Module std::os::unix::io

1.0.0 · source · []
Available on Unix only.
Expand description

Unix-specific extensions to general I/O primitives.

Just like raw pointers, raw file descriptors point to resources with dynamic lifetimes, and they can dangle if they outlive their resources or be forged if they’re created from invalid values.

This module provides three types for representing file descriptors, with different ownership properties: raw, borrowed, and owned, which are analogous to types used for representing pointers:

TypeAnalogous to
RawFd*const _
BorrowedFd<'a>&'a _

Like raw pointers, RawFd values are primitive values. And in new code, they should be considered unsafe to do I/O on (analogous to dereferencing them). Rust did not always provide this guidance, so existing code in the Rust ecosystem often doesn’t mark RawFd usage as unsafe. Once the io_safety feature is stable, libraries will be encouraged to migrate, either by adding unsafe to APIs that dereference RawFd values, or by using to BorrowedFd or OwnedFd instead.

Like references, BorrowedFd values are tied to a lifetime, to ensure that they don’t outlive the resource they point to. These are safe to use. BorrowedFd values may be used in APIs which provide safe access to any system call except for:

  • close, because that would end the dynamic lifetime of the resource without ending the lifetime of the file descriptor.
  • dup2/dup3, in the second argument, because this argument is closed and assigned a new resource, which may break the assumptions other code using that file descriptor. This list doesn’t include mmap, since mmap does do a proper borrow of its file descriptor argument. That said, mmap is unsafe for other reasons: it operates on raw pointers, and it can have undefined behavior if the underlying storage is mutated. Mutations may come from other processes, or from the same process if the API provides BorrowedFd access, since as mentioned earlier, BorrowedFd values may be used in APIs which provide safe access to any system call. Consequently, code using mmap and presenting a safe API must take full responsibility for ensuring that safe Rust code cannot evoke undefined behavior through it.

Like boxes, OwnedFd values conceptually own the resource they point to, and free (close) it when they are dropped.



A borrowed file descriptor.


An owned file descriptor.



A trait to borrow the file descriptor from an underlying object.

A trait to extract the raw file descriptor from an underlying object.

A trait to express the ability to construct an object from a raw file descriptor.

A trait to express the ability to consume an object and acquire ownership of its raw file descriptor.

Type Definitions

Raw file descriptors.