From 6b6bc9805ddeebf35fec63844e6f00f06059650a Mon Sep 17 00:00:00 2001 From: Ralf Jung Date: Fri, 26 Apr 2024 19:04:23 +0200 Subject: [PATCH] io safety: update Unix explanation --- library/std/src/io/mod.rs | 2 +- library/std/src/os/unix/io/mod.rs | 31 +++++++++++++++++++++---------- 2 files changed, 22 insertions(+), 11 deletions(-) diff --git a/library/std/src/io/mod.rs b/library/std/src/io/mod.rs index 98973a43e1ded..a3fdcdc2d07ba 100644 --- a/library/std/src/io/mod.rs +++ b/library/std/src/io/mod.rs @@ -266,7 +266,7 @@ //! its file descriptors with no operations being performed by any other part of the program. //! //! Note that exclusive ownership of a file descriptor does *not* imply exclusive ownership of the -//! underlying kernel object that the file descriptor references (also called "file description" on +//! underlying kernel object that the file descriptor references (also called "open file description" on //! some operating systems). File descriptors basically work like [`Arc`]: when you receive an owned //! file descriptor, you cannot know whether there are any other file descriptors that reference the //! same kernel object. However, when you create a new kernel object, you know that you are holding diff --git a/library/std/src/os/unix/io/mod.rs b/library/std/src/os/unix/io/mod.rs index 827278f8b2605..6d4090ee31cfc 100644 --- a/library/std/src/os/unix/io/mod.rs +++ b/library/std/src/os/unix/io/mod.rs @@ -12,8 +12,8 @@ //! | Type | Analogous to | //! | ------------------ | ------------ | //! | [`RawFd`] | `*const _` | -//! | [`BorrowedFd<'a>`] | `&'a _` | -//! | [`OwnedFd`] | `Box<_>` | +//! | [`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 @@ -23,22 +23,31 @@ //! 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. +//! without ending the lifetime of the file descriptor. (Equivalently: +//! an `&Arc<_>` cannot be `drop`ed.) //! //! - `dup2`/`dup3`, in the second argument, because this argument is -//! closed and assigned a new resource, which may break the assumptions +//! 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 types implementing `AsFd` or `From` should not -//! assume they always have exclusive access to the underlying file -//! description. +//! `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 @@ -52,8 +61,10 @@ //! 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. +//! 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][io-safety] for a general explanation of I/O safety. //!