> Self;

        impl OsStringExt for os_str::OsString { ... }

        trait OsStrExt {
            fn to_wide_vec(&self) -> Vec<u16>;

        impl OsStrExt for os_str::OsStr { ... }



By placing these APIs under os, using them requires a clear opt in to platform-specific functionality.

The future

Introducing an additional string type is a bit daunting, since many existing APIs take and consume only standard Rust strings. Today’s solution demands that strings coming from the OS be assumed or turned into Unicode, and the proposed API continues to allow that (with more explicit and finer-grained control).

In the long run, however, robust applications are likely to work opaquely with OS strings far beyond the boundary to the system to avoid data loss and ensure maximal compatibility. If this situation becomes common, it should be possible to introduce an abstraction over various string types and generalize most functions that work with String/str to instead work generically. This RFC does not propose taking any such steps now – but it’s important that we can do so later if Rust’s standard strings turn out to not be sufficient and OS strings become commonplace.


To be added in a follow-up PR.

Splitting streams and cancellation

To be added in a follow-up PR.


Now that we’ve covered the core principles and techniques used throughout IO, we can go on to explore the modules in detail.


Ideally, the io module will be split into the parts that can live in libcore (most of it) and the parts that are added in the std::io facade. This part of the organization is non-normative, since it requires changes to today’s IoError (which currently references String); if these changes cannot be performed, everything here will live in std::io.


The current std::io::util module offers a number of Reader and Writer “adapters”. This RFC refactors the design to more closely follow std::iter. Along the way, it generalizes the by_ref adapter:

trait ReadExt: Read {
    // ... eliding the methods already described above

    // Postfix version of `(&mut self)`
    fn by_ref(&mut self) -> &mut Self { ... }

    // Read everything from `self`, then read from `next`
    fn chain<R: Read>(self, next: R) -> Chain<Self, R> { ... }

    // Adapt `self` to yield only the first `limit` bytes
    fn take(self, limit: u64) -> Take<Self> { ... }

    // Whenever reading from `self`, push the bytes read to `out`
    #[unstable] // uncertain semantics of errors "halfway through the operation"
    fn tee<W: Write>(self, out: W) -> Tee<Self, W> { ... }

trait WriteExt: Write {
    // Postfix version of `(&mut self)`
    fn by_ref<'a>(&'a mut self) -> &mut Self { ... }

    // Whenever bytes are written to `self`, write them to `other` as well
    #[unstable] // uncertain semantics of errors "halfway through the operation"
    fn broadcast<W: Write>(self, other: W) -> Broadcast<Self, W> { ... }

// An adaptor converting an `Iterator<u8>` to `Read`.
pub struct IterReader<T> { ... }

As with std::iter, these adapters are object unsafe and hence placed in an extension trait with a blanket impl.

Free functions

The current std::io::util module also includes a number of primitive readers and writers, as well as copy. These are updated as follows:

// A reader that yields no bytes
fn empty() -> Empty; // in theory just returns `impl Read`

impl Read for Empty { ... }

// A reader that yields `byte` repeatedly (generalizes today's ZeroReader)
fn repeat(byte: u8) -> Repeat;

impl Read for Repeat { ... }

// A writer that ignores the bytes written to it (/dev/null)
fn sink() -> Sink;

impl Write for Sink { ... }

// Copies all data from a `Read` to a `Write`, returning the amount of data
// copied.
pub fn copy<R, W>(r: &mut R, w: &mut W) -> Result<u64, Error>

Like write_all, the copy method will discard the amount of data already written on any error and also discard any partially read data on a write error. This method is intended to be a convenience and write should be used directly if this is not desirable.


The seeking infrastructure is largely the same as today’s, except that tell is removed and the seek signature is refactored with more precise types:

pub trait Seek {
    // returns the new position after seeking
    fn seek(&mut self, pos: SeekFrom) -> Result<u64, Error>;

pub enum SeekFrom {

The old tell function can be regained via seek(SeekFrom::Current(0)).


The current Buffer trait will be renamed to BufRead for clarity (and to open the door to BufWrite at some later point):

pub trait BufRead: Read {
    fn fill_buf(&mut self) -> Result<&[u8], Error>;
    fn consume(&mut self, amt: uint);

    fn read_until(&mut self, byte: u8, buf: &mut Vec<u8>) -> Result<(), Error> { ... }
    fn read_line(&mut self, buf: &mut String) -> Result<(), Error> { ... }

pub trait BufReadExt: BufRead {
    // Split is an iterator over Result<Vec<u8>, Error>
    fn split(&mut self, byte: u8) -> Split<Self> { ... }

    // Lines is an iterator over Result<String, Error>
    fn lines(&mut self) -> Lines<Self> { ... };

    // Chars is an iterator over Result<char, Error>
    fn chars(&mut self) -> Chars<Self> { ... }

The read_until and read_line methods are changed to take explicit, mutable buffers, for similar reasons to read_to_end. (Note that buffer reuse is particularly common for read_line). These functions include the delimiters in the strings they produce, both for easy cross-platform compatibility (in the case of read_line) and for ease in copying data without loss (in particular, distinguishing whether the last line included a final delimiter).

The split and lines methods provide iterator-based versions of read_until and read_line, and do not include the delimiter in their output. This matches conventions elsewhere (like split on strings) and is usually what you want when working with iterators.

The BufReader, BufWriter and BufStream types stay essentially as they are today, except that for streams and writers the into_inner method yields the structure back in the case of a write error, and its behavior is clarified to writing out the buffered data without flushing the underlying reader:

// If writing fails, you get the unwritten data back
fn into_inner(self) -> Result<W, IntoInnerError<Self>>;

pub struct IntoInnerError<W>(W, Error);

impl IntoInnerError<T> {
    pub fn error(&self) -> &Error { ... }
    pub fn into_inner(self) -> W { ... }
impl<W> FromError<IntoInnerError<W>> for Error { ... }


Many applications want to view in-memory data as either an implementor of Read or Write. This is often useful when composing streams or creating test cases. This functionality primarily comes from the following implementations:

impl<'a> Read for &'a [u8] { ... }
impl<'a> Write for &'a mut [u8] { ... }
impl Write for Vec<u8> { ... }

While efficient, none of these implementations support seeking (via an implementation of the Seek trait). The implementations of Read and Write for these types is not quite as efficient when Seek needs to be used, so the Seek-ability will be opted-in to with a new Cursor structure with the following API:

pub struct Cursor<T> {
    pos: u64,
    inner: T,
impl<T> Cursor<T> {
    pub fn new(inner: T) -> Cursor<T>;
    pub fn into_inner(self) -> T;
    pub fn get_ref(&self) -> &T;

// Error indicating that a negative offset was seeked to.
pub struct NegativeOffset;

impl Seek for Cursor<Vec<u8>> { ... }
impl<'a> Seek for Cursor<&'a [u8]> { ... }
impl<'a> Seek for Cursor<&'a mut [u8]> { ... }

impl Read for Cursor<Vec<u8>> { ... }
impl<'a> Read for Cursor<&'a [u8]> { ... }
impl<'a> Read for Cursor<&'a mut [u8]> { ... }

impl BufRead for Cursor<Vec<u8>> { ... }
impl<'a> BufRead for Cursor<&'a [u8]> { ... }
impl<'a> BufRead for Cursor<&'a mut [u8]> { ... }

impl<'a> Write for Cursor<&'a mut [u8]> { ... }
impl Write for Cursor<Vec<u8>> { ... }

A sample implementation can be found in a gist. Using one Cursor structure allows to emphasize that the only ability added is an implementation of Seek while still allowing all possible I/O operations for various types of buffers.

It is not currently proposed to unify these implementations via a trait. For example a Cursor<Rc<[u8]>> is a reasonable instance to have, but it will not have an implementation listed in the standard library to start out. It is considered a backwards-compatible addition to unify these various impl blocks with a trait.

The following types will be removed from the standard library and replaced as follows:

  • MemReader -> Cursor<Vec<u8>>
  • MemWriter -> Cursor<Vec<u8>>
  • BufReader -> Cursor<&[u8]> or Cursor<&mut [u8]>
  • BufWriter -> Cursor<&mut [u8]>

The std::io facade

The std::io module will largely be a facade over core::io, but it will add some functionality that can live only in std.


The IoError type will be renamed to std::io::Error, following our non-prefixing convention. It will remain largely as it is today, but its fields will be made private. It may eventually grow a field to track the underlying OS error code.

The std::io::IoErrorKind type will become std::io::ErrorKind, and ShortWrite will be dropped (it is no longer needed with the new Write semantics), which should decrease its footprint. The OtherIoError variant will become Other now that enums are namespaced. Other variants may be added over time, such as Interrupted, as more errors are classified from the system.

The EndOfFile variant will be removed in favor of returning Ok(0) from read on end of file (or write on an empty slice for example). This approach clarifies the meaning of the return value of read, matches Posix APIs, and makes it easier to use try! in the case that a “real” error should be bubbled out. (The main downside is that higher-level operations that might use Result<T, IoError> with some T != usize may need to wrap IoError in a further enum if they wish to forward unexpected EOF.)

Channel adapters

The ChanReader and ChanWriter adapters will be left as they are today, and they will remain #[unstable]. The channel adapters currently suffer from a few problems today, some of which are inherent to the design:

  • Construction is somewhat unergonomic. First a mpsc channel pair must be created and then each half of the reader/writer needs to be created.
  • Each call to write involves moving memory onto the heap to be sent, which isn’t necessarily efficient.
  • The design of std::sync::mpsc allows for growing more channels in the future, but it’s unclear if we’ll want to continue to provide a reader/writer adapter for each channel we add to std::sync.

These types generally feel as if they’re from a different era of Rust (which they are!) and may take some time to fit into the current standard library. They can be reconsidered for stabilization after the dust settles from the I/O redesign as well as the recent std::sync redesign. At this time, however, this RFC recommends they remain unstable.

stdin, stdout, stderr

The current stdio module will be removed in favor of these constructors in the io module:

pub fn stdin() -> Stdin;
pub fn stdout() -> Stdout;
pub fn stderr() -> Stderr;
  • stdin - returns a handle to a globally shared standard input of the process which is buffered as well. Due to the globally shared nature of this handle, all operations on Stdin directly will acquire a lock internally to ensure access to the shared buffer is synchronized. This implementation detail is also exposed through a lock method where the handle can be explicitly locked for a period of time so relocking is not necessary.

    The Read trait will be implemented directly on the returned Stdin handle but the BufRead trait will not be (due to synchronization concerns). The locked version of Stdin (StdinLock) will provide an implementation of BufRead.

    The design will largely be the same as is today with the old_io module.

    impl Stdin {
        fn lock(&self) -> StdinLock;
        fn read_line(&mut self, into: &mut String) -> io::Result<()>;
        fn read_until(&mut self, byte: u8, into: &mut Vec<u8>) -> io::Result<()>;
    impl Read for Stdin { ... }
    impl Read for StdinLock { ... }
    impl BufRead for StdinLock { ... }
  • stderr - returns a non buffered handle to the standard error output stream for the process. Each call to write will roughly translate to a system call to output data when written to stderr. This handle is locked like stdin to ensure, for example, that calls to write_all are atomic with respect to one another. There will also be an RAII guard to lock the handle and use the result as an instance of Write.

    impl Stderr {
        fn lock(&self) -> StderrLock;
    impl Write for Stderr { ... }
    impl Write for StderrLock { ... }
  • stdout - returns a globally buffered handle to the standard output of the current process. The amount of buffering can be decided at runtime to allow for different situations such as being attached to a TTY or being redirected to an output file. The Write trait will be implemented for this handle, and like stderr it will be possible to lock it and then use the result as an instance of Write as well.

    impl Stdout {
        fn lock(&self) -> StdoutLock;
    impl Write for Stdout { ... }
    impl Write for StdoutLock { ... }

Windows and stdio

On Windows, standard input and output handles can work with either arbitrary [u8] or [u16] depending on the state at runtime. For example a program attached to the console will work with arbitrary [u16], but a program attached to a pipe would work with arbitrary [u8].

To handle this difference, the following behavior will be enforced for the standard primitives listed above:

  • If attached to a pipe then no attempts at encoding or decoding will be done, the data will be ferried through as [u8].

  • If attached to a console, then stdin will attempt to interpret all input as UTF-16, re-encoding into UTF-8 and returning the UTF-8 data instead. This implies that data will be buffered internally to handle partial reads/writes. Invalid UTF-16 will simply be discarded returning an io::Error explaining why.

  • If attached to a console, then stdout and stderr will attempt to interpret input as UTF-8, re-encoding to UTF-16. If the input is not valid UTF-8 then an error will be returned and no data will be written.

Raw stdio

Note: This section is intended to be a sketch of possible raw stdio support, but it is not planned to implement or stabilize this implementation at this time.

The above standard input/output handles all involve some form of locking or buffering (or both). This cost is not always wanted, and hence raw variants will be provided. Due to platform differences across unix/windows, the following structure will be supported:

mod os {
    mod unix {
        mod stdio {
            struct Stdio { .. }

            impl Stdio {
                fn stdout() -> Stdio;
                fn stderr() -> Stdio;
                fn stdin() -> Stdio;

            impl Read for Stdio { ... }
            impl Write for Stdio { ... }

    mod windows {
        mod stdio {
            struct Stdio { ... }
            struct StdioConsole { ... }

            impl Stdio {
                fn stdout() -> io::Result<Stdio>;
                fn stderr() -> io::Result<Stdio>;
                fn stdin() -> io::Result<Stdio>;
            // same constructors StdioConsole

            impl Read for Stdio { ... }
            impl Write for Stdio { ... }

            impl StdioConsole {
                // returns slice of what was read
                fn read<'a>(&self, buf: &'a mut OsString) -> io::Result<&'a OsStr>;
                // returns remaining part of `buf` to be written
                fn write<'a>(&self, buf: &'a OsStr) -> io::Result<&'a OsStr>;

There are some key differences from today’s API:

  • On unix, the API has not changed much except that the handles have been consolidated into one type which implements both Read and Write (although writing to stdin is likely to generate an error).
  • On windows, there are two sets of handles representing the difference between “console mode” and not (e.g. a pipe). When not a console the normal I/O traits are implemented (delegating to ReadFile and WriteFile. The console mode operations work with OsStr, however, to show how they work with UCS-2 under the hood.

Printing functions

The current print, println, print_args, and println_args functions will all be “removed from the public interface” by prefixing them with __ and marking #[doc(hidden)]. These are all implementation details of the print! and println! macros and don’t need to be exposed in the public interface.

The set_stdout and set_stderr functions will be removed with no replacement for now. It’s unclear whether these functions should indeed control a thread local handle instead of a global handle as whether they’re justified in the first place. It is a backwards-compatible extension to allow this sort of output to be redirected and can be considered if the need arises.


Most of what’s available in std::os today will move to std::env, and the signatures will be updated to follow this RFC’s Design principles as follows.


  • args: change to yield an iterator rather than vector if possible; in any case, it should produce an OsString.

Environment variables:

  • vars (renamed from env): yields a vector of (OsString, OsString) pairs.

  • var (renamed from getenv): take a value bounded by AsOsStr, allowing Rust strings and slices to be ergonomically passed in. Yields an Option<OsString>.

  • var_string: take a value bounded by AsOsStr, returning Result<String, VarError> where VarError represents a non-unicode OsString or a “not present” value.

  • set_var (renamed from setenv): takes two AsOsStr-bounded values.

  • remove_var (renamed from unsetenv): takes a AsOsStr-bounded value.

  • join_paths: take an IntoIterator<T> where T: AsOsStr, yield a Result<OsString, JoinPathsError>.

  • split_paths take a AsOsStr, yield an Iterator<Path>.

Working directory:

  • current_dir (renamed from getcwd): yields a PathBuf.
  • set_current_dir (renamed from change_dir): takes an AsPath value.

Important locations:

  • home_dir (renamed from homedir): returns home directory as a PathBuf
  • temp_dir (renamed from tmpdir): returns a temporary directly as a PathBuf
  • current_exe (renamed from self_exe_name): returns the full path to the current binary as a PathBuf in an io::Result instead of an Option.

Exit status:

  • get_exit_status and set_exit_status stay as they are, but with updated docs that reflect that these only affect the return value of std::rt::start. These will remain #[unstable] for now and a future RFC will determine their stability.

Architecture information:

  • num_cpus, page_size: stay as they are, but remain #[unstable]. A future RFC will determine their stability and semantics.


  • Rename SYSNAME to OS.
  • Remove TMPBUF_SZ.

This brings the constants into line with our naming conventions elsewhere.

Items to move to os::platform

  • pipe will move to os::unix. It is currently primarily used for hooking to the IO of a child process, which will now be done behind a trait object abstraction.

Removed items

  • errno, error_string and last_os_error provide redundant, platform-specific functionality and will be removed for now. They may reappear later in os::unix and os::windows in a modified form.
  • dll_filename: deprecated in favor of working directly with the constants.
  • _NSGetArgc, _NSGetArgv: these should never have been public.
  • self_exe_path: deprecated in favor of current_exe plus path operations.
  • make_absolute: deprecated in favor of explicitly joining with the working directory.
  • all _as_bytes variants: deprecated in favor of yielding OsString values


The fs module will provide most of the functionality it does today, but with a stronger cross-platform orientation.

Note that all path-consuming functions will now take an AsPath-bounded parameter for ergonomic reasons (this will allow passing in Rust strings and literals directly, for example).

Free functions


  • copy. Take AsPath bound.

  • rename. Take AsPath bound.

  • remove_file (renamed from unlink). Take AsPath bound.

  • metadata (renamed from stat). Take AsPath bound. Yield a new struct, Metadata, with no public fields, but len, is_dir, is_file, perms, accessed and modified accessors. The various os::platform modules will offer extension methods on this structure.

  • set_perms (renamed from chmod). Take AsPath bound, and a Perms value. The Perms type will be revamped as a struct with private implementation; see below.


  • create_dir (renamed from mkdir). Take AsPath bound.
  • create_dir_all (renamed from mkdir_recursive). Take AsPath bound.
  • read_dir (renamed from readdir). Take AsPath bound. Yield a newtypes iterator, which yields a new type DirEntry which has an accessor for Path, but will eventually provide other information as well (possibly via platform-specific extensions).
  • remove_dir (renamed from rmdir). Take AsPath bound.
  • remove_dir_all (renamed from rmdir_recursive). Take AsPath bound.
  • walk_dir. Take AsPath bound. Yield an iterator over IoResult<DirEntry>.


  • hard_link (renamed from link). Take AsPath bound.
  • soft_link (renamed from symlink). Take AsPath bound.
  • read_link (renamed form readlink). Take AsPath bound.


The File type will largely stay as it is today, except that it will use the AsPath bound everywhere.

The stat method will be renamed to metadata, yield a Metadata structure (as described above), and take &self.

The fsync method will be renamed to sync_all, and datasync will be renamed to sync_data. (Although the latter is not available on Windows, it can be considered an optimization for flush and on Windows behave identically to sync_all, just as it does on some Unix filesystems.)

The path method will remain #[unstable], as we do not yet want to commit to its API.

The open_mode function will be removed in favor of and will take an OpenOptions struct, which will encompass today’s FileMode and FileAccess and support a builder-style API.

File kinds

The FileType type will be removed. As mentioned above, is_file and is_dir will be provided directly on Metadata; the other types need to be audited for compatibility across platforms. Platform-specific kinds will be relegated to extension traits in std::os::platform.

It’s possible that an extensible Kind will be added in the future.

File permissions

The permission models on Unix and Windows vary greatly – even between different filesystems within the same OS. Rather than offer an API that has no meaning on some platforms, we will initially provide a very limited Perms structure in std::fs, and then rich extension traits in std::os::unix and std::os::windows. Over time, if clear cross-platform patterns emerge for richer permissions, we can grow the Perms structure.

On the Unix side, the constructors and accessors for Perms will resemble the flags we have today; details are left to the implementation.

On the Windows side, initially there will be no extensions, as Windows has a very complex permissions model that will take some time to build out.

For std::fs itself, Perms will provide constructors and accessors for “world readable” – and that is all. At the moment, that is all that is known to be compatible across the platforms that Rust supports.


This trait will essentially remain stay as it is (renamed from PathExtensions), following the same changes made to fs free functions.

Items to move to os::platform

  • lstat will move to os::unix and remain #[unstable] for now since it is not yet implemented for Windows.

  • chown will move to os::unix (it currently does nothing on Windows), and eventually os::windows will grow support for Windows’s permission model. If at some point a reasonable intersection is found, we will re-introduce a cross-platform function in std::fs.

  • In general, offer all of the stat fields as an extension trait on Metadata (e.g. os::unix::MetadataExt).


The contents of std::io::net submodules tcp, udp, ip and addrinfo will be retained but moved into a single std::net module; the other modules are being moved or removed and are described elsewhere.


This structure will represent either a sockaddr_in or sockaddr_in6 which is commonly just a pairing of an IP address and a port.

enum SocketAddr {

impl SocketAddrV4 {
    fn new(addr: Ipv4Addr, port: u16) -> SocketAddrV4;
    fn ip(&self) -> &Ipv4Addr;
    fn port(&self) -> u16;

impl SocketAddrV6 {
    fn new(addr: Ipv6Addr, port: u16, flowinfo: u32, scope_id: u32) -> SocketAddrV6;
    fn ip(&self) -> &Ipv6Addr;
    fn port(&self) -> u16;
    fn flowinfo(&self) -> u32;
    fn scope_id(&self) -> u32;


Represents a version 4 IP address. It has the following interface:

impl Ipv4Addr {
    fn new(a: u8, b: u8, c: u8, d: u8) -> Ipv4Addr;
    fn any() -> Ipv4Addr;
    fn octets(&self) -> [u8; 4];
    fn to_ipv6_compatible(&self) -> Ipv6Addr;
    fn to_ipv6_mapped(&self) -> Ipv6Addr;


Represents a version 6 IP address. It has the following interface:

impl Ipv6Addr {
    fn new(a: u16, b: u16, c: u16, d: u16, e: u16, f: u16, g: u16, h: u16) -> Ipv6Addr;
    fn any() -> Ipv6Addr;
    fn segments(&self) -> [u16; 8]
    fn to_ipv4(&self) -> Option<Ipv4Addr>;


The current TcpStream struct will be pared back from where it is today to the following interface:

// TcpStream, which contains both a reader and a writer

impl TcpStream {
    fn connect<A: ToSocketAddrs>(addr: &A) -> io::Result<TcpStream>;
    fn peer_addr(&self) -> io::Result<SocketAddr>;
    fn local_addr(&self) -> io::Result<SocketAddr>;
    fn shutdown(&self, how: Shutdown) -> io::Result<()>;
    fn try_clone(&self) -> io::Result<TcpStream>;

impl Read for TcpStream { ... }
impl Write for TcpStream { ... }
impl<'a> Read for &'a TcpStream { ... }
impl<'a> Write for &'a TcpStream { ... }
#[cfg(unix)]    impl AsRawFd for TcpStream { ... }
#[cfg(windows)] impl AsRawSocket for TcpStream { ... }
  • clone has been replaced with a try_clone function. The implementation of try_clone will map to using dup on Unix platforms and WSADuplicateSocket on Windows platforms. The TcpStream itself will no longer be reference counted itself under the hood.
  • close_{read,write} are both removed in favor of binding the shutdown function directly on sockets. This will map to the shutdown function on both Unix and Windows.
  • set_timeout has been removed for now (as well as other timeout-related functions). It is likely that this may come back soon as a binding to setsockopt to the SO_RCVTIMEO and SO_SNDTIMEO options. This RFC does not currently proposed adding them just yet, however.
  • Implementations of Read and Write are provided for &TcpStream. These implementations are not necessarily ergonomic to call (requires taking an explicit reference), but they express the ability to concurrently read and write from a TcpStream

Various other options such as nodelay and keepalive will be left #[unstable] for now. The TcpStream structure will also adhere to both Send and Sync.

The TcpAcceptor struct will be removed and all functionality will be folded into the TcpListener structure. Specifically, this will be the resulting API:

impl TcpListener {
    fn bind<A: ToSocketAddrs>(addr: &A) -> io::Result<TcpListener>;
    fn local_addr(&self) -> io::Result<SocketAddr>;
    fn try_clone(&self) -> io::Result<TcpListener>;
    fn accept(&self) -> io::Result<(TcpStream, SocketAddr)>;
    fn incoming(&self) -> Incoming;

impl<'a> Iterator for Incoming<'a> {
    type Item = io::Result<TcpStream>;
#[cfg(unix)]    impl AsRawFd for TcpListener { ... }
#[cfg(windows)] impl AsRawSocket for TcpListener { ... }

Some major changes from today’s API include:

  • The static distinction between TcpAcceptor and TcpListener has been removed (more on this in the socket section).
  • The clone functionality has been removed in favor of try_clone (same caveats as TcpStream).
  • The close_accept functionality is removed entirely. This is not currently implemented via shutdown (not supported well across platforms) and is instead implemented via select. This functionality can return at a later date with a more robust interface.
  • The set_timeout functionality has also been removed in favor of returning at a later date in a more robust fashion with select.
  • The accept function no longer takes &mut self and returns SocketAddr. The change in mutability is done to express that multiple accept calls can happen concurrently.
  • For convenience the iterator does not yield the SocketAddr from accept.

The TcpListener type will also adhere to Send and Sync.


The UDP infrastructure will receive a similar face-lift as the TCP infrastructure will:

impl UdpSocket {
    fn bind<A: ToSocketAddrs>(addr: &A) -> io::Result<UdpSocket>;
    fn recv_from(&self, buf: &mut [u8]) -> io::Result<(usize, SocketAddr)>;
    fn send_to<A: ToSocketAddrs>(&self, buf: &[u8], addr: &A) -> io::Result<usize>;
    fn local_addr(&self) -> io::Result<SocketAddr>;
    fn try_clone(&self) -> io::Result<UdpSocket>;

#[cfg(unix)]    impl AsRawFd for UdpSocket { ... }
#[cfg(windows)] impl AsRawSocket for UdpSocket { ... }

Some important points of note are:

  • The send and recv function take &self instead of &mut self to indicate that they may be called safely in concurrent contexts.
  • All configuration options such as multicast and ttl are left as #[unstable] for now.
  • All timeout support is removed. This may come back in the form of setsockopt (as with TCP streams) or with a more general implementation of select.
  • clone functionality has been replaced with try_clone.

The UdpSocket type will adhere to both Send and Sync.


The current constructors for TcpStream, TcpListener, and UdpSocket are largely “convenience constructors” as they do not expose the underlying details that a socket can be configured before it is bound, connected, or listened on. One of the more frequent configuration options is SO_REUSEADDR which is set by default for TcpListener currently.

This RFC leaves it as an open question how best to implement this pre-configuration. The constructors today will likely remain no matter what as convenience constructors and a new structure would implement consuming methods to transform itself to each of the various TcpStream, TcpListener, and UdpSocket.

This RFC does, however, recommend not adding multiple constructors to the various types to set various configuration options. This pattern is best expressed via a flexible socket type to be added at a future date.


For the current addrinfo module:

  • The get_host_addresses should be renamed to lookup_host.
  • All other contents should be removed.

For the current ip module:

  • The ToSocketAddr trait should become ToSocketAddrs
  • The default to_socket_addr_all method should be removed.

The following implementations of ToSocketAddrs will be available:

impl ToSocketAddrs for SocketAddr { ... }
impl ToSocketAddrs for SocketAddrV4 { ... }
impl ToSocketAddrs for SocketAddrV6 { ... }
impl ToSocketAddrs for (Ipv4Addr, u16) { ... }
impl ToSocketAddrs for (Ipv6Addr, u16) { ... }
impl ToSocketAddrs for (&str, u16) { ... }
impl ToSocketAddrs for str { ... }
impl<T: ToSocketAddrs> ToSocketAddrs for &T { ... }


Currently std::io::process is used only for spawning new processes. The re-envisioned std::process will ultimately support inspecting currently-running processes, although this RFC does not propose any immediate support for doing so – it merely future-proofs the module.


The Command type is a builder API for processes, and is largely in good shape, modulo a few tweaks:

  • Replace ToCStr bounds with AsOsStr.
  • Replace env_set_all with env_clear
  • Rename cwd to current_dir, take AsPath.
  • Rename spawn to run
  • Move uid and gid to an extension trait in os::unix
  • Make detached take a bool (rather than always setting the command to detached mode).

The stdin, stdout, stderr methods will undergo a more significant change. By default, the corresponding options will be considered “unset”, the interpretation of which depends on how the process is launched:

  • For run or status, these will inherit from the current process by default.
  • For output, these will capture to new readers/writers by default.

The StdioContainer type will be renamed to Stdio, and will not be exposed directly as an enum (to enable growth and change over time). It will provide a Capture constructor for capturing input or output, an Inherit constructor (which just means to use the current IO object – it does not take an argument), and a Null constructor. The equivalent of today’s InheritFd will be added at a later point.


We propose renaming Process to Child so that we can add a more general notion of non-child Process later on (every Child will be able to give you a Process).

  • stdin, stdout and stderr will be retained as public fields, but their types will change to newtyped readers and writers to hide the internal pipe infrastructure.
  • The kill method is dropped, and id and signal will move to os::platform extension traits.
  • signal_exit, signal_kill, wait, and forget will all stay as they are.
  • set_timeout will be changed to use the with_deadline infrastructure.

There are also a few other related changes to the module:

  • Rename ProcessOutput to Output
  • Rename ProcessExit to ExitStatus, and hide its representation. Remove matches_exit_status, and add a status method yielding an Option<i32>
  • Remove MustDieSignal, PleaseExitSignal.
  • Remove EnvMap (which should never have been exposed).


Initially, this module will be empty except for the platform-specific unix and windows modules. It is expected to grow additional, more specific platform submodules (like linux, macos) over time.

Odds and ends

To be expanded in a follow-up PR.

The io prelude

The prelude submodule will contain most of the traits, types, and modules discussed in this RFC; it is meant to provide maximal convenience when working with IO of any kind. The exact contents of the module are left as an open question.


This RFC is largely about cleanup, normalization, and stabilization of our IO libraries – work that needs to be done, but that also represents nontrivial churn.

However, the actual implementation work involved is estimated to be reasonably contained, since all of the functionality is already in place in some form (including os_str, due to @SimonSapin’s WTF-8 implementation).


The main alternative design would be to continue staying with the Posix tradition in terms of naming and functionality (for which there is precedent in some other languages). However, Rust is already well-known for its strong cross-platform compatibility in std, and making the library more Windows-friendly will only increase its appeal.

More radically different designs (in terms of different design principles or visions) are outside the scope of this RFC.

Unresolved questions

To be expanded in follow-up PRs.

Wide string representation

(Text from @SimonSapin)

Rather than WTF-8, OsStr and OsString on Windows could use potentially-ill-formed UTF-16 (a.k.a. “wide” strings), with a different cost trade off.


  • No conversion between OsStr / OsString and OS calls.


  • More expensive conversions between OsStr / OsString and str / String.
  • These conversions have inconsistent performance characteristics between platforms. (Need to allocate on Windows, but not on Unix.)
  • Some of them return Cow, which has some ergonomic hit.

The API (only parts that differ) could look like:

pub mod os_str {
    mod imp {
        type Buf = Vec<u16>;
        type Slice = [u16];

    impl OsStr {
        pub fn from_str(&str) -> Cow<OsString, OsStr>;
        pub fn to_string(&self) -> Option<CowString>;
        pub fn to_string_lossy(&self) -> CowString;

    pub mod windows{
        trait OsStringExt {
            fn from_wide_slice(&[u16]) -> Self;
            fn from_wide_vec(Vec<u16>) -> Self;
            fn into_wide_vec(self) -> Vec<u16>;

        trait OsStrExt {
            fn from_wide_slice(&[u16]) -> Self;
            fn as_wide_slice(&self) -> &[u16];