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    //! Unix user, group, and process identifiers.
    //!
    //! # Safety
    //!
    //! The `Uid`, `Gid`, and `Pid` types can be constructed from raw integers,
    //! which is marked unsafe because actual OS's assign special meaning to some
    //! integer values.
    #![allow(unsafe_code)]

    use crate::{backend, io};
    #[cfg(any(target_os = "android", target_os = "linux"))]
    use backend::process::types::RawCpuid;

    /// The raw integer value of a Unix user ID.
    pub use backend::process::types::RawUid;

    /// The raw integer value of a Unix group ID.
    pub use backend::process::types::RawGid;

    /// The raw integer value of a Unix process ID.
    pub use backend::process::types::RawPid;

    /// The raw integer value of a Unix process ID.
    pub use backend::process::types::RawNonZeroPid;

    /// `uid_t`—A Unix user ID.
    #[repr(transparent)]
    #[derive(Copy, Clone, Eq, PartialEq, Debug, Hash)]
    pub struct Uid(RawUid);

    /// `gid_t`—A Unix group ID.
    #[repr(transparent)]
    #[derive(Copy, Clone, Eq, PartialEq, Debug, Hash)]
    pub struct Gid(RawGid);

    /// `pid_t`—A non-zero Unix process ID.
    ///
    /// This is a pid, and not a pidfd. It is not a file descriptor, and the
    /// process it refers to could disappear at any time and be replaced by
    /// another, unrelated, process.
    #[repr(transparent)]
    #[derive(Copy, Clone, Eq, PartialEq, Debug, Hash)]
    pub struct Pid(RawNonZeroPid);

    /// A Linux CPU ID.
    #[cfg(any(target_os = "android", target_os = "linux"))]
    #[repr(transparent)]
    #[derive(Copy, Clone, Eq, PartialEq, Debug, Hash)]
    pub struct Cpuid(RawCpuid);

    impl Uid {
        /// A `Uid` corresponding to the root user (uid 0).
        pub const ROOT: Self = Self(0);

        /// Converts a `RawUid` into a `Uid`.
        ///
        /// # Safety
        ///
        /// `raw` must be the value of a valid Unix user ID.
        #[inline]
        pub const unsafe fn from_raw(raw: RawUid) -> Self {
            Self(raw)
        }

        /// Converts a `Uid` into a `RawUid`.
        #[inline]
        pub const fn as_raw(self) -> RawUid {
            self.0
        }

        /// Test whether this uid represents the root user (uid 0).
        #[inline]
        pub const fn is_root(self) -> bool {
            self.0 == Self::ROOT.0
        }
    }

    impl Gid {
        /// A `Gid` corresponding to the root group (gid 0).
        pub const ROOT: Self = Self(0);

        /// Converts a `RawGid` into a `Gid`.
        ///
        /// # Safety
        ///
        /// `raw` must be the value of a valid Unix group ID.
        #[inline]
        pub const unsafe fn from_raw(raw: RawGid) -> Self {
            Self(raw)
        }

        /// Converts a `Gid` into a `RawGid`.
        #[inline]
        pub const fn as_raw(self) -> RawGid {
            self.0
        }

        /// Test whether this gid represents the root group (gid 0).
        #[inline]
        pub const fn is_root(self) -> bool {
            self.0 == Self::ROOT.0
        }
    }

    impl Pid {
        /// A `Pid` corresponding to the init process (pid 1).
        pub const INIT: Self = Self(
            // Safety: The init process' pid is always valid.
            unsafe { RawNonZeroPid::new_unchecked(1) },
        );

        /// Converts a `RawPid` into a `Pid`.
        ///
        /// # Safety
        ///
        /// `raw` must be the value of a valid Unix process ID, or zero.
        #[inline]
        pub const unsafe fn from_raw(raw: RawPid) -> Option<Self> {
            match RawNonZeroPid::new(raw) {
                Some(pid) => Some(Self(pid)),
                None => None,
            }
        }

        /// Converts a known non-zero `RawPid` into a `Pid`.
        ///
        /// # Safety
        ///
        /// `raw` must be the value of a valid Unix process ID. It must not be
        /// zero.
        #[inline]
        pub const unsafe fn from_raw_nonzero(raw: RawNonZeroPid) -> Self {
            Self(raw)
        }

        /// Creates a `Pid` holding the ID of the given child process.
        #[cfg(feature = "std")]
        #[inline]
        pub fn from_child(child: &std::process::Child) -> Self {
            let id = child.id();
            debug_assert_ne!(id, 0);

            // Safety: We know the returned ID is valid because it came directly
            // from an OS API.
            unsafe { Self::from_raw_nonzero(RawNonZeroPid::new_unchecked(id as _)) }
        }

        /// Converts a `Pid` into a `RawNonZeroPid`.
        #[inline]
        pub const fn as_raw_nonzero(self) -> RawNonZeroPid {
            self.0
        }

        /// Converts an `Option<Pid>` into a `RawPid`.
        #[inline]
        pub fn as_raw(pid: Option<Self>) -> RawPid {
            pid.map_or(0, |pid| pid.0.get())
        }

        /// Test whether this pid represents the init process (pid 0).
        #[inline]
        pub const fn is_init(self) -> bool {
            self.0.get() == Self::INIT.0.get()
        }
    }

    #[cfg(any(target_os = "android", target_os = "linux"))]
    impl Cpuid {
        /// Converts a `RawCpuid` into a `Cpuid`.
        ///
        /// # Safety
        ///
        /// `raw` must be the value of a valid Linux CPU ID.
        #[inline]
        pub const unsafe fn from_raw(raw: RawCpuid) -> Self {
            Self(raw)
        }

        /// Converts a `Cpuid` into a `RawCpuid`.
        #[inline]
        pub const fn as_raw(self) -> RawCpuid {
            self.0
        }
    }

    /// `getuid()`—Returns the process' real user ID.
    ///
    /// # References
    ///  - [POSIX]
    ///  - [Linux]
    ///
    /// [POSIX]: https://pubs.opengroup.org/onlinepubs/9699919799/functions/getuid.html
    /// [Linux]: https://man7.org/linux/man-pages/man2/getuid.2.html
    #[inline]
    #[must_use]
    pub fn getuid() -> Uid {
        backend::process::syscalls::getuid()
    }

    /// `geteuid()`—Returns the process' effective user ID.
    ///
    /// # References
    ///  - [POSIX]
    ///  - [Linux]
    ///
    /// [POSIX]: https://pubs.opengroup.org/onlinepubs/9699919799/functions/geteuid.html
    /// [Linux]: https://man7.org/linux/man-pages/man2/geteuid.2.html
    #[inline]
    #[must_use]
    pub fn geteuid() -> Uid {
        backend::process::syscalls::geteuid()
    }

    /// `getgid()`—Returns the process' real group ID.
    ///
    /// # References
    ///  - [POSIX]
    ///  - [Linux]
    ///
    /// [POSIX]: https://pubs.opengroup.org/onlinepubs/9699919799/functions/getgid.html
    /// [Linux]: https://man7.org/linux/man-pages/man2/getgid.2.html
    #[inline]
    #[must_use]
    pub fn getgid() -> Gid {
        backend::process::syscalls::getgid()
    }

    /// `getegid()`—Returns the process' effective group ID.
    ///
    /// # References
    ///  - [POSIX]
    ///  - [Linux]
    ///
    /// [POSIX]: https://pubs.opengroup.org/onlinepubs/9699919799/functions/getegid.html
    /// [Linux]: https://man7.org/linux/man-pages/man2/getegid.2.html
    #[inline]
    #[must_use]
    pub fn getegid() -> Gid {
        backend::process::syscalls::getegid()
    }

    /// `getpid()`—Returns the process' ID.
    ///
    /// # References
    ///  - [POSIX]
    ///  - [Linux]
    ///
    /// [POSIX]: https://pubs.opengroup.org/onlinepubs/9699919799/functions/getpid.html
    /// [Linux]: https://man7.org/linux/man-pages/man2/getpid.2.html
    #[inline]
    #[must_use]
    pub fn getpid() -> Pid {
        backend::process::syscalls::getpid()
    }

    /// `getppid()`—Returns the parent process' ID.
    ///
    /// # References
    ///  - [POSIX]
    ///  - [Linux]
    ///
    /// [POSIX]: https://pubs.opengroup.org/onlinepubs/9699919799/functions/getppid.html
    /// [Linux]: https://man7.org/linux/man-pages/man2/getppid.2.html
    #[inline]
    #[must_use]
    pub fn getppid() -> Option<Pid> {
        backend::process::syscalls::getppid()
    }

    /// `getpgid(pid)`—Returns the process group ID of the given process.
    ///
    /// # References
    ///  - [POSIX]
    ///  - [Linux]
    ///
    /// [POSIX]: https://pubs.opengroup.org/onlinepubs/9699919799/functions/getpgid.html
    /// [Linux]: https://man7.org/linux/man-pages/man2/getpgid.2.html
    #[inline]
    pub fn getpgid(pid: Option<Pid>) -> io::Result<Pid> {
        backend::process::syscalls::getpgid(pid)
    }

    /// `getpgrp()`—Returns the process' group ID.
    ///
    /// # References
    ///  - [POSIX]
    ///  - [Linux]
    ///
    /// [POSIX]: https://pubs.opengroup.org/onlinepubs/9699919799/functions/getpgrp.html
    /// [Linux]: https://man7.org/linux/man-pages/man2/getpgrp.2.html
    #[inline]
    #[must_use]
    pub fn getpgrp() -> Pid {
        backend::process::syscalls::getpgrp()
    }

    /// `setsid()`—Create a new session.
    ///
    /// # References
    ///  - [POSIX]
    ///  - [Linux]
    ///
    /// [POSIX]: https://pubs.opengroup.org/onlinepubs/9699919799/functions/setsid.html
    /// [Linux]: https://man7.org/linux/man-pages/man2/setsid.2.html
    #[inline]
    pub fn setsid() -> io::Result<Pid> {
        backend::process::syscalls::setsid()
    }

    // translate_fchown_args returns the raw value of the IDs. In case of `None`
    // it returns `u32::MAX` since it has the same bit pattern as `-1` indicating
    // no change to the owner/group ID.
    pub(crate) fn translate_fchown_args(owner: Option<Uid>, group: Option<Gid>) -> (u32, u32) {
        let ow = match owner {
            Some(o) => o.as_raw(),
            None => u32::MAX,
        };

        let gr = match group {
            Some(g) => g.as_raw(),
            None => u32::MAX,
        };

        (ow, gr)
    }