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    //! Safe wrappers around functions found in libc "unistd.h" header

    use errno::{self, Errno};
    use {Error, Result, NixPath};
    use fcntl::{AtFlags, at_rawfd, fcntl, FdFlag, OFlag};
    use fcntl::FcntlArg::F_SETFD;
    use libc::{self, c_char, c_void, c_int, c_long, c_uint, size_t, pid_t, off_t,
               uid_t, gid_t, mode_t};
    use std::{fmt, mem, ptr};
    use std::ffi::{CString, CStr, OsString, OsStr};
    use std::os::unix::ffi::{OsStringExt, OsStrExt};
    use std::os::unix::io::RawFd;
    use std::path::PathBuf;
    use void::Void;
    use sys::stat::Mode;

    #[cfg(any(target_os = "android", target_os = "linux"))]
    pub use self::pivot_root::*;

    #[cfg(any(target_os = "android", target_os = "freebsd",
              target_os = "linux", target_os = "openbsd"))]
    pub use self::setres::*;

    /// User identifier
    ///
    /// Newtype pattern around `uid_t` (which is just alias). It prevents bugs caused by accidentally
    /// passing wrong value.
    #[derive(Debug, Copy, Clone, Eq, PartialEq, Hash)]
    pub struct Uid(uid_t);

    impl Uid {
        /// Creates `Uid` from raw `uid_t`.
        pub fn from_raw(uid: uid_t) -> Self {
            Uid(uid)
        }

        /// Returns Uid of calling process. This is practically a more Rusty alias for `getuid`.
        pub fn current() -> Self {
            getuid()
        }

        /// Returns effective Uid of calling process. This is practically a more Rusty alias for `geteuid`.
        pub fn effective() -> Self {
            geteuid()
        }

        /// Returns true if the `Uid` represents privileged user - root. (If it equals zero.)
        pub fn is_root(&self) -> bool {
            *self == ROOT
        }

        /// Get the raw `uid_t` wrapped by `self`.
        pub fn as_raw(&self) -> uid_t {
            self.0
        }
    }

    impl From<Uid> for uid_t {
        fn from(uid: Uid) -> Self {
            uid.0
        }
    }

    impl fmt::Display for Uid {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            fmt::Display::fmt(&self.0, f)
        }
    }

    /// Constant for UID = 0
    pub const ROOT: Uid = Uid(0);

    /// Group identifier
    ///
    /// Newtype pattern around `gid_t` (which is just alias). It prevents bugs caused by accidentally
    /// passing wrong value.
    #[derive(Debug, Copy, Clone, Eq, PartialEq, Hash)]
    pub struct Gid(gid_t);

    impl Gid {
        /// Creates `Gid` from raw `gid_t`.
        pub fn from_raw(gid: gid_t) -> Self {
            Gid(gid)
        }

        /// Returns Gid of calling process. This is practically a more Rusty alias for `getgid`.
        pub fn current() -> Self {
            getgid()
        }

        /// Returns effective Gid of calling process. This is practically a more Rusty alias for `getgid`.
        pub fn effective() -> Self {
            getegid()
        }

        /// Get the raw `gid_t` wrapped by `self`.
        pub fn as_raw(&self) -> gid_t {
            self.0
        }
    }

    impl From<Gid> for gid_t {
        fn from(gid: Gid) -> Self {
            gid.0
        }
    }

    impl fmt::Display for Gid {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            fmt::Display::fmt(&self.0, f)
        }
    }

    /// Process identifier
    ///
    /// Newtype pattern around `pid_t` (which is just alias). It prevents bugs caused by accidentally
    /// passing wrong value.
    #[derive(Debug, Copy, Clone, Eq, PartialEq, Hash)]
    pub struct Pid(pid_t);

    impl Pid {
        /// Creates `Pid` from raw `pid_t`.
        pub fn from_raw(pid: pid_t) -> Self {
            Pid(pid)
        }

        /// Returns PID of calling process
        pub fn this() -> Self {
            getpid()
        }

        /// Returns PID of parent of calling process
        pub fn parent() -> Self {
            getppid()
        }

        /// Get the raw `pid_t` wrapped by `self`.
        pub fn as_raw(&self) -> pid_t {
            self.0
        }
    }

    impl From<Pid> for pid_t {
        fn from(pid: Pid) -> Self {
            pid.0
        }
    }

    impl fmt::Display for Pid {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            fmt::Display::fmt(&self.0, f)
        }
    }

    /// Represents the successful result of calling `fork`
    ///
    /// When `fork` is called, the process continues execution in the parent process
    /// and in the new child.  This return type can be examined to determine whether
    /// you are now executing in the parent process or in the child.
    #[derive(Clone, Copy, Debug)]
    pub enum ForkResult {
        Parent { child: Pid },
        Child,
    }

    impl ForkResult {

        /// Return `true` if this is the child process of the `fork()`
        #[inline]
        pub fn is_child(&self) -> bool {
            match *self {
                ForkResult::Child => true,
                _ => false
            }
        }

        /// Returns `true` if this is the parent process of the `fork()`
        #[inline]
        pub fn is_parent(&self) -> bool {
            !self.is_child()
        }
    }

    /// Create a new child process duplicating the parent process ([see
    /// fork(2)](http://pubs.opengroup.org/onlinepubs/9699919799/functions/fork.html)).
    ///
    /// After calling the fork system call (successfully) two processes will
    /// be created that are identical with the exception of their pid and the
    /// return value of this function.  As an example:
    ///
    /// ```no_run
    /// use nix::unistd::{fork, ForkResult};
    ///
    /// match fork() {
    ///    Ok(ForkResult::Parent { child, .. }) => {
    ///        println!("Continuing execution in parent process, new child has pid: {}", child);
    ///    }
    ///    Ok(ForkResult::Child) => println!("I'm a new child process"),
    ///    Err(_) => println!("Fork failed"),
    /// }
    /// ```
    ///
    /// This will print something like the following (order indeterministic).  The
    /// thing to note is that you end up with two processes continuing execution
    /// immediately after the fork call but with different match arms.
    ///
    /// ```text
    /// Continuing execution in parent process, new child has pid: 1234
    /// I'm a new child process
    /// ```
    ///
    /// # Safety
    ///
    /// In a multithreaded program, only [async-signal-safe] functions like `pause`
    /// and `_exit` may be called by the child (the parent isn't restricted). Note
    /// that memory allocation may **not** be async-signal-safe and thus must be
    /// prevented.
    ///
    /// Those functions are only a small subset of your operating system's API, so
    /// special care must be taken to only invoke code you can control and audit.
    ///
    /// [async-signal-safe]: http://man7.org/linux/man-pages/man7/signal-safety.7.html
    #[inline]
    pub fn fork() -> Result<ForkResult> {
        use self::ForkResult::*;
        let res = unsafe { libc::fork() };

        Errno::result(res).map(|res| match res {
            0 => Child,
            res => Parent { child: Pid(res) },
        })
    }

    /// Get the pid of this process (see
    /// [getpid(2)](http://pubs.opengroup.org/onlinepubs/9699919799/functions/getpid.html)).
    ///
    /// Since you are running code, there is always a pid to return, so there
    /// is no error case that needs to be handled.
    #[inline]
    pub fn getpid() -> Pid {
        Pid(unsafe { libc::getpid() })
    }

    /// Get the pid of this processes' parent (see
    /// [getpid(2)](http://pubs.opengroup.org/onlinepubs/9699919799/functions/getppid.html)).
    ///
    /// There is always a parent pid to return, so there is no error case that needs
    /// to be handled.
    #[inline]
    pub fn getppid() -> Pid {
        Pid(unsafe { libc::getppid() }) // no error handling, according to man page: "These functions are always successful."
    }

    /// Set a process group ID (see
    /// [setpgid(2)](http://pubs.opengroup.org/onlinepubs/9699919799/functions/setpgid.html)).
    ///
    /// Set the process group id (PGID) of a particular process.  If a pid of zero
    /// is specified, then the pid of the calling process is used.  Process groups
    /// may be used to group together a set of processes in order for the OS to
    /// apply some operations across the group.
    ///
    /// `setsid()` may be used to create a new process group.
    #[inline]
    pub fn setpgid(pid: Pid, pgid: Pid) -> Result<()> {
        let res = unsafe { libc::setpgid(pid.into(), pgid.into()) };
        Errno::result(res).map(drop)
    }
    #[inline]
    pub fn getpgid(pid: Option<Pid>) -> Result<Pid> {
        let res = unsafe { libc::getpgid(pid.unwrap_or(Pid(0)).into()) };
        Errno::result(res).map(Pid)
    }

    /// Create new session and set process group id (see
    /// [setsid(2)](http://pubs.opengroup.org/onlinepubs/9699919799/functions/setsid.html)).
    #[inline]
    pub fn setsid() -> Result<Pid> {
        Errno::result(unsafe { libc::setsid() }).map(Pid)
    }

    /// Get the process group ID of a session leader
    /// [getsid(2)](http://pubs.opengroup.org/onlinepubs/9699919799/functions/getsid.html).
    ///
    /// Obtain the process group ID of the process that is the session leader of the process specified
    /// by pid. If pid is zero, it specifies the calling process.
    #[inline]
    pub fn getsid(pid: Option<Pid>) -> Result<Pid> {
        let res = unsafe { libc::getsid(pid.unwrap_or(Pid(0)).into()) };
        Errno::result(res).map(Pid)
    }

    /// Get the terminal foreground process group (see
    /// [tcgetpgrp(3)](http://pubs.opengroup.org/onlinepubs/9699919799/functions/tcgetpgrp.html)).
    ///
    /// Get the group process id (GPID) of the foreground process group on the
    /// terminal associated to file descriptor (FD).
    #[inline]
    pub fn tcgetpgrp(fd: c_int) -> Result<Pid> {
        let res = unsafe { libc::tcgetpgrp(fd) };
        Errno::result(res).map(Pid)
    }
    /// Set the terminal foreground process group (see
    /// [tcgetpgrp(3)](http://pubs.opengroup.org/onlinepubs/9699919799/functions/tcsetpgrp.html)).
    ///
    /// Get the group process id (PGID) to the foreground process group on the
    /// terminal associated to file descriptor (FD).
    #[inline]
    pub fn tcsetpgrp(fd: c_int, pgrp: Pid) -> Result<()> {
        let res = unsafe { libc::tcsetpgrp(fd, pgrp.into()) };
        Errno::result(res).map(drop)
    }

    /// Get the group id of the calling process (see
    ///[getpgrp(3)](http://pubs.opengroup.org/onlinepubs/9699919799/functions/getpgrp.html)).
    ///
    /// Get the process group id (PGID) of the calling process.
    /// According to the man page it is always successful.
    #[inline]
    pub fn getpgrp() -> Pid {
        Pid(unsafe { libc::getpgrp() })
    }

    /// Get the caller's thread ID (see
    /// [gettid(2)](http://man7.org/linux/man-pages/man2/gettid.2.html).
    ///
    /// This function is only available on Linux based systems.  In a single
    /// threaded process, the main thread will have the same ID as the process.  In
    /// a multithreaded process, each thread will have a unique thread id but the
    /// same process ID.
    ///
    /// No error handling is required as a thread id should always exist for any
    /// process, even if threads are not being used.
    #[cfg(any(target_os = "linux", target_os = "android"))]
    #[inline]
    pub fn gettid() -> Pid {
        Pid(unsafe { libc::syscall(libc::SYS_gettid) as pid_t })
    }

    /// Create a copy of the specified file descriptor (see
    /// [dup(2)](http://pubs.opengroup.org/onlinepubs/9699919799/functions/dup.html)).
    ///
    /// The new file descriptor will be have a new index but refer to the same
    /// resource as the old file descriptor and the old and new file descriptors may
    /// be used interchangeably.  The new and old file descriptor share the same
    /// underlying resource, offset, and file status flags.  The actual index used
    /// for the file descriptor will be the lowest fd index that is available.
    ///
    /// The two file descriptors do not share file descriptor flags (e.g. `OFlag::FD_CLOEXEC`).
    #[inline]
    pub fn dup(oldfd: RawFd) -> Result<RawFd> {
        let res = unsafe { libc::dup(oldfd) };

        Errno::result(res)
    }

    /// Create a copy of the specified file descriptor using the specified fd (see
    /// [dup(2)](http://pubs.opengroup.org/onlinepubs/9699919799/functions/dup.html)).
    ///
    /// This function behaves similar to `dup()` except that it will try to use the
    /// specified fd instead of allocating a new one.  See the man pages for more
    /// detail on the exact behavior of this function.
    #[inline]
    pub fn dup2(oldfd: RawFd, newfd: RawFd) -> Result<RawFd> {
        let res = unsafe { libc::dup2(oldfd, newfd) };

        Errno::result(res)
    }

    /// Create a new copy of the specified file descriptor using the specified fd
    /// and flags (see [dup(2)](http://man7.org/linux/man-pages/man2/dup.2.html)).
    ///
    /// This function behaves similar to `dup2()` but allows for flags to be
    /// specified.
    pub fn dup3(oldfd: RawFd, newfd: RawFd, flags: OFlag) -> Result<RawFd> {
        dup3_polyfill(oldfd, newfd, flags)
    }

    #[inline]
    fn dup3_polyfill(oldfd: RawFd, newfd: RawFd, flags: OFlag) -> Result<RawFd> {
        if oldfd == newfd {
            return Err(Error::Sys(Errno::EINVAL));
        }

        let fd = dup2(oldfd, newfd)?;

        if flags.contains(OFlag::O_CLOEXEC) {
            if let Err(e) = fcntl(fd, F_SETFD(FdFlag::FD_CLOEXEC)) {
                let _ = close(fd);
                return Err(e);
            }
        }

        Ok(fd)
    }

    /// Change the current working directory of the calling process (see
    /// [chdir(2)](http://pubs.opengroup.org/onlinepubs/9699919799/functions/chdir.html)).
    ///
    /// This function may fail in a number of different scenarios.  See the man
    /// pages for additional details on possible failure cases.
    #[inline]
    pub fn chdir<P: ?Sized + NixPath>(path: &P) -> Result<()> {
        let res = path.with_nix_path(|cstr| {
            unsafe { libc::chdir(cstr.as_ptr()) }
        })?;

        Errno::result(res).map(drop)
    }

    /// Change the current working directory of the process to the one
    /// given as an open file descriptor (see
    /// [fchdir(2)](http://pubs.opengroup.org/onlinepubs/9699919799/functions/fchdir.html)).
    ///
    /// This function may fail in a number of different scenarios.  See the man
    /// pages for additional details on possible failure cases.
    #[inline]
    pub fn fchdir(dirfd: RawFd) -> Result<()> {
        let res = unsafe { libc::fchdir(dirfd) };

        Errno::result(res).map(drop)
    }

    /// Creates new directory `path` with access rights `mode`.  (see [mkdir(2)](http://pubs.opengroup.org/onlinepubs/9699919799/functions/mkdir.html))
    ///
    /// # Errors
    ///
    /// There are several situations where mkdir might fail:
    ///
    /// - current user has insufficient rights in the parent directory
    /// - the path already exists
    /// - the path name is too long (longer than `PATH_MAX`, usually 4096 on linux, 1024 on OS X)
    ///
    /// # Example
    ///
    /// ```rust
    /// extern crate tempfile;
    /// extern crate nix;
    ///
    /// use nix::unistd;
    /// use nix::sys::stat;
    /// use tempfile::tempdir;
    ///
    /// fn main() {
    ///     let tmp_dir1 = tempdir().unwrap();
    ///     let tmp_dir2 = tmp_dir1.path().join("new_dir");
    ///
    ///     // create new directory and give read, write and execute rights to the owner
    ///     match unistd::mkdir(&tmp_dir2, stat::Mode::S_IRWXU) {
    ///        Ok(_) => println!("created {:?}", tmp_dir2),
    ///        Err(err) => println!("Error creating directory: {}", err),
    ///     }
    /// }
    /// ```
    #[inline]
    pub fn mkdir<P: ?Sized + NixPath>(path: &P, mode: Mode) -> Result<()> {
        let res = path.with_nix_path(|cstr| {
            unsafe { libc::mkdir(cstr.as_ptr(), mode.bits() as mode_t) }
        })?;

        Errno::result(res).map(drop)
    }

    /// Creates new fifo special file (named pipe) with path `path` and access rights `mode`.
    ///
    /// # Errors
    ///
    /// There are several situations where mkfifo might fail:
    ///
    /// - current user has insufficient rights in the parent directory
    /// - the path already exists
    /// - the path name is too long (longer than `PATH_MAX`, usually 4096 on linux, 1024 on OS X)
    ///
    /// For a full list consult
    /// [posix specification](http://pubs.opengroup.org/onlinepubs/9699919799/functions/mkfifo.html)
    ///
    /// # Example
    ///
    /// ```rust
    /// extern crate tempfile;
    /// extern crate nix;
    ///
    /// use nix::unistd;
    /// use nix::sys::stat;
    /// use tempfile::tempdir;
    ///
    /// fn main() {
    ///     let tmp_dir = tempdir().unwrap();
    ///     let fifo_path = tmp_dir.path().join("foo.pipe");
    ///
    ///     // create new fifo and give read, write and execute rights to the owner
    ///     match unistd::mkfifo(&fifo_path, stat::Mode::S_IRWXU) {
    ///        Ok(_) => println!("created {:?}", fifo_path),
    ///        Err(err) => println!("Error creating fifo: {}", err),
    ///     }
    /// }
    /// ```
    #[inline]
    pub fn mkfifo<P: ?Sized + NixPath>(path: &P, mode: Mode) -> Result<()> {
        let res = path.with_nix_path(|cstr| {
            unsafe { libc::mkfifo(cstr.as_ptr(), mode.bits() as mode_t) }
        })?;

        Errno::result(res).map(drop)
    }

    /// Creates a symbolic link at `path2` which points to `path1`.
    ///
    /// If `dirfd` has a value, then `path2` is relative to directory associated
    /// with the file descriptor.
    ///
    /// If `dirfd` is `None`, then `path2` is relative to the current working
    /// directory. This is identical to `libc::symlink(path1, path2)`.
    ///
    /// See also [symlinkat(2)](http://pubs.opengroup.org/onlinepubs/9699919799/functions/symlinkat.html).
    pub fn symlinkat<P1: ?Sized + NixPath, P2: ?Sized + NixPath>(
        path1: &P1,
        dirfd: Option<RawFd>,
        path2: &P2) -> Result<()> {
        let res =
            path1.with_nix_path(|path1| {
                path2.with_nix_path(|path2| {
                    unsafe {
                        libc::symlinkat(
                            path1.as_ptr(),
                            dirfd.unwrap_or(libc::AT_FDCWD),
                            path2.as_ptr()
                        )
                    }
                })
            })??;
        Errno::result(res).map(drop)
    }

    /// Returns the current directory as a `PathBuf`
    ///
    /// Err is returned if the current user doesn't have the permission to read or search a component
    /// of the current path.
    ///
    /// # Example
    ///
    /// ```rust
    /// extern crate nix;
    ///
    /// use nix::unistd;
    ///
    /// fn main() {
    ///     // assume that we are allowed to get current directory
    ///     let dir = unistd::getcwd().unwrap();
    ///     println!("The current directory is {:?}", dir);
    /// }
    /// ```
    #[inline]
    pub fn getcwd() -> Result<PathBuf> {
        let mut buf = Vec::with_capacity(512);
        loop {
            unsafe {
                let ptr = buf.as_mut_ptr() as *mut c_char;

                // The buffer must be large enough to store the absolute pathname plus
                // a terminating null byte, or else null is returned.
                // To safely handle this we start with a reasonable size (512 bytes)
                // and double the buffer size upon every error
                if !libc::getcwd(ptr, buf.capacity()).is_null() {
                    let len = CStr::from_ptr(buf.as_ptr() as *const c_char).to_bytes().len();
                    buf.set_len(len);
                    buf.shrink_to_fit();
                    return Ok(PathBuf::from(OsString::from_vec(buf)));
                } else {
                    let error = Errno::last();
                    // ERANGE means buffer was too small to store directory name
                    if error != Errno::ERANGE {
                        return Err(Error::Sys(error));
                    }
                }

                // Trigger the internal buffer resizing logic of `Vec` by requiring
                // more space than the current capacity.
                let cap = buf.capacity();
                buf.set_len(cap);
                buf.reserve(1);
            }
        }
    }

    /// Computes the raw UID and GID values to pass to a `*chown` call.
    fn chown_raw_ids(owner: Option<Uid>, group: Option<Gid>) -> (libc::uid_t, libc::gid_t) {
        // According to the POSIX specification, -1 is used to indicate that owner and group
        // are not to be changed.  Since uid_t and gid_t are unsigned types, we have to wrap
        // around to get -1.
        let uid = owner.map(Into::into).unwrap_or((0 as uid_t).wrapping_sub(1));
        let gid = group.map(Into::into).unwrap_or((0 as gid_t).wrapping_sub(1));
        (uid, gid)
    }

    /// Change the ownership of the file at `path` to be owned by the specified
    /// `owner` (user) and `group` (see
    /// [chown(2)](http://pubs.opengroup.org/onlinepubs/9699919799/functions/chown.html)).
    ///
    /// The owner/group for the provided path name will not be modified if `None` is
    /// provided for that argument.  Ownership change will be attempted for the path
    /// only if `Some` owner/group is provided.
    #[inline]
    pub fn chown<P: ?Sized + NixPath>(path: &P, owner: Option<Uid>, group: Option<Gid>) -> Result<()> {
        let res = path.with_nix_path(|cstr| {
            let (uid, gid) = chown_raw_ids(owner, group);
            unsafe { libc::chown(cstr.as_ptr(), uid, gid) }
        })?;

        Errno::result(res).map(drop)
    }

    /// Flags for `fchownat` function.
    #[derive(Clone, Copy, Debug)]
    pub enum FchownatFlags {
        FollowSymlink,
        NoFollowSymlink,
    }

    /// Change the ownership of the file at `path` to be owned by the specified
    /// `owner` (user) and `group`.
    ///
    /// The owner/group for the provided path name will not be modified if `None` is
    /// provided for that argument.  Ownership change will be attempted for the path
    /// only if `Some` owner/group is provided.
    ///
    /// The file to be changed is determined relative to the directory associated
    /// with the file descriptor `dirfd` or the current working directory
    /// if `dirfd` is `None`.
    ///
    /// If `flag` is `FchownatFlags::NoFollowSymlink` and `path` names a symbolic link,
    /// then the mode of the symbolic link is changed.
    ///
    /// `fchownat(None, path, mode, FchownatFlags::NoFollowSymlink)` is identical to
    /// a call `libc::lchown(path, mode)`.  That's why `lchmod` is unimplemented in
    /// the `nix` crate.
    ///
    /// # References
    ///
    /// [fchownat(2)](http://pubs.opengroup.org/onlinepubs/9699919799/functions/fchownat.html).
    pub fn fchownat<P: ?Sized + NixPath>(
        dirfd: Option<RawFd>,
        path: &P,
        owner: Option<Uid>,
        group: Option<Gid>,
        flag: FchownatFlags,
    ) -> Result<()> {
        let atflag =
            match flag {
                FchownatFlags::FollowSymlink => AtFlags::empty(),
                FchownatFlags::NoFollowSymlink => AtFlags::AT_SYMLINK_NOFOLLOW,
            };
        let res = path.with_nix_path(|cstr| unsafe {
            let (uid, gid) = chown_raw_ids(owner, group);
            libc::fchownat(at_rawfd(dirfd), cstr.as_ptr(), uid, gid,
                           atflag.bits() as libc::c_int)
        })?;

        Errno::result(res).map(drop)
    }

    fn to_exec_array(args: &[CString]) -> Vec<*const c_char> {
        let mut args_p: Vec<*const c_char> = args.iter().map(|s| s.as_ptr()).collect();
        args_p.push(ptr::null());
        args_p
    }

    /// Replace the current process image with a new one (see
    /// [exec(3)](http://pubs.opengroup.org/onlinepubs/9699919799/functions/exec.html)).
    ///
    /// See the `::nix::unistd::execve` system call for additional details.  `execv`
    /// performs the same action but does not allow for customization of the
    /// environment for the new process.
    #[inline]
    pub fn execv(path: &CString, argv: &[CString]) -> Result<Void> {
        let args_p = to_exec_array(argv);

        unsafe {
            libc::execv(path.as_ptr(), args_p.as_ptr())
        };

        Err(Error::Sys(Errno::last()))
    }

    /// Replace the current process image with a new one (see
    /// [execve(2)](http://pubs.opengroup.org/onlinepubs/9699919799/functions/exec.html)).
    ///
    /// The execve system call allows for another process to be "called" which will
    /// replace the current process image.  That is, this process becomes the new
    /// command that is run. On success, this function will not return. Instead,
    /// the new program will run until it exits.
    ///
    /// `::nix::unistd::execv` and `::nix::unistd::execve` take as arguments a slice
    /// of `::std::ffi::CString`s for `args` and `env` (for `execve`). Each element
    /// in the `args` list is an argument to the new process. Each element in the
    /// `env` list should be a string in the form "key=value".
    #[inline]
    pub fn execve(path: &CString, args: &[CString], env: &[CString]) -> Result<Void> {
        let args_p = to_exec_array(args);
        let env_p = to_exec_array(env);

        unsafe {
            libc::execve(path.as_ptr(), args_p.as_ptr(), env_p.as_ptr())
        };

        Err(Error::Sys(Errno::last()))
    }

    /// Replace the current process image with a new one and replicate shell `PATH`
    /// searching behavior (see
    /// [exec(3)](http://pubs.opengroup.org/onlinepubs/9699919799/functions/exec.html)).
    ///
    /// See `::nix::unistd::execve` for additional details.  `execvp` behaves the
    /// same as execv except that it will examine the `PATH` environment variables
    /// for file names not specified with a leading slash.  For example, `execv`
    /// would not work if "bash" was specified for the path argument, but `execvp`
    /// would assuming that a bash executable was on the system `PATH`.
    #[inline]
    pub fn execvp(filename: &CString, args: &[CString]) -> Result<Void> {
        let args_p = to_exec_array(args);

        unsafe {
            libc::execvp(filename.as_ptr(), args_p.as_ptr())
        };

        Err(Error::Sys(Errno::last()))
    }

    /// Replace the current process image with a new one and replicate shell `PATH`
    /// searching behavior (see
    /// [`execvpe(3)`](http://man7.org/linux/man-pages/man3/exec.3.html)).
    ///
    /// This functions like a combination of `execvp(2)` and `execve(2)` to pass an
    /// environment and have a search path. See these two for additional
    /// information.
    #[cfg(any(target_os = "haiku",
              target_os = "linux",
              target_os = "openbsd"))]
    pub fn execvpe(filename: &CString, args: &[CString], env: &[CString]) -> Result<Void> {
        let args_p = to_exec_array(args);
        let env_p = to_exec_array(env);

        unsafe {
            libc::execvpe(filename.as_ptr(), args_p.as_ptr(), env_p.as_ptr())
        };

        Err(Error::Sys(Errno::last()))
    }

    /// Replace the current process image with a new one (see
    /// [fexecve(2)](http://pubs.opengroup.org/onlinepubs/9699919799/functions/fexecve.html)).
    ///
    /// The `fexecve` function allows for another process to be "called" which will
    /// replace the current process image.  That is, this process becomes the new
    /// command that is run. On success, this function will not return. Instead,
    /// the new program will run until it exits.
    ///
    /// This function is similar to `execve`, except that the program to be executed
    /// is referenced as a file descriptor instead of a path.
    // Note for NetBSD and OpenBSD: although rust-lang/libc includes it (under
    // unix/bsd/netbsdlike/) fexecve is not currently implemented on NetBSD nor on
    // OpenBSD.
    #[cfg(any(target_os = "android",
              target_os = "linux",
              target_os = "freebsd"))]
    #[inline]
    pub fn fexecve(fd: RawFd, args: &[CString], env: &[CString]) -> Result<Void> {
        let args_p = to_exec_array(args);
        let env_p = to_exec_array(env);

        unsafe {
            libc::fexecve(fd, args_p.as_ptr(), env_p.as_ptr())
        };

        Err(Error::Sys(Errno::last()))
    }

    /// Execute program relative to a directory file descriptor (see
    /// [execveat(2)](http://man7.org/linux/man-pages/man2/execveat.2.html)).
    ///
    /// The `execveat` function allows for another process to be "called" which will
    /// replace the current process image.  That is, this process becomes the new
    /// command that is run. On success, this function will not return. Instead,
    /// the new program will run until it exits.
    ///
    /// This function is similar to `execve`, except that the program to be executed
    /// is referenced as a file descriptor to the base directory plus a path.
    #[cfg(any(target_os = "android", target_os = "linux"))]
    #[inline]
    pub fn execveat(dirfd: RawFd, pathname: &CString, args: &[CString],
                    env: &[CString], flags: super::fcntl::AtFlags) -> Result<Void> {
        let args_p = to_exec_array(args);
        let env_p = to_exec_array(env);

        unsafe {
            libc::syscall(libc::SYS_execveat, dirfd, pathname.as_ptr(),
                          args_p.as_ptr(), env_p.as_ptr(), flags);
        };

        Err(Error::Sys(Errno::last()))
    }

    /// Daemonize this process by detaching from the controlling terminal (see
    /// [daemon(3)](http://man7.org/linux/man-pages/man3/daemon.3.html)).
    ///
    /// When a process is launched it is typically associated with a parent and it,
    /// in turn, by its controlling terminal/process.  In order for a process to run
    /// in the "background" it must daemonize itself by detaching itself.  Under
    /// posix, this is done by doing the following:
    ///
    /// 1. Parent process (this one) forks
    /// 2. Parent process exits
    /// 3. Child process continues to run.
    ///
    /// `nochdir`:
    ///
    /// * `nochdir = true`: The current working directory after daemonizing will
    ///    be the current working directory.
    /// *  `nochdir = false`: The current working directory after daemonizing will
    ///    be the root direcory, `/`.
    ///
    /// `noclose`:
    ///
    /// * `noclose = true`: The process' current stdin, stdout, and stderr file
    ///   descriptors will remain identical after daemonizing.
    /// * `noclose = false`: The process' stdin, stdout, and stderr will point to
    ///   `/dev/null` after daemonizing.
    #[cfg_attr(any(target_os = "macos", target_os = "ios"), deprecated(
        since="0.14.0",
        note="Deprecated in MacOSX 10.5"
    ))]
    #[cfg_attr(any(target_os = "macos", target_os = "ios"), allow(deprecated))]
    pub fn daemon(nochdir: bool, noclose: bool) -> Result<()> {
        let res = unsafe { libc::daemon(nochdir as c_int, noclose as c_int) };
        Errno::result(res).map(drop)
    }

    /// Set the system host name (see
    /// [sethostname(2)](http://man7.org/linux/man-pages/man2/gethostname.2.html)).
    ///
    /// Given a name, attempt to update the system host name to the given string.
    /// On some systems, the host name is limited to as few as 64 bytes.  An error
    /// will be return if the name is not valid or the current process does not have
    /// permissions to update the host name.
    pub fn sethostname<S: AsRef<OsStr>>(name: S) -> Result<()> {
        // Handle some differences in type of the len arg across platforms.
        cfg_if! {
            if #[cfg(any(target_os = "dragonfly",
                         target_os = "freebsd",
                         target_os = "ios",
                         target_os = "macos", ))] {
                type sethostname_len_t = c_int;
            } else {
                type sethostname_len_t = size_t;
            }
        }
        let ptr = name.as_ref().as_bytes().as_ptr() as *const c_char;
        let len = name.as_ref().len() as sethostname_len_t;

        let res = unsafe { libc::sethostname(ptr, len) };
        Errno::result(res).map(drop)
    }

    /// Get the host name and store it in the provided buffer, returning a pointer
    /// the `CStr` in that buffer on success (see
    /// [gethostname(2)](http://pubs.opengroup.org/onlinepubs/9699919799/functions/gethostname.html)).
    ///
    /// This function call attempts to get the host name for the running system and
    /// store it in a provided buffer.  The buffer will be populated with bytes up
    /// to the length of the provided slice including a NUL terminating byte.  If
    /// the hostname is longer than the length provided, no error will be provided.
    /// The posix specification does not specify whether implementations will
    /// null-terminate in this case, but the nix implementation will ensure that the
    /// buffer is null terminated in this case.
    ///
    /// ```no_run
    /// use nix::unistd;
    ///
    /// let mut buf = [0u8; 64];
    /// let hostname_cstr = unistd::gethostname(&mut buf).expect("Failed getting hostname");
    /// let hostname = hostname_cstr.to_str().expect("Hostname wasn't valid UTF-8");
    /// println!("Hostname: {}", hostname);
    /// ```
    pub fn gethostname(buffer: &mut [u8]) -> Result<&CStr> {
        let ptr = buffer.as_mut_ptr() as *mut c_char;
        let len = buffer.len() as size_t;

        let res = unsafe { libc::gethostname(ptr, len) };
        Errno::result(res).map(|_| {
            buffer[len - 1] = 0; // ensure always null-terminated
            unsafe { CStr::from_ptr(buffer.as_ptr() as *const c_char) }
        })
    }

    /// Close a raw file descriptor
    ///
    /// Be aware that many Rust types implicitly close-on-drop, including
    /// `std::fs::File`.  Explicitly closing them with this method too can result in
    /// a double-close condition, which can cause confusing `EBADF` errors in
    /// seemingly unrelated code.  Caveat programmer.  See also
    /// [close(2)](http://pubs.opengroup.org/onlinepubs/9699919799/functions/close.html).
    ///
    /// # Examples
    ///
    /// ```no_run
    /// extern crate tempfile;
    /// extern crate nix;
    ///
    /// use std::os::unix::io::AsRawFd;
    /// use nix::unistd::close;
    ///
    /// fn main() {
    ///     let f = tempfile::tempfile().unwrap();
    ///     close(f.as_raw_fd()).unwrap();   // Bad!  f will also close on drop!
    /// }
    /// ```
    ///
    /// ```rust
    /// extern crate tempfile;
    /// extern crate nix;
    ///
    /// use std::os::unix::io::IntoRawFd;
    /// use nix::unistd::close;
    ///
    /// fn main() {
    ///     let f = tempfile::tempfile().unwrap();
    ///     close(f.into_raw_fd()).unwrap(); // Good.  into_raw_fd consumes f
    /// }
    /// ```
    pub fn close(fd: RawFd) -> Result<()> {
        let res = unsafe { libc::close(fd) };
        Errno::result(res).map(drop)
    }

    /// Read from a raw file descriptor.
    ///
    /// See also [read(2)](http://pubs.opengroup.org/onlinepubs/9699919799/functions/read.html)
    pub fn read(fd: RawFd, buf: &mut [u8]) -> Result<usize> {
        let res = unsafe { libc::read(fd, buf.as_mut_ptr() as *mut c_void, buf.len() as size_t) };

        Errno::result(res).map(|r| r as usize)
    }

    /// Write to a raw file descriptor.
    ///
    /// See also [write(2)](http://pubs.opengroup.org/onlinepubs/9699919799/functions/write.html)
    pub fn write(fd: RawFd, buf: &[u8]) -> Result<usize> {
        let res = unsafe { libc::write(fd, buf.as_ptr() as *const c_void, buf.len() as size_t) };

        Errno::result(res).map(|r| r as usize)
    }

    /// Directive that tells [`lseek`] and [`lseek64`] what the offset is relative to.
    ///
    /// [`lseek`]: ./fn.lseek.html
    /// [`lseek64`]: ./fn.lseek64.html
    #[repr(i32)]
    #[derive(Clone, Copy, Debug)]
    pub enum Whence {
        /// Specify an offset relative to the start of the file.
        SeekSet = libc::SEEK_SET,
        /// Specify an offset relative to the current file location.
        SeekCur = libc::SEEK_CUR,
        /// Specify an offset relative to the end of the file.
        SeekEnd = libc::SEEK_END,
        /// Specify an offset relative to the next location in the file greater than or
        /// equal to offset that contains some data. If offset points to
        /// some data, then the file offset is set to offset.
        #[cfg(any(target_os = "dragonfly", target_os = "freebsd",
              all(target_os = "linux", not(any(target_env = "musl",
                                               target_arch = "mips",
                                               target_arch = "mips64")))))]
        SeekData = libc::SEEK_DATA,
        /// Specify an offset relative to the next hole in the file greater than
        /// or equal to offset. If offset points into the middle of a hole, then
        /// the file offset should be set to offset. If there is no hole past offset,
        /// then the file offset should be adjusted to the end of the file (i.e., there
        /// is an implicit hole at the end of any file).
        #[cfg(any(target_os = "dragonfly", target_os = "freebsd",
              all(target_os = "linux", not(any(target_env = "musl",
                                               target_arch = "mips",
                                               target_arch = "mips64")))))]
        SeekHole = libc::SEEK_HOLE
    }

    /// Move the read/write file offset.
    ///
    /// See also [lseek(2)](http://pubs.opengroup.org/onlinepubs/9699919799/functions/lseek.html)
    pub fn lseek(fd: RawFd, offset: off_t, whence: Whence) -> Result<off_t> {
        let res = unsafe { libc::lseek(fd, offset, whence as i32) };

        Errno::result(res).map(|r| r as off_t)
    }

    #[cfg(any(target_os = "linux", target_os = "android"))]
    pub fn lseek64(fd: RawFd, offset: libc::off64_t, whence: Whence) -> Result<libc::off64_t> {
        let res = unsafe { libc::lseek64(fd, offset, whence as i32) };

        Errno::result(res).map(|r| r as libc::off64_t)
    }

    /// Create an interprocess channel.
    ///
    /// See also [pipe(2)](http://pubs.opengroup.org/onlinepubs/9699919799/functions/pipe.html)
    pub fn pipe() -> Result<(RawFd, RawFd)> {
        unsafe {
            let mut fds: [c_int; 2] = mem::uninitialized();

            let res = libc::pipe(fds.as_mut_ptr());

            Errno::result(res)?;

            Ok((fds[0], fds[1]))
        }
    }

    /// Like `pipe`, but allows setting certain file descriptor flags.
    ///
    /// The following flags are supported, and will be set atomically as the pipe is
    /// created:
    ///
    /// `O_CLOEXEC`:    Set the close-on-exec flag for the new file descriptors.
    /// `O_NONBLOCK`:   Set the non-blocking flag for the ends of the pipe.
    ///
    /// See also [pipe(2)](http://man7.org/linux/man-pages/man2/pipe.2.html)
    #[cfg(any(target_os = "android",
              target_os = "dragonfly",
              target_os = "emscripten",
              target_os = "freebsd",
              target_os = "linux",
              target_os = "netbsd",
              target_os = "openbsd"))]
    pub fn pipe2(flags: OFlag) -> Result<(RawFd, RawFd)> {
        let mut fds: [c_int; 2] = unsafe { mem::uninitialized() };

        let res = unsafe { libc::pipe2(fds.as_mut_ptr(), flags.bits()) };

        Errno::result(res)?;

        Ok((fds[0], fds[1]))
    }

    /// Like `pipe`, but allows setting certain file descriptor flags.
    ///
    /// The following flags are supported, and will be set after the pipe is
    /// created:
    ///
    /// `O_CLOEXEC`:    Set the close-on-exec flag for the new file descriptors.
    /// `O_NONBLOCK`:   Set the non-blocking flag for the ends of the pipe.
    #[cfg(any(target_os = "ios", target_os = "macos"))]
    #[deprecated(
        since="0.10.0",
        note="pipe2(2) is not actually atomic on these platforms.  Use pipe(2) and fcntl(2) instead"
    )]
    pub fn pipe2(flags: OFlag) -> Result<(RawFd, RawFd)> {
        let mut fds: [c_int; 2] = unsafe { mem::uninitialized() };

        let res = unsafe { libc::pipe(fds.as_mut_ptr()) };

        Errno::result(res)?;

        pipe2_setflags(fds[0], fds[1], flags)?;

        Ok((fds[0], fds[1]))
    }

    #[cfg(any(target_os = "ios", target_os = "macos"))]
    fn pipe2_setflags(fd1: RawFd, fd2: RawFd, flags: OFlag) -> Result<()> {
        use fcntl::FcntlArg::F_SETFL;

        let mut res = Ok(0);

        if flags.contains(OFlag::O_CLOEXEC) {
            res = res
                .and_then(|_| fcntl(fd1, F_SETFD(FdFlag::FD_CLOEXEC)))
                .and_then(|_| fcntl(fd2, F_SETFD(FdFlag::FD_CLOEXEC)));
        }

        if flags.contains(OFlag::O_NONBLOCK) {
            res = res
                .and_then(|_| fcntl(fd1, F_SETFL(OFlag::O_NONBLOCK)))
                .and_then(|_| fcntl(fd2, F_SETFL(OFlag::O_NONBLOCK)));
        }

        match res {
            Ok(_) => Ok(()),
            Err(e) => {
                let _ = close(fd1);
                let _ = close(fd2);
                Err(e)
            }
        }
    }

    /// Truncate a file to a specified length
    ///
    /// See also
    /// [truncate(2)](http://pubs.opengroup.org/onlinepubs/9699919799/functions/truncate.html)
    pub fn truncate<P: ?Sized + NixPath>(path: &P, len: off_t) -> Result<()> {
        let res = path.with_nix_path(|cstr| {
            unsafe {
                libc::truncate(cstr.as_ptr(), len)
            }
        })?;

        Errno::result(res).map(drop)
    }

    /// Truncate a file to a specified length
    ///
    /// See also
    /// [ftruncate(2)](http://pubs.opengroup.org/onlinepubs/9699919799/functions/ftruncate.html)
    pub fn ftruncate(fd: RawFd, len: off_t) -> Result<()> {
        Errno::result(unsafe { libc::ftruncate(fd, len) }).map(drop)
    }

    pub fn isatty(fd: RawFd) -> Result<bool> {
        unsafe {
            // ENOTTY means `fd` is a valid file descriptor, but not a TTY, so
            // we return `Ok(false)`
            if libc::isatty(fd) == 1 {
                Ok(true)
            } else {
                match Errno::last() {
                    Errno::ENOTTY => Ok(false),
                    err => Err(Error::Sys(err)),
                }
            }
        }
    }

    /// Remove a directory entry
    ///
    /// See also [unlink(2)](http://pubs.opengroup.org/onlinepubs/9699919799/functions/unlink.html)
    pub fn unlink<P: ?Sized + NixPath>(path: &P) -> Result<()> {
        let res = path.with_nix_path(|cstr| {
            unsafe {
                libc::unlink(cstr.as_ptr())
            }
        })?;
        Errno::result(res).map(drop)
    }

    #[inline]
    pub fn chroot<P: ?Sized + NixPath>(path: &P) -> Result<()> {
        let res = path.with_nix_path(|cstr| {
            unsafe { libc::chroot(cstr.as_ptr()) }
        })?;

        Errno::result(res).map(drop)
    }

    /// Commit filesystem caches to disk
    ///
    /// See also [sync(2)](http://pubs.opengroup.org/onlinepubs/9699919799/functions/sync.html)
    #[cfg(any(
        target_os = "dragonfly",
        target_os = "freebsd",
        target_os = "linux",
        target_os = "netbsd",
        target_os = "openbsd"
    ))]
    pub fn sync() -> () {
        unsafe { libc::sync() };
    }

    /// Synchronize changes to a file
    ///
    /// See also [fsync(2)](http://pubs.opengroup.org/onlinepubs/9699919799/functions/fsync.html)
    #[inline]
    pub fn fsync(fd: RawFd) -> Result<()> {
        let res = unsafe { libc::fsync(fd) };

        Errno::result(res).map(drop)
    }

    /// Synchronize the data of a file
    ///
    /// See also
    /// [fdatasync(2)](http://pubs.opengroup.org/onlinepubs/9699919799/functions/fdatasync.html)
    // `fdatasync(2) is in POSIX, but in libc it is only defined in `libc::notbsd`.
    // TODO: exclude only Apple systems after https://github.com/rust-lang/libc/pull/211
    #[cfg(any(target_os = "linux",
              target_os = "android",
              target_os = "emscripten"))]
    #[inline]
    pub fn fdatasync(fd: RawFd) -> Result<()> {
        let res = unsafe { libc::fdatasync(fd) };

        Errno::result(res).map(drop)
    }

    /// Get a real user ID
    ///
    /// See also [getuid(2)](http://pubs.opengroup.org/onlinepubs/9699919799/functions/getuid.html)
    // POSIX requires that getuid is always successful, so no need to check return
    // value or errno.
    #[inline]
    pub fn getuid() -> Uid {
        Uid(unsafe { libc::getuid() })
    }

    /// Get the effective user ID
    ///
    /// See also [geteuid(2)](http://pubs.opengroup.org/onlinepubs/9699919799/functions/geteuid.html)
    // POSIX requires that geteuid is always successful, so no need to check return
    // value or errno.
    #[inline]
    pub fn geteuid() -> Uid {
        Uid(unsafe { libc::geteuid() })
    }

    /// Get the real group ID
    ///
    /// See also [getgid(2)](http://pubs.opengroup.org/onlinepubs/9699919799/functions/getgid.html)
    // POSIX requires that getgid is always successful, so no need to check return
    // value or errno.
    #[inline]
    pub fn getgid() -> Gid {
        Gid(unsafe { libc::getgid() })
    }

    /// Get the effective group ID
    ///
    /// See also [getegid(2)](http://pubs.opengroup.org/onlinepubs/9699919799/functions/getegid.html)
    // POSIX requires that getegid is always successful, so no need to check return
    // value or errno.
    #[inline]
    pub fn getegid() -> Gid {
        Gid(unsafe { libc::getegid() })
    }

    /// Set the effective user ID
    ///
    /// See also [seteuid(2)](http://pubs.opengroup.org/onlinepubs/9699919799/functions/seteuid.html)
    #[inline]
    pub fn seteuid(euid: Uid) -> Result<()> {
        let res = unsafe { libc::seteuid(euid.into()) };

        Errno::result(res).map(drop)
    }

    /// Set the effective group ID
    ///
    /// See also [setegid(2)](http://pubs.opengroup.org/onlinepubs/9699919799/functions/setegid.html)
    #[inline]
    pub fn setegid(egid: Gid) -> Result<()> {
        let res = unsafe { libc::setegid(egid.into()) };

        Errno::result(res).map(drop)
    }

    /// Set the user ID
    ///
    /// See also [setuid(2)](http://pubs.opengroup.org/onlinepubs/9699919799/functions/setuid.html)
    #[inline]
    pub fn setuid(uid: Uid) -> Result<()> {
        let res = unsafe { libc::setuid(uid.into()) };

        Errno::result(res).map(drop)
    }

    /// Set the group ID
    ///
    /// See also [setgid(2)](http://pubs.opengroup.org/onlinepubs/9699919799/functions/setgid.html)
    #[inline]
    pub fn setgid(gid: Gid) -> Result<()> {
        let res = unsafe { libc::setgid(gid.into()) };

        Errno::result(res).map(drop)
    }

    /// Get the list of supplementary group IDs of the calling process.
    ///
    /// [Further reading](http://pubs.opengroup.org/onlinepubs/009695399/functions/getgroups.html)
    ///
    /// **Note:** This function is not available for Apple platforms. On those
    /// platforms, checking group membership should be achieved via communication
    /// with the `opendirectoryd` service.
    #[cfg(not(any(target_os = "ios", target_os = "macos")))]
    pub fn getgroups() -> Result<Vec<Gid>> {
        // First get the number of groups so we can size our Vec
        let ret = unsafe { libc::getgroups(0, ptr::null_mut()) };

        // Now actually get the groups. We try multiple times in case the number of
        // groups has changed since the first call to getgroups() and the buffer is
        // now too small.
        let mut groups = Vec::<Gid>::with_capacity(Errno::result(ret)? as usize);
        loop {
            // FIXME: On the platforms we currently support, the `Gid` struct has
            // the same representation in memory as a bare `gid_t`. This is not
            // necessarily the case on all Rust platforms, though. See RFC 1785.
            let ret = unsafe {
                libc::getgroups(groups.capacity() as c_int, groups.as_mut_ptr() as *mut gid_t)
            };

            match Errno::result(ret) {
                Ok(s) => {
                    unsafe { groups.set_len(s as usize) };
                    return Ok(groups);
                },
                Err(Error::Sys(Errno::EINVAL)) => {
                    // EINVAL indicates that the buffer size was too small. Trigger
                    // the internal buffer resizing logic of `Vec` by requiring
                    // more space than the current capacity.
                    let cap = groups.capacity();
                    unsafe { groups.set_len(cap) };
                    groups.reserve(1);
                },
                Err(e) => return Err(e)
            }
        }
    }

    /// Set the list of supplementary group IDs for the calling process.
    ///
    /// [Further reading](http://man7.org/linux/man-pages/man2/getgroups.2.html)
    ///
    /// **Note:** This function is not available for Apple platforms. On those
    /// platforms, group membership management should be achieved via communication
    /// with the `opendirectoryd` service.
    ///
    /// # Examples
    ///
    /// `setgroups` can be used when dropping privileges from the root user to a
    /// specific user and group. For example, given the user `www-data` with UID
    /// `33` and the group `backup` with the GID `34`, one could switch the user as
    /// follows:
    ///
    /// ```rust,no_run
    /// # use std::error::Error;
    /// # use nix::unistd::*;
    /// #
    /// # fn try_main() -> Result<(), Box<Error>> {
    /// let uid = Uid::from_raw(33);
    /// let gid = Gid::from_raw(34);
    /// setgroups(&[gid])?;
    /// setgid(gid)?;
    /// setuid(uid)?;
    /// #
    /// #     Ok(())
    /// # }
    /// #
    /// # fn main() {
    /// #     try_main().unwrap();
    /// # }
    /// ```
    #[cfg(not(any(target_os = "ios", target_os = "macos")))]
    pub fn setgroups(groups: &[Gid]) -> Result<()> {
        cfg_if! {
            if #[cfg(any(target_os = "dragonfly",
                         target_os = "freebsd",
                         target_os = "ios",
                         target_os = "macos",
                         target_os = "netbsd",
                         target_os = "openbsd"))] {
                type setgroups_ngroups_t = c_int;
            } else {
                type setgroups_ngroups_t = size_t;
            }
        }
        // FIXME: On the platforms we currently support, the `Gid` struct has the
        // same representation in memory as a bare `gid_t`. This is not necessarily
        // the case on all Rust platforms, though. See RFC 1785.
        let res = unsafe {
            libc::setgroups(groups.len() as setgroups_ngroups_t, groups.as_ptr() as *const gid_t)
        };

        Errno::result(res).map(drop)
    }

    /// Calculate the supplementary group access list.
    ///
    /// Gets the group IDs of all groups that `user` is a member of. The additional
    /// group `group` is also added to the list.
    ///
    /// [Further reading](http://man7.org/linux/man-pages/man3/getgrouplist.3.html)
    ///
    /// **Note:** This function is not available for Apple platforms. On those
    /// platforms, checking group membership should be achieved via communication
    /// with the `opendirectoryd` service.
    ///
    /// # Errors
    ///
    /// Although the `getgrouplist()` call does not return any specific
    /// errors on any known platforms, this implementation will return a system
    /// error of `EINVAL` if the number of groups to be fetched exceeds the
    /// `NGROUPS_MAX` sysconf value. This mimics the behaviour of `getgroups()`
    /// and `setgroups()`. Additionally, while some implementations will return a
    /// partial list of groups when `NGROUPS_MAX` is exceeded, this implementation
    /// will only ever return the complete list or else an error.
    #[cfg(not(any(target_os = "ios", target_os = "macos")))]
    pub fn getgrouplist(user: &CStr, group: Gid) -> Result<Vec<Gid>> {
        let ngroups_max = match sysconf(SysconfVar::NGROUPS_MAX) {
            Ok(Some(n)) => n as c_int,
            Ok(None) | Err(_) => <c_int>::max_value(),
        };
        use std::cmp::min;
        let mut ngroups = min(ngroups_max, 8);
        let mut groups = Vec::<Gid>::with_capacity(ngroups as usize);
        cfg_if! {
            if #[cfg(any(target_os = "ios", target_os = "macos"))] {
                type getgrouplist_group_t = c_int;
            } else {
                type getgrouplist_group_t = gid_t;
            }
        }
        let gid: gid_t = group.into();
        loop {
            let ret = unsafe {
                libc::getgrouplist(user.as_ptr(),
                                   gid as getgrouplist_group_t,
                                   groups.as_mut_ptr() as *mut getgrouplist_group_t,
                                   &mut ngroups)
            };

            // BSD systems only return 0 or -1, Linux returns ngroups on success.
            if ret >= 0 {
                unsafe { groups.set_len(ngroups as usize) };
                return Ok(groups);
            } else if ret == -1 {
                // Returns -1 if ngroups is too small, but does not set errno.
                // BSD systems will still fill the groups buffer with as many
                // groups as possible, but Linux manpages do not mention this
                // behavior.

                let cap = groups.capacity();
                if cap >= ngroups_max as usize {
                    // We already have the largest capacity we can, give up
                    return Err(Error::invalid_argument());
                }

                // Reserve space for at least ngroups
                groups.reserve(ngroups as usize);

                // Even if the buffer gets resized to bigger than ngroups_max,
                // don't ever ask for more than ngroups_max groups
                ngroups = min(ngroups_max, groups.capacity() as c_int);
            }
        }
    }

    /// Initialize the supplementary group access list.
    ///
    /// Sets the supplementary group IDs for the calling process using all groups
    /// that `user` is a member of. The additional group `group` is also added to
    /// the list.
    ///
    /// [Further reading](http://man7.org/linux/man-pages/man3/initgroups.3.html)
    ///
    /// **Note:** This function is not available for Apple platforms. On those
    /// platforms, group membership management should be achieved via communication
    /// with the `opendirectoryd` service.
    ///
    /// # Examples
    ///
    /// `initgroups` can be used when dropping privileges from the root user to
    /// another user. For example, given the user `www-data`, we could look up the
    /// UID and GID for the user in the system's password database (usually found
    /// in `/etc/passwd`). If the `www-data` user's UID and GID were `33` and `33`,
    /// respectively, one could switch the user as follows:
    ///
    /// ```rust,no_run
    /// # use std::error::Error;
    /// # use std::ffi::CString;
    /// # use nix::unistd::*;
    /// #
    /// # fn try_main() -> Result<(), Box<Error>> {
    /// let user = CString::new("www-data").unwrap();
    /// let uid = Uid::from_raw(33);
    /// let gid = Gid::from_raw(33);
    /// initgroups(&user, gid)?;
    /// setgid(gid)?;
    /// setuid(uid)?;
    /// #
    /// #     Ok(())
    /// # }
    /// #
    /// # fn main() {
    /// #     try_main().unwrap();
    /// # }
    /// ```
    #[cfg(not(any(target_os = "ios", target_os = "macos")))]
    pub fn initgroups(user: &CStr, group: Gid) -> Result<()> {
        cfg_if! {
            if #[cfg(any(target_os = "ios", target_os = "macos"))] {
                type initgroups_group_t = c_int;
            } else {
                type initgroups_group_t = gid_t;
            }
        }
        let gid: gid_t = group.into();
        let res = unsafe { libc::initgroups(user.as_ptr(), gid as initgroups_group_t) };

        Errno::result(res).map(drop)
    }

    /// Suspend the thread until a signal is received.
    ///
    /// See also [pause(2)](http://pubs.opengroup.org/onlinepubs/9699919799/functions/pause.html).
    #[inline]
    pub fn pause() {
        unsafe { libc::pause() };
    }

    pub mod alarm {
        //! Alarm signal scheduling.
        //!
        //! Scheduling an alarm will trigger a `SIGALRM` signal when the time has
        //! elapsed, which has to be caught, because the default action for the
        //! signal is to terminate the program. This signal also can't be ignored
        //! because the system calls like `pause` will not be interrupted, see the
        //! second example below.
        //!
        //! # Examples
        //!
        //! Canceling an alarm:
        //!
        //! ```
        //! use nix::unistd::alarm;
        //!
        //! // Set an alarm for 60 seconds from now.
        //! alarm::set(60);
        //!
        //! // Cancel the above set alarm, which returns the number of seconds left
        //! // of the previously set alarm.
        //! assert_eq!(alarm::cancel(), Some(60));
        //! ```
        //!
        //! Scheduling an alarm and waiting for the signal:
        //!
        //! ```
        //! use std::time::{Duration, Instant};
        //!
        //! use nix::unistd::{alarm, pause};
        //! use nix::sys::signal::*;
        //!
        //! // We need to setup an empty signal handler to catch the alarm signal,
        //! // otherwise the program will be terminated once the signal is delivered.
        //! extern fn signal_handler(_: nix::libc::c_int) { }
        //! unsafe { sigaction(Signal::SIGALRM, &SigAction::new(SigHandler::Handler(signal_handler), SaFlags::empty(), SigSet::empty())); }
        //!
        //! // Set an alarm for 1 second from now.
        //! alarm::set(1);
        //!
        //! let start = Instant::now();
        //! // Pause the process until the alarm signal is received.
        //! pause();
        //!
        //! assert!(start.elapsed() >= Duration::from_secs(1));
        //! ```
        //!
        //! # References
        //!
        //! See also [alarm(2)](http://pubs.opengroup.org/onlinepubs/9699919799/functions/alarm.html).

        use libc;

        /// Schedule an alarm signal.
        ///
        /// This will cause the system to generate a `SIGALRM` signal for the
        /// process after the specified number of seconds have elapsed.
        ///
        /// Returns the leftover time of a previously set alarm if there was one.
        pub fn set(secs: libc::c_uint) -> Option<libc::c_uint> {
            assert!(secs != 0, "passing 0 to `alarm::set` is not allowed, to cancel an alarm use `alarm::cancel`");
            alarm(secs)
        }

        /// Cancel an previously set alarm signal.
        ///
        /// Returns the leftover time of a previously set alarm if there was one.
        pub fn cancel() -> Option<libc::c_uint> {
            alarm(0)
        }

        fn alarm(secs: libc::c_uint) -> Option<libc::c_uint> {
            match unsafe { libc::alarm(secs) } {
                0 => None,
                secs => Some(secs),
            }
        }
    }

    /// Suspend execution for an interval of time
    ///
    /// See also [sleep(2)](http://pubs.opengroup.org/onlinepubs/009695399/functions/sleep.html#tag_03_705_05)
    // Per POSIX, does not fail
    #[inline]
    pub fn sleep(seconds: c_uint) -> c_uint {
        unsafe { libc::sleep(seconds) }
    }

    pub mod acct {
        use libc;
        use {Result, NixPath};
        use errno::Errno;
        use std::ptr;

        /// Enable process accounting
        ///
        /// See also [acct(2)](https://linux.die.net/man/2/acct)
        pub fn enable<P: ?Sized + NixPath>(filename: &P) -> Result<()> {
            let res = filename.with_nix_path(|cstr| {
                unsafe { libc::acct(cstr.as_ptr()) }
            })?;

            Errno::result(res).map(drop)
        }

        /// Disable process accounting
        pub fn disable() -> Result<()> {
            let res = unsafe { libc::acct(ptr::null()) };

            Errno::result(res).map(drop)
        }
    }

    /// Creates a regular file which persists even after process termination
    ///
    /// * `template`: a path whose 6 rightmost characters must be X, e.g. `/tmp/tmpfile_XXXXXX`
    /// * returns: tuple of file descriptor and filename
    ///
    /// Err is returned either if no temporary filename could be created or the template doesn't
    /// end with XXXXXX
    ///
    /// See also [mkstemp(2)](http://pubs.opengroup.org/onlinepubs/9699919799/functions/mkstemp.html)
    ///
    /// # Example
    ///
    /// ```rust
    /// use nix::unistd;
    ///
    /// let _ = match unistd::mkstemp("/tmp/tempfile_XXXXXX") {
    ///     Ok((fd, path)) => {
    ///         unistd::unlink(path.as_path()).unwrap(); // flag file to be deleted at app termination
    ///         fd
    ///     }
    ///     Err(e) => panic!("mkstemp failed: {}", e)
    /// };
    /// // do something with fd
    /// ```
    #[inline]
    pub fn mkstemp<P: ?Sized + NixPath>(template: &P) -> Result<(RawFd, PathBuf)> {
        let mut path = template.with_nix_path(|path| {path.to_bytes_with_nul().to_owned()})?;
        let p = path.as_mut_ptr() as *mut _;
        let fd = unsafe { libc::mkstemp(p) };
        let last = path.pop(); // drop the trailing nul
        debug_assert!(last == Some(b'\0'));
        let pathname = OsString::from_vec(path);
        Errno::result(fd)?;
        Ok((fd, PathBuf::from(pathname)))
    }

    /// Variable names for `pathconf`
    ///
    /// Nix uses the same naming convention for these variables as the
    /// [getconf(1)](http://pubs.opengroup.org/onlinepubs/9699919799/utilities/getconf.html) utility.
    /// That is, `PathconfVar` variables have the same name as the abstract
    /// variables  shown in the `pathconf(2)` man page.  Usually, it's the same as
    /// the C variable name without the leading `_PC_`.
    ///
    /// POSIX 1003.1-2008 standardizes all of these variables, but some OSes choose
    /// not to implement variables that cannot change at runtime.
    ///
    /// # References
    ///
    /// - [pathconf(2)](http://pubs.opengroup.org/onlinepubs/9699919799/functions/pathconf.html)
    /// - [limits.h](http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/limits.h.html)
    /// - [unistd.h](http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/unistd.h.html)
    #[derive(Clone, Copy, Debug, Eq, Hash, PartialEq)]
    #[repr(i32)]
    pub enum PathconfVar {
        #[cfg(any(target_os = "dragonfly", target_os = "freebsd", target_os = "linux",
                  target_os = "netbsd", target_os = "openbsd"))]
        /// Minimum number of bits needed to represent, as a signed integer value,
        /// the maximum size of a regular file allowed in the specified directory.
        FILESIZEBITS = libc::_PC_FILESIZEBITS,
        /// Maximum number of links to a single file.
        LINK_MAX = libc::_PC_LINK_MAX,
        /// Maximum number of bytes in a terminal canonical input line.
        MAX_CANON = libc::_PC_MAX_CANON,
        /// Minimum number of bytes for which space is available in a terminal input
        /// queue; therefore, the maximum number of bytes a conforming application
        /// may require to be typed as input before reading them.
        MAX_INPUT = libc::_PC_MAX_INPUT,
        /// Maximum number of bytes in a filename (not including the terminating
        /// null of a filename string).
        NAME_MAX = libc::_PC_NAME_MAX,
        /// Maximum number of bytes the implementation will store as a pathname in a
        /// user-supplied buffer of unspecified size, including the terminating null
        /// character. Minimum number the implementation will accept as the maximum
        /// number of bytes in a pathname.
        PATH_MAX = libc::_PC_PATH_MAX,
        /// Maximum number of bytes that is guaranteed to be atomic when writing to
        /// a pipe.
        PIPE_BUF = libc::_PC_PIPE_BUF,
        #[cfg(any(target_os = "android", target_os = "dragonfly", target_os = "linux",
                  target_os = "netbsd", target_os = "openbsd"))]
        /// Symbolic links can be created.
        POSIX2_SYMLINKS = libc::_PC_2_SYMLINKS,
        #[cfg(any(target_os = "android", target_os = "dragonfly", target_os = "freebsd",
                  target_os = "linux", target_os = "openbsd"))]
        /// Minimum number of bytes of storage actually allocated for any portion of
        /// a file.
        POSIX_ALLOC_SIZE_MIN = libc::_PC_ALLOC_SIZE_MIN,
        #[cfg(any(target_os = "android", target_os = "dragonfly", target_os = "freebsd",
                  target_os = "linux", target_os = "openbsd"))]
        /// Recommended increment for file transfer sizes between the
        /// `POSIX_REC_MIN_XFER_SIZE` and `POSIX_REC_MAX_XFER_SIZE` values.
        POSIX_REC_INCR_XFER_SIZE = libc::_PC_REC_INCR_XFER_SIZE,
        #[cfg(any(target_os = "android", target_os = "dragonfly", target_os = "freebsd",
                  target_os = "linux", target_os = "openbsd"))]
        /// Maximum recommended file transfer size.
        POSIX_REC_MAX_XFER_SIZE = libc::_PC_REC_MAX_XFER_SIZE,
        #[cfg(any(target_os = "android", target_os = "dragonfly", target_os = "freebsd",
                  target_os = "linux", target_os = "openbsd"))]
        /// Minimum recommended file transfer size.
        POSIX_REC_MIN_XFER_SIZE = libc::_PC_REC_MIN_XFER_SIZE,
        #[cfg(any(target_os = "android", target_os = "dragonfly", target_os = "freebsd",
                  target_os = "linux", target_os = "openbsd"))]
        ///  Recommended file transfer buffer alignment.
        POSIX_REC_XFER_ALIGN = libc::_PC_REC_XFER_ALIGN,
        #[cfg(any(target_os = "android", target_os = "dragonfly", target_os = "freebsd",
                  target_os = "linux", target_os = "netbsd", target_os = "openbsd"))]
        /// Maximum number of bytes in a symbolic link.
        SYMLINK_MAX = libc::_PC_SYMLINK_MAX,
        /// The use of `chown` and `fchown` is restricted to a process with
        /// appropriate privileges, and to changing the group ID of a file only to
        /// the effective group ID of the process or to one of its supplementary
        /// group IDs.
        _POSIX_CHOWN_RESTRICTED = libc::_PC_CHOWN_RESTRICTED,
        /// Pathname components longer than {NAME_MAX} generate an error.
        _POSIX_NO_TRUNC = libc::_PC_NO_TRUNC,
        /// This symbol shall be defined to be the value of a character that shall
        /// disable terminal special character handling.
        _POSIX_VDISABLE = libc::_PC_VDISABLE,
        #[cfg(any(target_os = "android", target_os = "dragonfly", target_os = "freebsd",
                  target_os = "linux", target_os = "openbsd"))]
        /// Asynchronous input or output operations may be performed for the
        /// associated file.
        _POSIX_ASYNC_IO = libc::_PC_ASYNC_IO,
        #[cfg(any(target_os = "android", target_os = "dragonfly", target_os = "freebsd",
                  target_os = "linux", target_os = "openbsd"))]
        /// Prioritized input or output operations may be performed for the
        /// associated file.
        _POSIX_PRIO_IO = libc::_PC_PRIO_IO,
        #[cfg(any(target_os = "android", target_os = "dragonfly", target_os = "freebsd",
                  target_os = "linux", target_os = "netbsd", target_os = "openbsd"))]
        /// Synchronized input or output operations may be performed for the
        /// associated file.
        _POSIX_SYNC_IO = libc::_PC_SYNC_IO,
        #[cfg(any(target_os = "dragonfly", target_os = "openbsd"))]
        /// The resolution in nanoseconds for all file timestamps.
        _POSIX_TIMESTAMP_RESOLUTION = libc::_PC_TIMESTAMP_RESOLUTION
    }

    /// Like `pathconf`, but works with file descriptors instead of paths (see
    /// [fpathconf(2)](http://pubs.opengroup.org/onlinepubs/9699919799/functions/pathconf.html))
    ///
    /// # Parameters
    ///
    /// - `fd`:   The file descriptor whose variable should be interrogated
    /// - `var`:  The pathconf variable to lookup
    ///
    /// # Returns
    ///
    /// - `Ok(Some(x))`: the variable's limit (for limit variables) or its
    ///     implementation level (for option variables).  Implementation levels are
    ///     usually a decimal-coded date, such as 200112 for POSIX 2001.12
    /// - `Ok(None)`: the variable has no limit (for limit variables) or is
    ///     unsupported (for option variables)
    /// - `Err(x)`: an error occurred
    pub fn fpathconf(fd: RawFd, var: PathconfVar) -> Result<Option<c_long>> {
        let raw = unsafe {
            Errno::clear();
            libc::fpathconf(fd, var as c_int)
        };
        if raw == -1 {
            if errno::errno() == 0 {
                Ok(None)
            } else {
                Err(Error::Sys(Errno::last()))
            }
        } else {
            Ok(Some(raw))
        }
    }

    /// Get path-dependent configurable system variables (see
    /// [pathconf(2)](http://pubs.opengroup.org/onlinepubs/9699919799/functions/pathconf.html))
    ///
    /// Returns the value of a path-dependent configurable system variable.  Most
    /// supported variables also have associated compile-time constants, but POSIX
    /// allows their values to change at runtime.  There are generally two types of
    /// `pathconf` variables: options and limits.  See [pathconf(2)](http://pubs.opengroup.org/onlinepubs/9699919799/functions/pathconf.html) for more details.
    ///
    /// # Parameters
    ///
    /// - `path`: Lookup the value of `var` for this file or directory
    /// - `var`:  The `pathconf` variable to lookup
    ///
    /// # Returns
    ///
    /// - `Ok(Some(x))`: the variable's limit (for limit variables) or its
    ///     implementation level (for option variables).  Implementation levels are
    ///     usually a decimal-coded date, such as 200112 for POSIX 2001.12
    /// - `Ok(None)`: the variable has no limit (for limit variables) or is
    ///     unsupported (for option variables)
    /// - `Err(x)`: an error occurred
    pub fn pathconf<P: ?Sized + NixPath>(path: &P, var: PathconfVar) -> Result<Option<c_long>> {
        let raw = path.with_nix_path(|cstr| {
            unsafe {
                Errno::clear();
                libc::pathconf(cstr.as_ptr(), var as c_int)
            }
        })?;
        if raw == -1 {
            if errno::errno() == 0 {
                Ok(None)
            } else {
                Err(Error::Sys(Errno::last()))
            }
        } else {
            Ok(Some(raw))
        }
    }

    /// Variable names for `sysconf`
    ///
    /// Nix uses the same naming convention for these variables as the
    /// [getconf(1)](http://pubs.opengroup.org/onlinepubs/9699919799/utilities/getconf.html) utility.
    /// That is, `SysconfVar` variables have the same name as the abstract variables
    /// shown in the `sysconf(3)` man page.  Usually, it's the same as the C
    /// variable name without the leading `_SC_`.
    ///
    /// All of these symbols are standardized by POSIX 1003.1-2008, but haven't been
    /// implemented by all platforms.
    ///
    /// # References
    ///
    /// - [sysconf(3)](http://pubs.opengroup.org/onlinepubs/9699919799/functions/sysconf.html)
    /// - [unistd.h](http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/unistd.h.html)
    /// - [limits.h](http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/limits.h.html)
    #[derive(Clone, Copy, Debug, Eq, Hash, PartialEq)]
    #[repr(i32)]
    pub enum SysconfVar {
        /// Maximum number of I/O operations in a single list I/O call supported by
        /// the implementation.
        AIO_LISTIO_MAX = libc::_SC_AIO_LISTIO_MAX,
        /// Maximum number of outstanding asynchronous I/O operations supported by
        /// the implementation.
        AIO_MAX = libc::_SC_AIO_MAX,
        #[cfg(any(target_os="android", target_os="dragonfly", target_os="freebsd",
                  target_os = "ios", target_os="linux", target_os = "macos",
                  target_os="openbsd"))]
        /// The maximum amount by which a process can decrease its asynchronous I/O
        /// priority level from its own scheduling priority.
        AIO_PRIO_DELTA_MAX = libc::_SC_AIO_PRIO_DELTA_MAX,
        /// Maximum length of argument to the exec functions including environment data.
        ARG_MAX = libc::_SC_ARG_MAX,
        /// Maximum number of functions that may be registered with `atexit`.
        ATEXIT_MAX = libc::_SC_ATEXIT_MAX,
        /// Maximum obase values allowed by the bc utility.
        BC_BASE_MAX = libc::_SC_BC_BASE_MAX,
        /// Maximum number of elements permitted in an array by the bc utility.
        BC_DIM_MAX = libc::_SC_BC_DIM_MAX,
        /// Maximum scale value allowed by the bc utility.
        BC_SCALE_MAX = libc::_SC_BC_SCALE_MAX,
        /// Maximum length of a string constant accepted by the bc utility.
        BC_STRING_MAX = libc::_SC_BC_STRING_MAX,
        /// Maximum number of simultaneous processes per real user ID.
        CHILD_MAX = libc::_SC_CHILD_MAX,
        // _SC_CLK_TCK is obsolete
        /// Maximum number of weights that can be assigned to an entry of the
        /// LC_COLLATE order keyword in the locale definition file
        COLL_WEIGHTS_MAX = libc::_SC_COLL_WEIGHTS_MAX,
        /// Maximum number of timer expiration overruns.
        DELAYTIMER_MAX = libc::_SC_DELAYTIMER_MAX,
        /// Maximum number of expressions that can be nested within parentheses by
        /// the expr utility.
        EXPR_NEST_MAX = libc::_SC_EXPR_NEST_MAX,
        #[cfg(any(target_os="dragonfly", target_os="freebsd", target_os = "ios",
                  target_os="linux", target_os = "macos", target_os="netbsd",
                  target_os="openbsd"))]
        /// Maximum length of a host name (not including the terminating null) as
        /// returned from the `gethostname` function
        HOST_NAME_MAX = libc::_SC_HOST_NAME_MAX,
        /// Maximum number of iovec structures that one process has available for
        /// use with `readv` or `writev`.
        IOV_MAX = libc::_SC_IOV_MAX,
        /// Unless otherwise noted, the maximum length, in bytes, of a utility's
        /// input line (either standard input or another file), when the utility is
        /// described as processing text files. The length includes room for the
        /// trailing <newline>.
        LINE_MAX = libc::_SC_LINE_MAX,
        /// Maximum length of a login name.
        LOGIN_NAME_MAX = libc::_SC_LOGIN_NAME_MAX,
        /// Maximum number of simultaneous supplementary group IDs per process.
        NGROUPS_MAX = libc::_SC_NGROUPS_MAX,
        /// Initial size of `getgrgid_r` and `getgrnam_r` data buffers
        GETGR_R_SIZE_MAX = libc::_SC_GETGR_R_SIZE_MAX,
        /// Initial size of `getpwuid_r` and `getpwnam_r` data buffers
        GETPW_R_SIZE_MAX = libc::_SC_GETPW_R_SIZE_MAX,
        /// The maximum number of open message queue descriptors a process may hold.
        MQ_OPEN_MAX = libc::_SC_MQ_OPEN_MAX,
        /// The maximum number of message priorities supported by the implementation.
        MQ_PRIO_MAX = libc::_SC_MQ_PRIO_MAX,
        /// A value one greater than the maximum value that the system may assign to
        /// a newly-created file descriptor.
        OPEN_MAX = libc::_SC_OPEN_MAX,
        #[cfg(any(target_os="dragonfly", target_os="freebsd", target_os = "ios",
                  target_os="linux", target_os = "macos", target_os="openbsd"))]
        /// The implementation supports the Advisory Information option.
        _POSIX_ADVISORY_INFO = libc::_SC_ADVISORY_INFO,
        #[cfg(any(target_os="dragonfly", target_os="freebsd", target_os = "ios",
                  target_os="linux", target_os = "macos", target_os="netbsd",
                  target_os="openbsd"))]
        /// The implementation supports barriers.
        _POSIX_BARRIERS = libc::_SC_BARRIERS,
        /// The implementation supports asynchronous input and output.
        _POSIX_ASYNCHRONOUS_IO = libc::_SC_ASYNCHRONOUS_IO,
        #[cfg(any(target_os="dragonfly", target_os="freebsd", target_os = "ios",
                  target_os="linux", target_os = "macos", target_os="netbsd",
                  target_os="openbsd"))]
        /// The implementation supports clock selection.
        _POSIX_CLOCK_SELECTION = libc::_SC_CLOCK_SELECTION,
        #[cfg(any(target_os="dragonfly", target_os="freebsd", target_os = "ios",
                  target_os="linux", target_os = "macos", target_os="netbsd",
                  target_os="openbsd"))]
        /// The implementation supports the Process CPU-Time Clocks option.
        _POSIX_CPUTIME = libc::_SC_CPUTIME,
        /// The implementation supports the File Synchronization option.
        _POSIX_FSYNC = libc::_SC_FSYNC,
        #[cfg(any(target_os="dragonfly", target_os="freebsd", target_os = "ios",
                  target_os="linux", target_os = "macos", target_os="openbsd"))]
        /// The implementation supports the IPv6 option.
        _POSIX_IPV6 = libc::_SC_IPV6,
        /// The implementation supports job control.
        _POSIX_JOB_CONTROL = libc::_SC_JOB_CONTROL,
        /// The implementation supports memory mapped Files.
        _POSIX_MAPPED_FILES = libc::_SC_MAPPED_FILES,
        /// The implementation supports the Process Memory Locking option.
        _POSIX_MEMLOCK = libc::_SC_MEMLOCK,
        /// The implementation supports the Range Memory Locking option.
        _POSIX_MEMLOCK_RANGE = libc::_SC_MEMLOCK_RANGE,
        /// The implementation supports memory protection.
        _POSIX_MEMORY_PROTECTION = libc::_SC_MEMORY_PROTECTION,
        /// The implementation supports the Message Passing option.
        _POSIX_MESSAGE_PASSING = libc::_SC_MESSAGE_PASSING,
        /// The implementation supports the Monotonic Clock option.
        _POSIX_MONOTONIC_CLOCK = libc::_SC_MONOTONIC_CLOCK,
        #[cfg(any(target_os="android", target_os="dragonfly", target_os="freebsd",
                  target_os = "ios", target_os="linux", target_os = "macos",
                  target_os="openbsd"))]
        /// The implementation supports the Prioritized Input and Output option.
        _POSIX_PRIORITIZED_IO = libc::_SC_PRIORITIZED_IO,
        /// The implementation supports the Process Scheduling option.
        _POSIX_PRIORITY_SCHEDULING = libc::_SC_PRIORITY_SCHEDULING,
        #[cfg(any(target_os="dragonfly", target_os="freebsd", target_os = "ios",
                  target_os="linux", target_os = "macos", target_os="openbsd"))]
        /// The implementation supports the Raw Sockets option.
        _POSIX_RAW_SOCKETS = libc::_SC_RAW_SOCKETS,
        #[cfg(any(target_os="dragonfly", target_os="freebsd", target_os = "ios",
                  target_os="linux", target_os = "macos", target_os="netbsd",
                  target_os="openbsd"))]
        /// The implementation supports read-write locks.
        _POSIX_READER_WRITER_LOCKS = libc::_SC_READER_WRITER_LOCKS,
        #[cfg(any(target_os = "android", target_os="dragonfly", target_os="freebsd",
                  target_os = "ios", target_os="linux", target_os = "macos",
                  target_os = "openbsd"))]
        /// The implementation supports realtime signals.
        _POSIX_REALTIME_SIGNALS = libc::_SC_REALTIME_SIGNALS,
        #[cfg(any(target_os="dragonfly", target_os="freebsd", target_os = "ios",
                  target_os="linux", target_os = "macos", target_os="netbsd",
                  target_os="openbsd"))]
        /// The implementation supports the Regular Expression Handling option.
        _POSIX_REGEXP = libc::_SC_REGEXP,
        /// Each process has a saved set-user-ID and a saved set-group-ID.
        _POSIX_SAVED_IDS = libc::_SC_SAVED_IDS,
        /// The implementation supports semaphores.
        _POSIX_SEMAPHORES = libc::_SC_SEMAPHORES,
        /// The implementation supports the Shared Memory Objects option.
        _POSIX_SHARED_MEMORY_OBJECTS = libc::_SC_SHARED_MEMORY_OBJECTS,
        #[cfg(any(target_os="dragonfly", target_os="freebsd", target_os = "ios",
                  target_os="linux", target_os = "macos", target_os="netbsd",
                  target_os="openbsd"))]
        /// The implementation supports the POSIX shell.
        _POSIX_SHELL = libc::_SC_SHELL,
        #[cfg(any(target_os="dragonfly", target_os="freebsd", target_os = "ios",
                  target_os="linux", target_os = "macos", target_os="netbsd",
                  target_os="openbsd"))]
        /// The implementation supports the Spawn option.
        _POSIX_SPAWN = libc::_SC_SPAWN,
        #[cfg(any(target_os="dragonfly", target_os="freebsd", target_os = "ios",
                  target_os="linux", target_os = "macos", target_os="netbsd",
                  target_os="openbsd"))]
        /// The implementation supports spin locks.
        _POSIX_SPIN_LOCKS = libc::_SC_SPIN_LOCKS,
        #[cfg(any(target_os="dragonfly", target_os="freebsd", target_os = "ios",
                  target_os="linux", target_os = "macos", target_os="openbsd"))]
        /// The implementation supports the Process Sporadic Server option.
        _POSIX_SPORADIC_SERVER = libc::_SC_SPORADIC_SERVER,
        #[cfg(any(target_os = "ios", target_os="linux", target_os = "macos",
                  target_os="openbsd"))]
        _POSIX_SS_REPL_MAX = libc::_SC_SS_REPL_MAX,
        /// The implementation supports the Synchronized Input and Output option.
        _POSIX_SYNCHRONIZED_IO = libc::_SC_SYNCHRONIZED_IO,
        /// The implementation supports the Thread Stack Address Attribute option.
        _POSIX_THREAD_ATTR_STACKADDR = libc::_SC_THREAD_ATTR_STACKADDR,
        /// The implementation supports the Thread Stack Size Attribute option.
        _POSIX_THREAD_ATTR_STACKSIZE = libc::_SC_THREAD_ATTR_STACKSIZE,
        #[cfg(any(target_os = "ios", target_os="linux", target_os = "macos",
                  target_os="netbsd", target_os="openbsd"))]
        /// The implementation supports the Thread CPU-Time Clocks option.
        _POSIX_THREAD_CPUTIME = libc::_SC_THREAD_CPUTIME,
        /// The implementation supports the Non-Robust Mutex Priority Inheritance
        /// option.
        _POSIX_THREAD_PRIO_INHERIT = libc::_SC_THREAD_PRIO_INHERIT,
        /// The implementation supports the Non-Robust Mutex Priority Protection option.
        _POSIX_THREAD_PRIO_PROTECT = libc::_SC_THREAD_PRIO_PROTECT,
        /// The implementation supports the Thread Execution Scheduling option.
        _POSIX_THREAD_PRIORITY_SCHEDULING = libc::_SC_THREAD_PRIORITY_SCHEDULING,
        #[cfg(any(target_os="dragonfly", target_os="freebsd", target_os = "ios",
                  target_os="linux", target_os = "macos", target_os="netbsd",
                  target_os="openbsd"))]
        /// The implementation supports the Thread Process-Shared Synchronization
        /// option.
        _POSIX_THREAD_PROCESS_SHARED = libc::_SC_THREAD_PROCESS_SHARED,
        #[cfg(any(target_os="dragonfly", target_os="linux", target_os="openbsd"))]
        /// The implementation supports the Robust Mutex Priority Inheritance option.
        _POSIX_THREAD_ROBUST_PRIO_INHERIT = libc::_SC_THREAD_ROBUST_PRIO_INHERIT,
        #[cfg(any(target_os="dragonfly", target_os="linux", target_os="openbsd"))]
        /// The implementation supports the Robust Mutex Priority Protection option.
        _POSIX_THREAD_ROBUST_PRIO_PROTECT = libc::_SC_THREAD_ROBUST_PRIO_PROTECT,
        /// The implementation supports thread-safe functions.
        _POSIX_THREAD_SAFE_FUNCTIONS = libc::_SC_THREAD_SAFE_FUNCTIONS,
        #[cfg(any(target_os="dragonfly", target_os="freebsd", target_os = "ios",
                  target_os="linux", target_os = "macos", target_os="openbsd"))]
        /// The implementation supports the Thread Sporadic Server option.
        _POSIX_THREAD_SPORADIC_SERVER = libc::_SC_THREAD_SPORADIC_SERVER,
        /// The implementation supports threads.
        _POSIX_THREADS = libc::_SC_THREADS,
        #[cfg(any(target_os="dragonfly", target_os="freebsd", target_os = "ios",
                  target_os="linux", target_os = "macos", target_os="openbsd"))]
        /// The implementation supports timeouts.
        _POSIX_TIMEOUTS = libc::_SC_TIMEOUTS,
        /// The implementation supports timers.
        _POSIX_TIMERS = libc::_SC_TIMERS,
        #[cfg(any(target_os="dragonfly", target_os="freebsd", target_os = "ios",
                  target_os="linux", target_os = "macos", target_os="openbsd"))]
        /// The implementation supports the Trace option.
        _POSIX_TRACE = libc::_SC_TRACE,
        #[cfg(any(target_os="dragonfly", target_os="freebsd", target_os = "ios",
                  target_os="linux", target_os = "macos", target_os="openbsd"))]
        /// The implementation supports the Trace Event Filter option.
        _POSIX_TRACE_EVENT_FILTER = libc::_SC_TRACE_EVENT_FILTER,
        #[cfg(any(target_os = "ios", target_os="linux", target_os = "macos",
                  target_os="openbsd"))]
        _POSIX_TRACE_EVENT_NAME_MAX = libc::_SC_TRACE_EVENT_NAME_MAX,
        #[cfg(any(target_os="dragonfly", target_os="freebsd", target_os = "ios",
                  target_os="linux", target_os = "macos", target_os="openbsd"))]
        /// The implementation supports the Trace Inherit option.
        _POSIX_TRACE_INHERIT = libc::_SC_TRACE_INHERIT,
        #[cfg(any(target_os="dragonfly", target_os="freebsd", target_os = "ios",
                  target_os="linux", target_os = "macos", target_os="openbsd"))]
        /// The implementation supports the Trace Log option.
        _POSIX_TRACE_LOG = libc::_SC_TRACE_LOG,
        #[cfg(any(target_os = "ios", target_os="linux", target_os = "macos",
                  target_os="openbsd"))]
        _POSIX_TRACE_NAME_MAX = libc::_SC_TRACE_NAME_MAX,
        #[cfg(any(target_os = "ios", target_os="linux", target_os = "macos",
                  target_os="openbsd"))]
        _POSIX_TRACE_SYS_MAX = libc::_SC_TRACE_SYS_MAX,
        #[cfg(any(target_os = "ios", target_os="linux", target_os = "macos",
                  target_os="openbsd"))]
        _POSIX_TRACE_USER_EVENT_MAX = libc::_SC_TRACE_USER_EVENT_MAX,
        #[cfg(any(target_os="dragonfly", target_os="freebsd", target_os = "ios",
                  target_os="linux", target_os = "macos", target_os="openbsd"))]
        /// The implementation supports the Typed Memory Objects option.
        _POSIX_TYPED_MEMORY_OBJECTS = libc::_SC_TYPED_MEMORY_OBJECTS,
        /// Integer value indicating version of this standard (C-language binding)
        /// to which the implementation conforms. For implementations conforming to
        /// POSIX.1-2008, the value shall be 200809L.
        _POSIX_VERSION = libc::_SC_VERSION,
        #[cfg(any(target_os="dragonfly", target_os="freebsd", target_os = "ios",
                  target_os="linux", target_os = "macos", target_os="netbsd",
                  target_os="openbsd"))]
        /// The implementation provides a C-language compilation environment with
        /// 32-bit `int`, `long`, `pointer`, and `off_t` types.
        _POSIX_V6_ILP32_OFF32 = libc::_SC_V6_ILP32_OFF32,
        #[cfg(any(target_os="dragonfly", target_os="freebsd", target_os = "ios",
                  target_os="linux", target_os = "macos", target_os="netbsd",
                  target_os="openbsd"))]
        /// The implementation provides a C-language compilation environment with
        /// 32-bit `int`, `long`, and pointer types and an `off_t` type using at
        /// least 64 bits.
        _POSIX_V6_ILP32_OFFBIG = libc::_SC_V6_ILP32_OFFBIG,
        #[cfg(any(target_os="dragonfly", target_os="freebsd", target_os = "ios",
                  target_os="linux", target_os = "macos", target_os="netbsd",
                  target_os="openbsd"))]
        /// The implementation provides a C-language compilation environment with
        /// 32-bit `int` and 64-bit `long`, `pointer`, and `off_t` types.
        _POSIX_V6_LP64_OFF64 = libc::_SC_V6_LP64_OFF64,
        #[cfg(any(target_os="dragonfly", target_os="freebsd", target_os = "ios",
                  target_os="linux", target_os = "macos", target_os="netbsd",
                  target_os="openbsd"))]
        /// The implementation provides a C-language compilation environment with an
        /// `int` type using at least 32 bits and `long`, pointer, and `off_t` types
        /// using at least 64 bits.
        _POSIX_V6_LPBIG_OFFBIG = libc::_SC_V6_LPBIG_OFFBIG,
        /// The implementation supports the C-Language Binding option.
        _POSIX2_C_BIND = libc::_SC_2_C_BIND,
        /// The implementation supports the C-Language Development Utilities option.
        _POSIX2_C_DEV = libc::_SC_2_C_DEV,
        /// The implementation supports the Terminal Characteristics option.
        _POSIX2_CHAR_TERM = libc::_SC_2_CHAR_TERM,
        /// The implementation supports the FORTRAN Development Utilities option.
        _POSIX2_FORT_DEV = libc::_SC_2_FORT_DEV,
        /// The implementation supports the FORTRAN Runtime Utilities option.
        _POSIX2_FORT_RUN = libc::_SC_2_FORT_RUN,
        /// The implementation supports the creation of locales by the localedef
        /// utility.
        _POSIX2_LOCALEDEF = libc::_SC_2_LOCALEDEF,
        #[cfg(any(target_os="dragonfly", target_os="freebsd", target_os = "ios",
                  target_os="linux", target_os = "macos", target_os="netbsd",
                  target_os="openbsd"))]
        /// The implementation supports the Batch Environment Services and Utilities
        /// option.
        _POSIX2_PBS = libc::_SC_2_PBS,
        #[cfg(any(target_os="dragonfly", target_os="freebsd", target_os = "ios",
                  target_os="linux", target_os = "macos", target_os="netbsd",
                  target_os="openbsd"))]
        /// The implementation supports the Batch Accounting option.
        _POSIX2_PBS_ACCOUNTING = libc::_SC_2_PBS_ACCOUNTING,
        #[cfg(any(target_os="dragonfly", target_os="freebsd", target_os = "ios",
                  target_os="linux", target_os = "macos", target_os="netbsd",
                  target_os="openbsd"))]
        /// The implementation supports the Batch Checkpoint/Restart option.
        _POSIX2_PBS_CHECKPOINT = libc::_SC_2_PBS_CHECKPOINT,
        #[cfg(any(target_os="dragonfly", target_os="freebsd", target_os = "ios",
                  target_os="linux", target_os = "macos", target_os="netbsd",
                  target_os="openbsd"))]
        /// The implementation supports the Locate Batch Job Request option.
        _POSIX2_PBS_LOCATE = libc::_SC_2_PBS_LOCATE,
        #[cfg(any(target_os="dragonfly", target_os="freebsd", target_os = "ios",
                  target_os="linux", target_os = "macos", target_os="netbsd",
                  target_os="openbsd"))]
        /// The implementation supports the Batch Job Message Request option.
        _POSIX2_PBS_MESSAGE = libc::_SC_2_PBS_MESSAGE,
        #[cfg(any(target_os="dragonfly", target_os="freebsd", target_os = "ios",
                  target_os="linux", target_os = "macos", target_os="netbsd",
                  target_os="openbsd"))]
        /// The implementation supports the Track Batch Job Request option.
        _POSIX2_PBS_TRACK = libc::_SC_2_PBS_TRACK,
        /// The implementation supports the Software Development Utilities option.
        _POSIX2_SW_DEV = libc::_SC_2_SW_DEV,
        /// The implementation supports the User Portability Utilities option.
        _POSIX2_UPE = libc::_SC_2_UPE,
        /// Integer value indicating version of the Shell and Utilities volume of
        /// POSIX.1 to which the implementation conforms.
        _POSIX2_VERSION = libc::_SC_2_VERSION,
        /// The size of a system page in bytes.
        ///
        /// POSIX also defines an alias named `PAGESIZE`, but Rust does not allow two
        /// enum constants to have the same value, so nix omits `PAGESIZE`.
        PAGE_SIZE = libc::_SC_PAGE_SIZE,
        PTHREAD_DESTRUCTOR_ITERATIONS = libc::_SC_THREAD_DESTRUCTOR_ITERATIONS,
        PTHREAD_KEYS_MAX = libc::_SC_THREAD_KEYS_MAX,
        PTHREAD_STACK_MIN = libc::_SC_THREAD_STACK_MIN,
        PTHREAD_THREADS_MAX = libc::_SC_THREAD_THREADS_MAX,
        RE_DUP_MAX = libc::_SC_RE_DUP_MAX,
        #[cfg(any(target_os="android", target_os="dragonfly", target_os="freebsd",
                  target_os = "ios", target_os="linux", target_os = "macos",
                  target_os="openbsd"))]
        RTSIG_MAX = libc::_SC_RTSIG_MAX,
        SEM_NSEMS_MAX = libc::_SC_SEM_NSEMS_MAX,
        #[cfg(any(target_os="android", target_os="dragonfly", target_os="freebsd",
                  target_os = "ios", target_os="linux", target_os = "macos",
                  target_os="openbsd"))]
        SEM_VALUE_MAX = libc::_SC_SEM_VALUE_MAX,
        #[cfg(any(target_os = "android", target_os="dragonfly", target_os="freebsd",
                  target_os = "ios", target_os="linux", target_os = "macos",
                  target_os = "openbsd"))]
        SIGQUEUE_MAX = libc::_SC_SIGQUEUE_MAX,
        STREAM_MAX = libc::_SC_STREAM_MAX,
        #[cfg(any(target_os="dragonfly", target_os="freebsd", target_os = "ios",
                  target_os="linux", target_os = "macos", target_os="netbsd",
                  target_os="openbsd"))]
        SYMLOOP_MAX = libc::_SC_SYMLOOP_MAX,
        TIMER_MAX = libc::_SC_TIMER_MAX,
        TTY_NAME_MAX = libc::_SC_TTY_NAME_MAX,
        TZNAME_MAX = libc::_SC_TZNAME_MAX,
        #[cfg(any(target_os="android", target_os="dragonfly", target_os="freebsd",
                  target_os = "ios", target_os="linux", target_os = "macos",
                  target_os="openbsd"))]
        /// The implementation supports the X/Open Encryption Option Group.
        _XOPEN_CRYPT = libc::_SC_XOPEN_CRYPT,
        #[cfg(any(target_os="android", target_os="dragonfly", target_os="freebsd",
                  target_os = "ios", target_os="linux", target_os = "macos",
                  target_os="openbsd"))]
        /// The implementation supports the Issue 4, Version 2 Enhanced
        /// Internationalization Option Group.
        _XOPEN_ENH_I18N = libc::_SC_XOPEN_ENH_I18N,
        #[cfg(any(target_os="android", target_os="dragonfly", target_os="freebsd",
                  target_os = "ios", target_os="linux", target_os = "macos",
                  target_os="openbsd"))]
        _XOPEN_LEGACY = libc::_SC_XOPEN_LEGACY,
        #[cfg(any(target_os="android", target_os="dragonfly", target_os="freebsd",
                  target_os = "ios", target_os="linux", target_os = "macos",
                  target_os="openbsd"))]
        /// The implementation supports the X/Open Realtime Option Group.
        _XOPEN_REALTIME = libc::_SC_XOPEN_REALTIME,
        #[cfg(any(target_os="android", target_os="dragonfly", target_os="freebsd",
                  target_os = "ios", target_os="linux", target_os = "macos",
                  target_os="openbsd"))]
        /// The implementation supports the X/Open Realtime Threads Option Group.
        _XOPEN_REALTIME_THREADS = libc::_SC_XOPEN_REALTIME_THREADS,
        /// The implementation supports the Issue 4, Version 2 Shared Memory Option
        /// Group.
        _XOPEN_SHM = libc::_SC_XOPEN_SHM,
        #[cfg(any(target_os="dragonfly", target_os="freebsd", target_os = "ios",
                  target_os="linux", target_os = "macos", target_os="openbsd"))]
        /// The implementation supports the XSI STREAMS Option Group.
        _XOPEN_STREAMS = libc::_SC_XOPEN_STREAMS,
        #[cfg(any(target_os="android", target_os="dragonfly", target_os="freebsd",
                  target_os = "ios", target_os="linux", target_os = "macos",
                  target_os="openbsd"))]
        /// The implementation supports the XSI option
        _XOPEN_UNIX = libc::_SC_XOPEN_UNIX,
        #[cfg(any(target_os="android", target_os="dragonfly", target_os="freebsd",
                  target_os = "ios", target_os="linux", target_os = "macos",
                  target_os="openbsd"))]
        /// Integer value indicating version of the X/Open Portability Guide to
        /// which the implementation conforms.
        _XOPEN_VERSION = libc::_SC_XOPEN_VERSION,
    }

    /// Get configurable system variables (see
    /// [sysconf(3)](http://pubs.opengroup.org/onlinepubs/9699919799/functions/sysconf.html))
    ///
    /// Returns the value of a configurable system variable.  Most supported
    /// variables also have associated compile-time constants, but POSIX
    /// allows their values to change at runtime.  There are generally two types of
    /// sysconf variables: options and limits.  See sysconf(3) for more details.
    ///
    /// # Returns
    ///
    /// - `Ok(Some(x))`: the variable's limit (for limit variables) or its
    ///     implementation level (for option variables).  Implementation levels are
    ///     usually a decimal-coded date, such as 200112 for POSIX 2001.12
    /// - `Ok(None)`: the variable has no limit (for limit variables) or is
    ///     unsupported (for option variables)
    /// - `Err(x)`: an error occurred
    pub fn sysconf(var: SysconfVar) -> Result<Option<c_long>> {
        let raw = unsafe {
            Errno::clear();
            libc::sysconf(var as c_int)
        };
        if raw == -1 {
            if errno::errno() == 0 {
                Ok(None)
            } else {
                Err(Error::Sys(Errno::last()))
            }
        } else {
            Ok(Some(raw))
        }
    }

    #[cfg(any(target_os = "android", target_os = "linux"))]
    mod pivot_root {
        use libc;
        use {Result, NixPath};
        use errno::Errno;

        pub fn pivot_root<P1: ?Sized + NixPath, P2: ?Sized + NixPath>(
                new_root: &P1, put_old: &P2) -> Result<()> {
            let res = new_root.with_nix_path(|new_root| {
                put_old.with_nix_path(|put_old| {
                    unsafe {
                        libc::syscall(libc::SYS_pivot_root, new_root.as_ptr(), put_old.as_ptr())
                    }
                })
            })??;

            Errno::result(res).map(drop)
        }
    }

    #[cfg(any(target_os = "android", target_os = "freebsd",
              target_os = "linux", target_os = "openbsd"))]
    mod setres {
        use libc;
        use Result;
        use errno::Errno;
        use super::{Uid, Gid};

        /// Sets the real, effective, and saved uid.
        /// ([see setresuid(2)](http://man7.org/linux/man-pages/man2/setresuid.2.html))
        ///
        /// * `ruid`: real user id
        /// * `euid`: effective user id
        /// * `suid`: saved user id
        /// * returns: Ok or libc error code.
        ///
        /// Err is returned if the user doesn't have permission to set this UID.
        #[inline]
        pub fn setresuid(ruid: Uid, euid: Uid, suid: Uid) -> Result<()> {
            let res = unsafe { libc::setresuid(ruid.into(), euid.into(), suid.into()) };

            Errno::result(res).map(drop)
        }

        /// Sets the real, effective, and saved gid.
        /// ([see setresuid(2)](http://man7.org/linux/man-pages/man2/setresuid.2.html))
        ///
        /// * `rgid`: real group id
        /// * `egid`: effective group id
        /// * `sgid`: saved group id
        /// * returns: Ok or libc error code.
        ///
        /// Err is returned if the user doesn't have permission to set this GID.
        #[inline]
        pub fn setresgid(rgid: Gid, egid: Gid, sgid: Gid) -> Result<()> {
            let res = unsafe { libc::setresgid(rgid.into(), egid.into(), sgid.into()) };

            Errno::result(res).map(drop)
        }
    }

    libc_bitflags!{
        /// Options for access()
        pub struct AccessFlags : c_int {
            /// Test for existence of file.
            F_OK;
            /// Test for read permission.
            R_OK;
            /// Test for write permission.
            W_OK;
            /// Test for execute (search) permission.
            X_OK;
        }
    }

    /// Checks the file named by `path` for accessibility according to the flags given by `amode`
    /// See [access(2)](http://pubs.opengroup.org/onlinepubs/9699919799/functions/access.html)
    pub fn access<P: ?Sized + NixPath>(path: &P, amode: AccessFlags) -> Result<()> {
        let res = path.with_nix_path(|cstr| {
            unsafe {
                libc::access(cstr.as_ptr(), amode.bits)
            }
        })?;
        Errno::result(res).map(drop)
    }

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