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    use os::unix::prelude::*;

    use ffi::{OsString, OsStr, CString, CStr};
    use fmt;
    use io;
    use libc::{self, c_int, gid_t, uid_t, c_char, EXIT_SUCCESS, EXIT_FAILURE};
    use ptr;
    use sys::fd::FileDesc;
    use sys::fs::{File, OpenOptions};
    use sys::pipe::{self, AnonPipe};
    use sys_common::process::{CommandEnv, DefaultEnvKey};
    use collections::BTreeMap;

    ////////////////////////////////////////////////////////////////////////////////
    // Command
    ////////////////////////////////////////////////////////////////////////////////

    pub struct Command {
        // Currently we try hard to ensure that the call to `.exec()` doesn't
        // actually allocate any memory. While many platforms try to ensure that
        // memory allocation works after a fork in a multithreaded process, it's
        // been observed to be buggy and somewhat unreliable, so we do our best to
        // just not do it at all!
        //
        // Along those lines, the `argv` and `envp` raw pointers here are exactly
        // what's gonna get passed to `execvp`. The `argv` array starts with the
        // `program` and ends with a NULL, and the `envp` pointer, if present, is
        // also null-terminated.
        //
        // Right now we don't support removing arguments, so there's no much fancy
        // support there, but we support adding and removing environment variables,
        // so a side table is used to track where in the `envp` array each key is
        // located. Whenever we add a key we update it in place if it's already
        // present, and whenever we remove a key we update the locations of all
        // other keys.
        program: CString,
        args: Vec<CString>,
        argv: Argv,
        env: CommandEnv<DefaultEnvKey>,

        cwd: Option<CString>,
        uid: Option<uid_t>,
        gid: Option<gid_t>,
        saw_nul: bool,
        closures: Vec<Box<dyn FnMut() -> io::Result<()> + Send + Sync>>,
        stdin: Option<Stdio>,
        stdout: Option<Stdio>,
        stderr: Option<Stdio>,
    }

    // Create a new type for argv, so that we can make it `Send`
    struct Argv(Vec<*const c_char>);

    // It is safe to make Argv Send, because it contains pointers to memory owned by `Command.args`
    unsafe impl Send for Argv {}

    // passed back to std::process with the pipes connected to the child, if any
    // were requested
    pub struct StdioPipes {
        pub stdin: Option<AnonPipe>,
        pub stdout: Option<AnonPipe>,
        pub stderr: Option<AnonPipe>,
    }

    // passed to do_exec() with configuration of what the child stdio should look
    // like
    pub struct ChildPipes {
        pub stdin: ChildStdio,
        pub stdout: ChildStdio,
        pub stderr: ChildStdio,
    }

    pub enum ChildStdio {
        Inherit,
        Explicit(c_int),
        Owned(FileDesc),
    }

    pub enum Stdio {
        Inherit,
        Null,
        MakePipe,
        Fd(FileDesc),
    }

    impl Command {
        pub fn new(program: &OsStr) -> Command {
            let mut saw_nul = false;
            let program = os2c(program, &mut saw_nul);
            Command {
                argv: Argv(vec![program.as_ptr(), ptr::null()]),
                program,
                args: Vec::new(),
                env: Default::default(),
                cwd: None,
                uid: None,
                gid: None,
                saw_nul,
                closures: Vec::new(),
                stdin: None,
                stdout: None,
                stderr: None,
            }
        }

        pub fn arg(&mut self, arg: &OsStr) {
            // Overwrite the trailing NULL pointer in `argv` and then add a new null
            // pointer.
            let arg = os2c(arg, &mut self.saw_nul);
            self.argv.0[self.args.len() + 1] = arg.as_ptr();
            self.argv.0.push(ptr::null());

            // Also make sure we keep track of the owned value to schedule a
            // destructor for this memory.
            self.args.push(arg);
        }

        pub fn cwd(&mut self, dir: &OsStr) {
            self.cwd = Some(os2c(dir, &mut self.saw_nul));
        }
        pub fn uid(&mut self, id: uid_t) {
            self.uid = Some(id);
        }
        pub fn gid(&mut self, id: gid_t) {
            self.gid = Some(id);
        }

        pub fn saw_nul(&self) -> bool {
            self.saw_nul
        }
        pub fn get_argv(&self) -> &Vec<*const c_char> {
            &self.argv.0
        }

        #[allow(dead_code)]
        pub fn get_cwd(&self) -> &Option<CString> {
            &self.cwd
        }
        #[allow(dead_code)]
        pub fn get_uid(&self) -> Option<uid_t> {
            self.uid
        }
        #[allow(dead_code)]
        pub fn get_gid(&self) -> Option<gid_t> {
            self.gid
        }

        pub fn get_closures(&mut self) -> &mut Vec<Box<dyn FnMut() -> io::Result<()> + Send + Sync>> {
            &mut self.closures
        }

        pub fn before_exec(&mut self,
                           f: Box<dyn FnMut() -> io::Result<()> + Send + Sync>) {
            self.closures.push(f);
        }

        pub fn stdin(&mut self, stdin: Stdio) {
            self.stdin = Some(stdin);
        }

        pub fn stdout(&mut self, stdout: Stdio) {
            self.stdout = Some(stdout);
        }

        pub fn stderr(&mut self, stderr: Stdio) {
            self.stderr = Some(stderr);
        }

        pub fn env_mut(&mut self) -> &mut CommandEnv<DefaultEnvKey> {
            &mut self.env
        }

        pub fn capture_env(&mut self) -> Option<CStringArray> {
            let maybe_env = self.env.capture_if_changed();
            maybe_env.map(|env| construct_envp(env, &mut self.saw_nul))
        }
        #[allow(dead_code)]
        pub fn env_saw_path(&self) -> bool {
            self.env.have_changed_path()
        }

        pub fn setup_io(&self, default: Stdio, needs_stdin: bool)
                    -> io::Result<(StdioPipes, ChildPipes)> {
            let null = Stdio::Null;
            let default_stdin = if needs_stdin {&default} else {&null};
            let stdin = self.stdin.as_ref().unwrap_or(default_stdin);
            let stdout = self.stdout.as_ref().unwrap_or(&default);
            let stderr = self.stderr.as_ref().unwrap_or(&default);
            let (their_stdin, our_stdin) = stdin.to_child_stdio(true)?;
            let (their_stdout, our_stdout) = stdout.to_child_stdio(false)?;
            let (their_stderr, our_stderr) = stderr.to_child_stdio(false)?;
            let ours = StdioPipes {
                stdin: our_stdin,
                stdout: our_stdout,
                stderr: our_stderr,
            };
            let theirs = ChildPipes {
                stdin: their_stdin,
                stdout: their_stdout,
                stderr: their_stderr,
            };
            Ok((ours, theirs))
        }
    }

    fn os2c(s: &OsStr, saw_nul: &mut bool) -> CString {
        CString::new(s.as_bytes()).unwrap_or_else(|_e| {
            *saw_nul = true;
            CString::new("<string-with-nul>").unwrap()
        })
    }

    // Helper type to manage ownership of the strings within a C-style array.
    pub struct CStringArray {
        items: Vec<CString>,
        ptrs: Vec<*const c_char>
    }

    impl CStringArray {
        pub fn with_capacity(capacity: usize) -> Self {
            let mut result = CStringArray {
                items: Vec::with_capacity(capacity),
                ptrs: Vec::with_capacity(capacity+1)
            };
            result.ptrs.push(ptr::null());
            result
        }
        pub fn push(&mut self, item: CString) {
            let l = self.ptrs.len();
            self.ptrs[l-1] = item.as_ptr();
            self.ptrs.push(ptr::null());
            self.items.push(item);
        }
        pub fn as_ptr(&self) -> *const *const c_char {
            self.ptrs.as_ptr()
        }
    }

    fn construct_envp(env: BTreeMap<DefaultEnvKey, OsString>, saw_nul: &mut bool) -> CStringArray {
        let mut result = CStringArray::with_capacity(env.len());
        for (k, v) in env {
            let mut k: OsString = k.into();

            // Reserve additional space for '=' and null terminator
            k.reserve_exact(v.len() + 2);
            k.push("=");
            k.push(&v);

            // Add the new entry into the array
            if let Ok(item) = CString::new(k.into_vec()) {
                result.push(item);
            } else {
                *saw_nul = true;
            }
        }

        result
    }

    impl Stdio {
        pub fn to_child_stdio(&self, readable: bool)
                          -> io::Result<(ChildStdio, Option<AnonPipe>)> {
            match *self {
                Stdio::Inherit => {
                    Ok((ChildStdio::Inherit, None))
                },

                // Make sure that the source descriptors are not an stdio
                // descriptor, otherwise the order which we set the child's
                // descriptors may blow away a descriptor which we are hoping to
                // save. For example, suppose we want the child's stderr to be the
                // parent's stdout, and the child's stdout to be the parent's
                // stderr. No matter which we dup first, the second will get
                // overwritten prematurely.
                Stdio::Fd(ref fd) => {
                    if fd.raw() >= 0 && fd.raw() <= libc::STDERR_FILENO {
                        Ok((ChildStdio::Owned(fd.duplicate()?), None))
                    } else {
                        Ok((ChildStdio::Explicit(fd.raw()), None))
                    }
                }

                Stdio::MakePipe => {
                    let (reader, writer) = pipe::anon_pipe()?;
                    let (ours, theirs) = if readable {
                        (writer, reader)
                    } else {
                        (reader, writer)
                    };
                    Ok((ChildStdio::Owned(theirs.into_fd()), Some(ours)))
                }

                Stdio::Null => {
                    let mut opts = OpenOptions::new();
                    opts.read(readable);
                    opts.write(!readable);
                    let path = unsafe {
                        CStr::from_ptr("/dev/null\0".as_ptr() as *const _)
                    };
                    let fd = File::open_c(&path, &opts)?;
                    Ok((ChildStdio::Owned(fd.into_fd()), None))
                }
            }
        }
    }

    impl From<AnonPipe> for Stdio {
        fn from(pipe: AnonPipe) -> Stdio {
            Stdio::Fd(pipe.into_fd())
        }
    }

    impl From<File> for Stdio {
        fn from(file: File) -> Stdio {
            Stdio::Fd(file.into_fd())
        }
    }

    impl ChildStdio {
        pub fn fd(&self) -> Option<c_int> {
            match *self {
                ChildStdio::Inherit => None,
                ChildStdio::Explicit(fd) => Some(fd),
                ChildStdio::Owned(ref fd) => Some(fd.raw()),
            }
        }
    }

    impl fmt::Debug for Command {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            write!(f, "{:?}", self.program)?;
            for arg in &self.args {
                write!(f, " {:?}", arg)?;
            }
            Ok(())
        }
    }

    /// Unix exit statuses
    #[derive(PartialEq, Eq, Clone, Copy, Debug)]
    pub struct ExitStatus(c_int);

    impl ExitStatus {
        pub fn new(status: c_int) -> ExitStatus {
            ExitStatus(status)
        }

        fn exited(&self) -> bool {
            unsafe { libc::WIFEXITED(self.0) }
        }

        pub fn success(&self) -> bool {
            self.code() == Some(0)
        }

        pub fn code(&self) -> Option<i32> {
            if self.exited() {
                Some(unsafe { libc::WEXITSTATUS(self.0) })
            } else {
                None
            }
        }

        pub fn signal(&self) -> Option<i32> {
            if !self.exited() {
                Some(unsafe { libc::WTERMSIG(self.0) })
            } else {
                None
            }
        }
    }

    impl From<c_int> for ExitStatus {
        fn from(a: c_int) -> ExitStatus {
            ExitStatus(a)
        }
    }

    impl fmt::Display for ExitStatus {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            if let Some(code) = self.code() {
                write!(f, "exit code: {}", code)
            } else {
                let signal = self.signal().unwrap();
                write!(f, "signal: {}", signal)
            }
        }
    }

    #[derive(PartialEq, Eq, Clone, Copy, Debug)]
    pub struct ExitCode(u8);

    impl ExitCode {
        pub const SUCCESS: ExitCode = ExitCode(EXIT_SUCCESS as _);
        pub const FAILURE: ExitCode = ExitCode(EXIT_FAILURE as _);

        #[inline]
        pub fn as_i32(&self) -> i32 {
            self.0 as i32
        }
    }

    #[cfg(all(test, not(target_os = "emscripten")))]
    mod tests {
        use super::*;

        use ffi::OsStr;
        use mem;
        use ptr;
        use libc;
        use sys::cvt;

        macro_rules! t {
            ($e:expr) => {
                match $e {
                    Ok(t) => t,
                    Err(e) => panic!("received error for `{}`: {}", stringify!($e), e),
                }
            }
        }

        // Android with api less than 21 define sig* functions inline, so it is not
        // available for dynamic link. Implementing sigemptyset and sigaddset allow us
        // to support older Android version (independent of libc version).
        // The following implementations are based on https://git.io/vSkNf

        #[cfg(not(target_os = "android"))]
        extern {
            #[cfg_attr(target_os = "netbsd", link_name = "__sigemptyset14")]
            fn sigemptyset(set: *mut libc::sigset_t) -> libc::c_int;

            #[cfg_attr(target_os = "netbsd", link_name = "__sigaddset14")]
            fn sigaddset(set: *mut libc::sigset_t, signum: libc::c_int) -> libc::c_int;
        }

        #[cfg(target_os = "android")]
        unsafe fn sigemptyset(set: *mut libc::sigset_t) -> libc::c_int {
            libc::memset(set as *mut _, 0, mem::size_of::<libc::sigset_t>());
            return 0;
        }

        #[cfg(target_os = "android")]
        unsafe fn sigaddset(set: *mut libc::sigset_t, signum: libc::c_int) -> libc::c_int {
            use slice;

            let raw = slice::from_raw_parts_mut(set as *mut u8, mem::size_of::<libc::sigset_t>());
            let bit = (signum - 1) as usize;
            raw[bit / 8] |= 1 << (bit % 8);
            return 0;
        }

        // See #14232 for more information, but it appears that signal delivery to a
        // newly spawned process may just be raced in the macOS, so to prevent this
        // test from being flaky we ignore it on macOS.
        #[test]
        #[cfg_attr(target_os = "macos", ignore)]
        // When run under our current QEMU emulation test suite this test fails,
        // although the reason isn't very clear as to why. For now this test is
        // ignored there.
        #[cfg_attr(target_arch = "arm", ignore)]
        #[cfg_attr(target_arch = "aarch64", ignore)]
        fn test_process_mask() {
            unsafe {
                // Test to make sure that a signal mask does not get inherited.
                let mut cmd = Command::new(OsStr::new("cat"));

                let mut set: libc::sigset_t = mem::uninitialized();
                let mut old_set: libc::sigset_t = mem::uninitialized();
                t!(cvt(sigemptyset(&mut set)));
                t!(cvt(sigaddset(&mut set, libc::SIGINT)));
                t!(cvt(libc::pthread_sigmask(libc::SIG_SETMASK, &set, &mut old_set)));

                cmd.stdin(Stdio::MakePipe);
                cmd.stdout(Stdio::MakePipe);

                let (mut cat, mut pipes) = t!(cmd.spawn(Stdio::Null, true));
                let stdin_write = pipes.stdin.take().unwrap();
                let stdout_read = pipes.stdout.take().unwrap();

                t!(cvt(libc::pthread_sigmask(libc::SIG_SETMASK, &old_set,
                                             ptr::null_mut())));

                t!(cvt(libc::kill(cat.id() as libc::pid_t, libc::SIGINT)));
                // We need to wait until SIGINT is definitely delivered. The
                // easiest way is to write something to cat, and try to read it
                // back: if SIGINT is unmasked, it'll get delivered when cat is
                // next scheduled.
                let _ = stdin_write.write(b"Hello");
                drop(stdin_write);

                // Either EOF or failure (EPIPE) is okay.
                let mut buf = [0; 5];
                if let Ok(ret) = stdout_read.read(&mut buf) {
                    assert_eq!(ret, 0);
                }

                t!(cat.wait());
            }
        }
    }

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