Module io

Available on Unix only.

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 be droped.)

• 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.

Re-exports

• pub use crate::os::fd::*;