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    //! Implementation of `std::os` functionality for unix systems

    #![allow(unused_imports)] // lots of cfg code here

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

    use error::Error as StdError;
    use ffi::{CString, CStr, OsString, OsStr};
    use fmt;
    use io;
    use iter;
    use libc::{self, c_int, c_char, c_void};
    use marker::PhantomData;
    use mem;
    use memchr;
    use path::{self, PathBuf};
    use ptr;
    use slice;
    use str;
    use sys_common::mutex::{Mutex, MutexGuard};
    use sys::cvt;
    use sys::fd;
    use vec;

    const TMPBUF_SZ: usize = 128;

    extern {
        #[cfg(not(target_os = "dragonfly"))]
        #[cfg_attr(any(target_os = "linux",
                       target_os = "emscripten",
                       target_os = "fuchsia",
                       target_os = "l4re"),
                   link_name = "__errno_location")]
        #[cfg_attr(any(target_os = "bitrig",
                       target_os = "netbsd",
                       target_os = "openbsd",
                       target_os = "android",
                       target_os = "hermit",
                       target_env = "newlib"),
                   link_name = "__errno")]
        #[cfg_attr(target_os = "solaris", link_name = "___errno")]
        #[cfg_attr(any(target_os = "macos",
                       target_os = "ios",
                       target_os = "freebsd"),
                   link_name = "__error")]
        #[cfg_attr(target_os = "haiku", link_name = "_errnop")]
        fn errno_location() -> *mut c_int;
    }

    /// Returns the platform-specific value of errno
    #[cfg(not(target_os = "dragonfly"))]
    pub fn errno() -> i32 {
        unsafe {
            (*errno_location()) as i32
        }
    }

    /// Sets the platform-specific value of errno
    #[cfg(all(not(target_os = "linux"),
              not(target_os = "dragonfly")))] // needed for readdir and syscall!
    pub fn set_errno(e: i32) {
        unsafe {
            *errno_location() = e as c_int
        }
    }

    #[cfg(target_os = "dragonfly")]
    pub fn errno() -> i32 {
        extern {
            #[thread_local]
            static errno: c_int;
        }

        unsafe { errno as i32 }
    }

    #[cfg(target_os = "dragonfly")]
    pub fn set_errno(e: i32) {
        extern {
            #[thread_local]
            static mut errno: c_int;
        }

        unsafe {
            errno = e;
        }
    }

    /// Gets a detailed string description for the given error number.
    pub fn error_string(errno: i32) -> String {
        extern {
            #[cfg_attr(any(target_os = "linux", target_env = "newlib"),
                       link_name = "__xpg_strerror_r")]
            fn strerror_r(errnum: c_int, buf: *mut c_char,
                          buflen: libc::size_t) -> c_int;
        }

        let mut buf = [0 as c_char; TMPBUF_SZ];

        let p = buf.as_mut_ptr();
        unsafe {
            if strerror_r(errno as c_int, p, buf.len()) < 0 {
                panic!("strerror_r failure");
            }

            let p = p as *const _;
            str::from_utf8(CStr::from_ptr(p).to_bytes()).unwrap().to_owned()
        }
    }

    pub fn getcwd() -> io::Result<PathBuf> {
        let mut buf = Vec::with_capacity(512);
        loop {
            unsafe {
                let ptr = buf.as_mut_ptr() as *mut libc::c_char;
                if !libc::getcwd(ptr, buf.capacity()).is_null() {
                    let len = CStr::from_ptr(buf.as_ptr() as *const libc::c_char).to_bytes().len();
                    buf.set_len(len);
                    buf.shrink_to_fit();
                    return Ok(PathBuf::from(OsString::from_vec(buf)));
                } else {
                    let error = io::Error::last_os_error();
                    if error.raw_os_error() != Some(libc::ERANGE) {
                        return Err(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);
            }
        }
    }

    pub fn chdir(p: &path::Path) -> io::Result<()> {
        let p: &OsStr = p.as_ref();
        let p = CString::new(p.as_bytes())?;
        unsafe {
            match libc::chdir(p.as_ptr()) == (0 as c_int) {
                true => Ok(()),
                false => Err(io::Error::last_os_error()),
            }
        }
    }

    pub struct SplitPaths<'a> {
        iter: iter::Map<slice::Split<'a, u8, fn(&u8) -> bool>,
                        fn(&'a [u8]) -> PathBuf>,
    }

    pub fn split_paths(unparsed: &OsStr) -> SplitPaths {
        fn bytes_to_path(b: &[u8]) -> PathBuf {
            PathBuf::from(<OsStr as OsStrExt>::from_bytes(b))
        }
        fn is_colon(b: &u8) -> bool { *b == b':' }
        let unparsed = unparsed.as_bytes();
        SplitPaths {
            iter: unparsed.split(is_colon as fn(&u8) -> bool)
                          .map(bytes_to_path as fn(&[u8]) -> PathBuf)
        }
    }

    impl<'a> Iterator for SplitPaths<'a> {
        type Item = PathBuf;
        fn next(&mut self) -> Option<PathBuf> { self.iter.next() }
        fn size_hint(&self) -> (usize, Option<usize>) { self.iter.size_hint() }
    }

    #[derive(Debug)]
    pub struct JoinPathsError;

    pub fn join_paths<I, T>(paths: I) -> Result<OsString, JoinPathsError>
        where I: Iterator<Item=T>, T: AsRef<OsStr>
    {
        let mut joined = Vec::new();
        let sep = b':';

        for (i, path) in paths.enumerate() {
            let path = path.as_ref().as_bytes();
            if i > 0 { joined.push(sep) }
            if path.contains(&sep) {
                return Err(JoinPathsError)
            }
            joined.extend_from_slice(path);
        }
        Ok(OsStringExt::from_vec(joined))
    }

    impl fmt::Display for JoinPathsError {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            "path segment contains separator `:`".fmt(f)
        }
    }

    impl StdError for JoinPathsError {
        fn description(&self) -> &str { "failed to join paths" }
    }

    #[cfg(any(target_os = "freebsd", target_os = "dragonfly"))]
    pub fn current_exe() -> io::Result<PathBuf> {
        unsafe {
            let mut mib = [libc::CTL_KERN as c_int,
                           libc::KERN_PROC as c_int,
                           libc::KERN_PROC_PATHNAME as c_int,
                           -1 as c_int];
            let mut sz = 0;
            cvt(libc::sysctl(mib.as_mut_ptr(), mib.len() as ::libc::c_uint,
                             ptr::null_mut(), &mut sz, ptr::null_mut(), 0))?;
            if sz == 0 {
                return Err(io::Error::last_os_error())
            }
            let mut v: Vec<u8> = Vec::with_capacity(sz);
            cvt(libc::sysctl(mib.as_mut_ptr(), mib.len() as ::libc::c_uint,
                             v.as_mut_ptr() as *mut libc::c_void, &mut sz,
                             ptr::null_mut(), 0))?;
            if sz == 0 {
                return Err(io::Error::last_os_error());
            }
            v.set_len(sz - 1); // chop off trailing NUL
            Ok(PathBuf::from(OsString::from_vec(v)))
        }
    }

    #[cfg(target_os = "netbsd")]
    pub fn current_exe() -> io::Result<PathBuf> {
        fn sysctl() -> io::Result<PathBuf> {
            unsafe {
                let mib = [libc::CTL_KERN, libc::KERN_PROC_ARGS, -1, libc::KERN_PROC_PATHNAME];
                let mut path_len: usize = 0;
                cvt(libc::sysctl(mib.as_ptr(), mib.len() as ::libc::c_uint,
                                 ptr::null_mut(), &mut path_len,
                                 ptr::null(), 0))?;
                if path_len <= 1 {
                    return Err(io::Error::new(io::ErrorKind::Other,
                               "KERN_PROC_PATHNAME sysctl returned zero-length string"))
                }
                let mut path: Vec<u8> = Vec::with_capacity(path_len);
                cvt(libc::sysctl(mib.as_ptr(), mib.len() as ::libc::c_uint,
                                 path.as_ptr() as *mut libc::c_void, &mut path_len,
                                 ptr::null(), 0))?;
                path.set_len(path_len - 1); // chop off NUL
                Ok(PathBuf::from(OsString::from_vec(path)))
            }
        }
        fn procfs() -> io::Result<PathBuf> {
            let curproc_exe = path::Path::new("/proc/curproc/exe");
            if curproc_exe.is_file() {
                return ::fs::read_link(curproc_exe);
            }
            Err(io::Error::new(io::ErrorKind::Other,
                               "/proc/curproc/exe doesn't point to regular file."))
        }
        sysctl().or_else(|_| procfs())
    }

    #[cfg(any(target_os = "bitrig", target_os = "openbsd"))]
    pub fn current_exe() -> io::Result<PathBuf> {
        unsafe {
            let mut mib = [libc::CTL_KERN,
                           libc::KERN_PROC_ARGS,
                           libc::getpid(),
                           libc::KERN_PROC_ARGV];
            let mib = mib.as_mut_ptr();
            let mut argv_len = 0;
            cvt(libc::sysctl(mib, 4, ptr::null_mut(), &mut argv_len,
                             ptr::null_mut(), 0))?;
            let mut argv = Vec::<*const libc::c_char>::with_capacity(argv_len as usize);
            cvt(libc::sysctl(mib, 4, argv.as_mut_ptr() as *mut _,
                             &mut argv_len, ptr::null_mut(), 0))?;
            argv.set_len(argv_len as usize);
            if argv[0].is_null() {
                return Err(io::Error::new(io::ErrorKind::Other,
                                          "no current exe available"))
            }
            let argv0 = CStr::from_ptr(argv[0]).to_bytes();
            if argv0[0] == b'.' || argv0.iter().any(|b| *b == b'/') {
                ::fs::canonicalize(OsStr::from_bytes(argv0))
            } else {
                Ok(PathBuf::from(OsStr::from_bytes(argv0)))
            }
        }
    }

    #[cfg(any(target_os = "linux", target_os = "android", target_os = "emscripten"))]
    pub fn current_exe() -> io::Result<PathBuf> {
        match ::fs::read_link("/proc/self/exe") {
            Err(ref e) if e.kind() == io::ErrorKind::NotFound => {
                Err(io::Error::new(
                    io::ErrorKind::Other,
                    "no /proc/self/exe available. Is /proc mounted?"
                ))
            },
            other => other,
        }
    }

    #[cfg(any(target_os = "macos", target_os = "ios"))]
    pub fn current_exe() -> io::Result<PathBuf> {
        extern {
            fn _NSGetExecutablePath(buf: *mut libc::c_char,
                                    bufsize: *mut u32) -> libc::c_int;
        }
        unsafe {
            let mut sz: u32 = 0;
            _NSGetExecutablePath(ptr::null_mut(), &mut sz);
            if sz == 0 { return Err(io::Error::last_os_error()); }
            let mut v: Vec<u8> = Vec::with_capacity(sz as usize);
            let err = _NSGetExecutablePath(v.as_mut_ptr() as *mut i8, &mut sz);
            if err != 0 { return Err(io::Error::last_os_error()); }
            v.set_len(sz as usize - 1); // chop off trailing NUL
            Ok(PathBuf::from(OsString::from_vec(v)))
        }
    }

    #[cfg(any(target_os = "solaris"))]
    pub fn current_exe() -> io::Result<PathBuf> {
        extern {
            fn getexecname() -> *const c_char;
        }
        unsafe {
            let path = getexecname();
            if path.is_null() {
                Err(io::Error::last_os_error())
            } else {
                let filename = CStr::from_ptr(path).to_bytes();
                let path = PathBuf::from(<OsStr as OsStrExt>::from_bytes(filename));

                // Prepend a current working directory to the path if
                // it doesn't contain an absolute pathname.
                if filename[0] == b'/' {
                    Ok(path)
                } else {
                    getcwd().map(|cwd| cwd.join(path))
                }
            }
        }
    }

    #[cfg(target_os = "haiku")]
    pub fn current_exe() -> io::Result<PathBuf> {
        // Use Haiku's image info functions
        #[repr(C)]
        struct image_info {
            id: i32,
            type_: i32,
            sequence: i32,
            init_order: i32,
            init_routine: *mut libc::c_void,    // function pointer
            term_routine: *mut libc::c_void,    // function pointer
            device: libc::dev_t,
            node: libc::ino_t,
            name: [libc::c_char; 1024],         // MAXPATHLEN
            text: *mut libc::c_void,
            data: *mut libc::c_void,
            text_size: i32,
            data_size: i32,
            api_version: i32,
            abi: i32,
        }

        unsafe {
            extern {
                fn _get_next_image_info(team_id: i32, cookie: *mut i32,
                    info: *mut image_info, size: i32) -> i32;
            }

            let mut info: image_info = mem::zeroed();
            let mut cookie: i32 = 0;
            // the executable can be found at team id 0
            let result = _get_next_image_info(0, &mut cookie, &mut info,
                mem::size_of::<image_info>() as i32);
            if result != 0 {
                use io::ErrorKind;
                Err(io::Error::new(ErrorKind::Other, "Error getting executable path"))
            } else {
                let name = CStr::from_ptr(info.name.as_ptr()).to_bytes();
                Ok(PathBuf::from(OsStr::from_bytes(name)))
            }
        }
    }

    #[cfg(any(target_os = "fuchsia", target_os = "l4re", target_os = "hermit"))]
    pub fn current_exe() -> io::Result<PathBuf> {
        use io::ErrorKind;
        Err(io::Error::new(ErrorKind::Other, "Not yet implemented!"))
    }

    pub struct Env {
        iter: vec::IntoIter<(OsString, OsString)>,
        _dont_send_or_sync_me: PhantomData<*mut ()>,
    }

    impl Iterator for Env {
        type Item = (OsString, OsString);
        fn next(&mut self) -> Option<(OsString, OsString)> { self.iter.next() }
        fn size_hint(&self) -> (usize, Option<usize>) { self.iter.size_hint() }
    }

    #[cfg(target_os = "macos")]
    pub unsafe fn environ() -> *mut *const *const c_char {
        extern { fn _NSGetEnviron() -> *mut *const *const c_char; }
        _NSGetEnviron()
    }

    #[cfg(not(target_os = "macos"))]
    pub unsafe fn environ() -> *mut *const *const c_char {
        extern { static mut environ: *const *const c_char; }
        &mut environ
    }

    pub unsafe fn env_lock() -> MutexGuard<'static> {
        // We never call `ENV_LOCK.init()`, so it is UB to attempt to
        // acquire this mutex reentrantly!
        static ENV_LOCK: Mutex = Mutex::new();
        ENV_LOCK.lock()
    }

    /// Returns a vector of (variable, value) byte-vector pairs for all the
    /// environment variables of the current process.
    pub fn env() -> Env {
        unsafe {
            let _guard = env_lock();
            let mut environ = *environ();
            let mut result = Vec::new();
            while environ != ptr::null() && *environ != ptr::null() {
                if let Some(key_value) = parse(CStr::from_ptr(*environ).to_bytes()) {
                    result.push(key_value);
                }
                environ = environ.offset(1);
            }
            return Env {
                iter: result.into_iter(),
                _dont_send_or_sync_me: PhantomData,
            }
        }

        fn parse(input: &[u8]) -> Option<(OsString, OsString)> {
            // Strategy (copied from glibc): Variable name and value are separated
            // by an ASCII equals sign '='. Since a variable name must not be
            // empty, allow variable names starting with an equals sign. Skip all
            // malformed lines.
            if input.is_empty() {
                return None;
            }
            let pos = memchr::memchr(b'=', &input[1..]).map(|p| p + 1);
            pos.map(|p| (
                OsStringExt::from_vec(input[..p].to_vec()),
                OsStringExt::from_vec(input[p+1..].to_vec()),
            ))
        }
    }

    pub fn getenv(k: &OsStr) -> io::Result<Option<OsString>> {
        // environment variables with a nul byte can't be set, so their value is
        // always None as well
        let k = CString::new(k.as_bytes())?;
        unsafe {
            let _guard = env_lock();
            let s = libc::getenv(k.as_ptr()) as *const libc::c_char;
            let ret = if s.is_null() {
                None
            } else {
                Some(OsStringExt::from_vec(CStr::from_ptr(s).to_bytes().to_vec()))
            };
            Ok(ret)
        }
    }

    pub fn setenv(k: &OsStr, v: &OsStr) -> io::Result<()> {
        let k = CString::new(k.as_bytes())?;
        let v = CString::new(v.as_bytes())?;

        unsafe {
            let _guard = env_lock();
            cvt(libc::setenv(k.as_ptr(), v.as_ptr(), 1)).map(|_| ())
        }
    }

    pub fn unsetenv(n: &OsStr) -> io::Result<()> {
        let nbuf = CString::new(n.as_bytes())?;

        unsafe {
            let _guard = env_lock();
            cvt(libc::unsetenv(nbuf.as_ptr())).map(|_| ())
        }
    }

    pub fn page_size() -> usize {
        unsafe {
            libc::sysconf(libc::_SC_PAGESIZE) as usize
        }
    }

    pub fn temp_dir() -> PathBuf {
        ::env::var_os("TMPDIR").map(PathBuf::from).unwrap_or_else(|| {
            if cfg!(target_os = "android") {
                PathBuf::from("/data/local/tmp")
            } else {
                PathBuf::from("/tmp")
            }
        })
    }

    pub fn home_dir() -> Option<PathBuf> {
        return ::env::var_os("HOME").or_else(|| unsafe {
            fallback()
        }).map(PathBuf::from);

        #[cfg(any(target_os = "android",
                  target_os = "ios",
                  target_os = "emscripten"))]
        unsafe fn fallback() -> Option<OsString> { None }
        #[cfg(not(any(target_os = "android",
                      target_os = "ios",
                      target_os = "emscripten")))]
        unsafe fn fallback() -> Option<OsString> {
            let amt = match libc::sysconf(libc::_SC_GETPW_R_SIZE_MAX) {
                n if n < 0 => 512 as usize,
                n => n as usize,
            };
            let mut buf = Vec::with_capacity(amt);
            let mut passwd: libc::passwd = mem::zeroed();
            let mut result = ptr::null_mut();
            match libc::getpwuid_r(libc::getuid(), &mut passwd, buf.as_mut_ptr(),
                                   buf.capacity(), &mut result) {
                0 if !result.is_null() => {
                    let ptr = passwd.pw_dir as *const _;
                    let bytes = CStr::from_ptr(ptr).to_bytes().to_vec();
                    Some(OsStringExt::from_vec(bytes))
                },
                _ => None,
            }
        }
    }

    pub fn exit(code: i32) -> ! {
        unsafe { libc::exit(code as c_int) }
    }

    pub fn getpid() -> u32 {
        unsafe { libc::getpid() as u32 }
    }

    pub fn getppid() -> u32 {
        unsafe { libc::getppid() as u32 }
    }

    #[cfg(target_env = "gnu")]
    pub fn glibc_version() -> Option<(usize, usize)> {
        if let Some(Ok(version_str)) = glibc_version_cstr().map(CStr::to_str) {
            parse_glibc_version(version_str)
        } else {
            None
        }
    }

    #[cfg(target_env = "gnu")]
    fn glibc_version_cstr() -> Option<&'static CStr> {
        weak! {
            fn gnu_get_libc_version() -> *const libc::c_char
        }
        if let Some(f) = gnu_get_libc_version.get() {
            unsafe { Some(CStr::from_ptr(f())) }
        } else {
            None
        }
    }

    // Returns Some((major, minor)) if the string is a valid "x.y" version,
    // ignoring any extra dot-separated parts. Otherwise return None.
    #[cfg(target_env = "gnu")]
    fn parse_glibc_version(version: &str) -> Option<(usize, usize)> {
        let mut parsed_ints = version.split('.').map(str::parse::<usize>).fuse();
        match (parsed_ints.next(), parsed_ints.next()) {
            (Some(Ok(major)), Some(Ok(minor))) => Some((major, minor)),
            _ => None
        }
    }

    #[cfg(all(test, target_env = "gnu"))]
    mod test {
        use super::*;

        #[test]
        fn test_glibc_version() {
            // This mostly just tests that the weak linkage doesn't panic wildly...
            glibc_version();
        }

        #[test]
        fn test_parse_glibc_version() {
            let cases = [
                ("0.0", Some((0, 0))),
                ("01.+2", Some((1, 2))),
                ("3.4.5.six", Some((3, 4))),
                ("1", None),
                ("1.-2", None),
                ("1.foo", None),
                ("foo.1", None),
            ];
            for &(version_str, parsed) in cases.iter() {
                assert_eq!(parsed, parse_glibc_version(version_str));
            }
        }
    }

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