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. These types reflect concepts of I/O safety on Unix.
| Type | Analogous to |
|---|---|
RawFd | *const _ |
BorrowedFd<'a> | &'a Arc<_> |
OwnedFd | Arc<_> |
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.
Libraries are encouraged to migrate,
either by adding unsafe to APIs that dereference RawFd values, or by
using to BorrowedFd or OwnedFd instead.
The use of Arc for borrowed/owned file descriptors may be surprising. Unix file descriptors
are mere references to internal kernel objects called “open file descriptions”, and the same
open file description can be referenced by multiple file descriptors (e.g. if dup is used).
State such as the offset within the file is shared among all file descriptors that refer to the
same open file description, and the kernel internally does reference-counting to only close the
underlying resource once all file descriptors referencing it are closed. That’s why Arc (and
not Box) is the closest Rust analogy to an “owned” file descriptor.
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. (Equivalently: an&Arc<_>cannot bedroped.) -
dup2/dup3, in the second argument, because this argument is closed and assigned a new resource, which may break the assumptions of other code using that file descriptor.
BorrowedFd values may be used in APIs which provide safe access to dup system calls, so code
working with OwnedFd cannot assume to have exclusive access to the underlying open file
description. (Equivalently: &Arc may be used in APIs that provide safe access to clone, so
code working with an Arc cannot assume that the reference count is 1.)
BorrowedFd values may also be used with mmap, since mmap uses the
provided file descriptor in a manner similar to dup and does not require
the BorrowedFd passed to it to live for the lifetime of the resulting
mapping. 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 Arc, OwnedFd values conceptually own one reference to the resource they point to,
and decrement the reference count when they are dropped (by calling close).
When the reference count reaches 0, the underlying open file description will be freed
by the kernel.
See the io module docs for a general explanation of I/O safety.
§/proc/self/mem and similar OS features
Some platforms have special files, such as /proc/self/mem, which
provide read and write access to the process’s memory. Such reads
and writes happen outside the control of the Rust compiler, so they do not
uphold Rust’s memory safety guarantees.
This does not mean that all APIs that might allow /proc/self/mem
to be opened and read from or written must be unsafe. Rust’s safety guarantees
only cover what the program itself can do, and not what entities outside
the program can do to it. /proc/self/mem is considered to be such an
external entity, along with /proc/self/fd/*, debugging interfaces, and people with physical
access to the hardware. This is true even in cases where the program is controlling the external
entity.
If you desire to comprehensively prevent programs from reaching out and
causing external entities to reach back in and violate memory safety, it’s
necessary to use sandboxing, which is outside the scope of std.
Structs§
- A borrowed file descriptor.
- An owned file descriptor.
Traits§
- 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 Aliases§
- Raw file descriptors.