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+//! Unsafe `ioctl` API.
+//!
+//! Unix systems expose a number of `ioctl`'s. `ioctl`s have been adopted as a
+//! general purpose system call for making calls into the kernel. In addition
+//! to the wide variety of system calls that are included by default in the
+//! kernel, many drivers expose their own `ioctl`'s for controlling their
+//! behavior, some of which are proprietary. Therefore it is impossible to make
+//! a safe interface for every `ioctl` call, as they all have wildly varying
+//! semantics.
+//!
+//! This module provides an unsafe interface to write your own `ioctl` API. To
+//! start, create a type that implements [`Ioctl`]. Then, pass it to [`ioctl`]
+//! to make the `ioctl` call.
+
+#![allow(unsafe_code)]
+
+use crate::backend::c;
+use crate::fd::{AsFd, BorrowedFd};
+use crate::io::Result;
+
+#[cfg(any(linux_kernel, bsd))]
+use core::mem;
+
+pub use patterns::*;
+
+mod patterns;
+
+#[cfg(linux_kernel)]
+mod linux;
+
+#[cfg(bsd)]
+mod bsd;
+
+#[cfg(linux_kernel)]
+use linux as platform;
+
+#[cfg(bsd)]
+use bsd as platform;
+
+/// Perform an `ioctl` call.
+///
+/// `ioctl` was originally intended to act as a way of modifying the behavior
+/// of files, but has since been adopted as a general purpose system call for
+/// making calls into the kernel. In addition to the default calls exposed by
+/// generic file descriptors, many drivers expose their own `ioctl` calls for
+/// controlling their behavior, some of which are proprietary.
+///
+/// This crate exposes many other `ioctl` interfaces with safe and idiomatic
+/// wrappers, like [`ioctl_fionbio`](crate::io::ioctl_fionbio) and
+/// [`ioctl_fionread`](crate::io::ioctl_fionread). It is recommended to use
+/// those instead of this function, as they are safer and more idiomatic.
+/// For other cases, implement the [`Ioctl`] API and pass it to this function.
+///
+/// See documentation for [`Ioctl`] for more information.
+///
+/// # Safety
+///
+/// While [`Ioctl`] takes much of the unsafety out of `ioctl` calls, it is
+/// still unsafe to call this code with arbitrary device drivers, as it is up
+/// to the device driver to implement the `ioctl` call correctly. It is on the
+/// onus of the protocol between the user and the driver to ensure that the
+/// `ioctl` call is safe to make.
+///
+/// # References
+///
+/// - [Linux]
+/// - [WinSock2]
+/// - [FreeBSD]
+/// - [NetBSD]
+/// - [OpenBSD]
+/// - [Apple]
+/// - [Solaris]
+/// - [illumos]
+///
+/// [Linux]: https://man7.org/linux/man-pages/man2/ioctl.2.html
+/// [Winsock2]: https://learn.microsoft.com/en-us/windows/win32/api/winsock/nf-winsock-ioctlsocket
+/// [FreeBSD]: https://man.freebsd.org/cgi/man.cgi?query=ioctl&sektion=2
+/// [NetBSD]: https://man.netbsd.org/ioctl.2
+/// [OpenBSD]: https://man.openbsd.org/ioctl.2
+/// [Apple]: https://developer.apple.com/library/archive/documentation/System/Conceptual/ManPages_iPhoneOS/man2/ioctl.2.html
+/// [Solaris]: https://docs.oracle.com/cd/E23824_01/html/821-1463/ioctl-2.html
+/// [illumos]: https://illumos.org/man/2/ioctl
+#[inline]
+pub unsafe fn ioctl<F: AsFd, I: Ioctl>(fd: F, mut ioctl: I) -> Result<I::Output> {
+    let fd = fd.as_fd();
+    let request = I::OPCODE.raw();
+    let arg = ioctl.as_ptr();
+
+    // SAFETY: The variant of `Ioctl` asserts that this is a valid IOCTL call to
+    // make.
+    let output = if I::IS_MUTATING {
+        _ioctl(fd, request, arg)?
+    } else {
+        _ioctl_readonly(fd, request, arg)?
+    };
+
+    // SAFETY: The variant of `Ioctl` asserts that this is a valid pointer to the
+    // output data.
+    I::output_from_ptr(output, arg)
+}
+
+unsafe fn _ioctl(
+    fd: BorrowedFd<'_>,
+    request: RawOpcode,
+    arg: *mut c::c_void,
+) -> Result<IoctlOutput> {
+    crate::backend::io::syscalls::ioctl(fd, request, arg)
+}
+
+unsafe fn _ioctl_readonly(
+    fd: BorrowedFd<'_>,
+    request: RawOpcode,
+    arg: *mut c::c_void,
+) -> Result<IoctlOutput> {
+    crate::backend::io::syscalls::ioctl_readonly(fd, request, arg)
+}
+
+/// A trait defining the properties of an `ioctl` command.
+///
+/// Objects implementing this trait can be passed to [`ioctl`] to make an
+/// `ioctl` call. The contents of the object represent the inputs to the
+/// `ioctl` call. The inputs must be convertible to a pointer through the
+/// `as_ptr` method. In most cases, this involves either casting a number to a
+/// pointer, or creating a pointer to the actual data. The latter case is
+/// necessary for `ioctl` calls that modify userspace data.
+///
+/// # Safety
+///
+/// This trait is unsafe to implement because it is impossible to guarantee
+/// that the `ioctl` call is safe. The `ioctl` call may be proprietary, or it
+/// may be unsafe to call in certain circumstances.
+///
+/// By implementing this trait, you guarantee that:
+///
+/// - The `ioctl` call expects the input provided by `as_ptr` and produces the
+///   output as indicated by `output`.
+/// - That `output_from_ptr` can safely take the pointer from `as_ptr` and cast
+///   it to the correct type, *only* after the `ioctl` call.
+/// - That `OPCODE` uniquely identifies the `ioctl` call.
+/// - That, for whatever platforms you are targeting, the `ioctl` call is safe
+///   to make.
+/// - If `IS_MUTATING` is false, that no userspace data will be modified by the
+///   `ioctl` call.
+pub unsafe trait Ioctl {
+    /// The type of the output data.
+    ///
+    /// Given a pointer, one should be able to construct an instance of this
+    /// type.
+    type Output;
+
+    /// The opcode used by this `ioctl` command.
+    ///
+    /// There are different types of opcode depending on the operation. See
+    /// documentation for the [`Opcode`] struct for more information.
+    const OPCODE: Opcode;
+
+    /// Does the `ioctl` mutate any data in the userspace?
+    ///
+    /// If the `ioctl` call does not mutate any data in the userspace, then
+    /// making this `false` enables optimizations that can make the call
+    /// faster. When in doubt, set this to `true`.
+    ///
+    /// # Safety
+    ///
+    /// This should only be set to `false` if the `ioctl` call does not mutate
+    /// any data in the userspace. Undefined behavior may occur if this is set
+    /// to `false` when it should be `true`.
+    const IS_MUTATING: bool;
+
+    /// Get a pointer to the data to be passed to the `ioctl` command.
+    ///
+    /// See trait-level documentation for more information.
+    fn as_ptr(&mut self) -> *mut c::c_void;
+
+    /// Cast the output data to the correct type.
+    ///
+    /// # Safety
+    ///
+    /// The `extract_output` value must be the resulting value after a
+    /// successful `ioctl` call, and `out` is the direct return value of an
+    /// `ioctl` call that did not fail. In this case `extract_output` is the
+    /// pointer that was passed to the `ioctl` call.
+    unsafe fn output_from_ptr(
+        out: IoctlOutput,
+        extract_output: *mut c::c_void,
+    ) -> Result<Self::Output>;
+}
+
+/// The opcode used by an `Ioctl`.
+#[derive(Debug, Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
+pub struct Opcode {
+    /// The raw opcode.
+    raw: RawOpcode,
+}
+
+impl Opcode {
+    /// Create a new old `Opcode` from a raw opcode.
+    ///
+    /// Rather than being a composition of several attributes, old opcodes are
+    /// just numbers. In general most drivers follow stricter conventions, but
+    /// older drivers may still use this strategy.
+    #[inline]
+    pub const fn old(raw: RawOpcode) -> Self {
+        Self { raw }
+    }
+
+    /// Create a new opcode from a direction, group, number and size.
+    #[cfg(any(linux_kernel, bsd))]
+    #[inline]
+    pub const fn from_components(
+        direction: Direction,
+        group: u8,
+        number: u8,
+        data_size: usize,
+    ) -> Self {
+        if data_size > RawOpcode::MAX as usize {
+            panic!("data size is too large");
+        }
+
+        Self::old(platform::compose_opcode(
+            direction,
+            group as RawOpcode,
+            number as RawOpcode,
+            data_size as RawOpcode,
+        ))
+    }
+
+    /// Create a new non-mutating opcode from a group, a number and the type of
+    /// data.
+    #[cfg(any(linux_kernel, bsd))]
+    #[inline]
+    pub const fn none<T>(group: u8, number: u8) -> Self {
+        Self::from_components(Direction::None, group, number, mem::size_of::<T>())
+    }
+
+    /// Create a new reading opcode from a group, a number and the type of
+    /// data.
+    #[cfg(any(linux_kernel, bsd))]
+    #[inline]
+    pub const fn read<T>(group: u8, number: u8) -> Self {
+        Self::from_components(Direction::Read, group, number, mem::size_of::<T>())
+    }
+
+    /// Create a new writing opcode from a group, a number and the type of
+    /// data.
+    #[cfg(any(linux_kernel, bsd))]
+    #[inline]
+    pub const fn write<T>(group: u8, number: u8) -> Self {
+        Self::from_components(Direction::Write, group, number, mem::size_of::<T>())
+    }
+
+    /// Create a new reading and writing opcode from a group, a number and the
+    /// type of data.
+    #[cfg(any(linux_kernel, bsd))]
+    #[inline]
+    pub const fn read_write<T>(group: u8, number: u8) -> Self {
+        Self::from_components(Direction::ReadWrite, group, number, mem::size_of::<T>())
+    }
+
+    /// Get the raw opcode.
+    #[inline]
+    pub fn raw(self) -> RawOpcode {
+        self.raw
+    }
+}
+
+/// The direction that an `ioctl` is going.
+///
+/// Note that this is relative to userspace. `Read` means reading data from the
+/// kernel, and write means the kernel writing data to userspace.
+#[derive(Debug, Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
+pub enum Direction {
+    /// None of the above.
+    None,
+
+    /// Read data from the kernel.
+    Read,
+
+    /// Write data to the kernel.
+    Write,
+
+    /// Read and write data to the kernel.
+    ReadWrite,
+}
+
+/// The type used by the `ioctl` to signify the output.
+pub type IoctlOutput = c::c_int;
+
+/// The type used by the `ioctl` to signify the command.
+pub type RawOpcode = _RawOpcode;
+
+// Under raw Linux, this is an `unsigned int`.
+#[cfg(linux_raw)]
+type _RawOpcode = c::c_uint;
+
+// On libc Linux with GNU libc or uclibc, this is an `unsigned long`.
+#[cfg(all(
+    not(linux_raw),
+    target_os = "linux",
+    any(target_env = "gnu", target_env = "uclibc")
+))]
+type _RawOpcode = c::c_ulong;
+
+// Musl uses `c_int`.
+#[cfg(all(
+    not(linux_raw),
+    target_os = "linux",
+    not(target_env = "gnu"),
+    not(target_env = "uclibc")
+))]
+type _RawOpcode = c::c_int;
+
+// Android uses `c_int`.
+#[cfg(all(not(linux_raw), target_os = "android"))]
+type _RawOpcode = c::c_int;
+
+// BSD, Haiku, and Redox use `unsigned long`.
+#[cfg(any(bsd, target_os = "redox", target_os = "haiku"))]
+type _RawOpcode = c::c_ulong;
+
+// AIX, Emscripten, Fuchsia, Solaris, and WASI use a `int`.
+#[cfg(any(
+    solarish,
+    target_os = "aix",
+    target_os = "fuchsia",
+    target_os = "emscripten",
+    target_os = "wasi",
+    target_os = "nto"
+))]
+type _RawOpcode = c::c_int;
+
+// ESP-IDF uses a `c_uint`.
+#[cfg(target_os = "espidf")]
+type _RawOpcode = c::c_uint;
+
+// Windows has `ioctlsocket`, which uses `i32`.
+#[cfg(windows)]
+type _RawOpcode = i32;