std: begin moving platform support modules into `pal`

1 files changed, 783 insertions, 0 deletions

diff --git a/library/std/src/sys/pal/unix/os.rs b/library/std/src/sys/pal/unix/os.rs
new file mode 100644
index 00000000000..881b3a25c51
--- /dev/null
+++ b/library/std/src/sys/pal/unix/os.rs
@@ -0,0 +1,783 @@
+//! Implementation of `std::os` functionality for unix systems
+
+#![allow(unused_imports)] // lots of cfg code here
+
+#[cfg(test)]
+mod tests;
+
+use crate::os::unix::prelude::*;
+
+use crate::error::Error as StdError;
+use crate::ffi::{CStr, CString, OsStr, OsString};
+use crate::fmt;
+use crate::io;
+use crate::iter;
+use crate::mem;
+use crate::path::{self, PathBuf};
+use crate::ptr;
+use crate::slice;
+use crate::str;
+use crate::sync::{PoisonError, RwLock};
+use crate::sys::common::small_c_string::{run_path_with_cstr, run_with_cstr};
+use crate::sys::cvt;
+use crate::sys::fd;
+use crate::sys::memchr;
+use crate::vec;
+
+#[cfg(all(target_env = "gnu", not(target_os = "vxworks")))]
+use crate::sys::weak::weak;
+
+use libc::{c_char, c_int, c_void};
+
+const TMPBUF_SZ: usize = 128;
+
+cfg_if::cfg_if! {
+    if #[cfg(target_os = "redox")] {
+        const PATH_SEPARATOR: u8 = b';';
+    } else {
+        const PATH_SEPARATOR: u8 = b':';
+    }
+}
+
+extern "C" {
+    #[cfg(not(any(target_os = "dragonfly", target_os = "vxworks")))]
+    #[cfg_attr(
+        any(
+            target_os = "linux",
+            target_os = "emscripten",
+            target_os = "fuchsia",
+            target_os = "l4re",
+            target_os = "hurd",
+        ),
+        link_name = "__errno_location"
+    )]
+    #[cfg_attr(
+        any(
+            target_os = "netbsd",
+            target_os = "openbsd",
+            target_os = "android",
+            target_os = "redox",
+            target_env = "newlib"
+        ),
+        link_name = "__errno"
+    )]
+    #[cfg_attr(any(target_os = "solaris", target_os = "illumos"), link_name = "___errno")]
+    #[cfg_attr(target_os = "nto", link_name = "__get_errno_ptr")]
+    #[cfg_attr(
+        any(
+            target_os = "macos",
+            target_os = "ios",
+            target_os = "tvos",
+            target_os = "freebsd",
+            target_os = "watchos"
+        ),
+        link_name = "__error"
+    )]
+    #[cfg_attr(target_os = "haiku", link_name = "_errnop")]
+    #[cfg_attr(target_os = "aix", link_name = "_Errno")]
+    fn errno_location() -> *mut c_int;
+}
+
+/// Returns the platform-specific value of errno
+#[cfg(not(any(target_os = "dragonfly", target_os = "vxworks")))]
+pub fn errno() -> i32 {
+    unsafe { (*errno_location()) as i32 }
+}
+
+/// Sets the platform-specific value of errno
+#[cfg(all(not(target_os = "dragonfly"), not(target_os = "vxworks")))] // needed for readdir and syscall!
+#[allow(dead_code)] // but not all target cfgs actually end up using it
+pub fn set_errno(e: i32) {
+    unsafe { *errno_location() = e as c_int }
+}
+
+#[cfg(target_os = "vxworks")]
+pub fn errno() -> i32 {
+    unsafe { libc::errnoGet() }
+}
+
+#[cfg(target_os = "dragonfly")]
+pub fn errno() -> i32 {
+    extern "C" {
+        #[thread_local]
+        static errno: c_int;
+    }
+
+    unsafe { errno as i32 }
+}
+
+#[cfg(target_os = "dragonfly")]
+#[allow(dead_code)]
+pub fn set_errno(e: i32) {
+    extern "C" {
+        #[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 "C" {
+        #[cfg_attr(
+            all(
+                any(target_os = "linux", target_os = "hurd", target_env = "newlib"),
+                not(target_env = "ohos")
+            ),
+            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 _;
+        // We can't always expect a UTF-8 environment. When we don't get that luxury,
+        // it's better to give a low-quality error message than none at all.
+        String::from_utf8_lossy(CStr::from_ptr(p).to_bytes()).into()
+    }
+}
+
+#[cfg(target_os = "espidf")]
+pub fn getcwd() -> io::Result<PathBuf> {
+    Ok(PathBuf::from("/"))
+}
+
+#[cfg(not(target_os = "espidf"))]
+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);
+        }
+    }
+}
+
+#[cfg(target_os = "espidf")]
+pub fn chdir(_p: &path::Path) -> io::Result<()> {
+    super::unsupported::unsupported()
+}
+
+#[cfg(not(target_os = "espidf"))]
+pub fn chdir(p: &path::Path) -> io::Result<()> {
+    let result = run_path_with_cstr(p, |p| unsafe { Ok(libc::chdir(p.as_ptr())) })?;
+    if result == 0 { Ok(()) } else { 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_separator(b: &u8) -> bool {
+        *b == PATH_SEPARATOR
+    }
+    let unparsed = unparsed.as_bytes();
+    SplitPaths {
+        iter: unparsed
+            .split(is_separator 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();
+
+    for (i, path) in paths.enumerate() {
+        let path = path.as_ref().as_bytes();
+        if i > 0 {
+            joined.push(PATH_SEPARATOR)
+        }
+        if path.contains(&PATH_SEPARATOR) {
+            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 {
+        write!(f, "path segment contains separator `{}`", char::from(PATH_SEPARATOR))
+    }
+}
+
+impl StdError for JoinPathsError {
+    #[allow(deprecated)]
+    fn description(&self) -> &str {
+        "failed to join paths"
+    }
+}
+
+#[cfg(target_os = "aix")]
+pub fn current_exe() -> io::Result<PathBuf> {
+    use crate::io::ErrorKind;
+
+    #[cfg(test)]
+    use realstd::env;
+
+    #[cfg(not(test))]
+    use crate::env;
+
+    let exe_path = env::args().next().ok_or(io::const_io_error!(
+        ErrorKind::NotFound,
+        "an executable path was not found because no arguments were provided through argv"
+    ))?;
+    let path = PathBuf::from(exe_path);
+    if path.is_absolute() {
+        return path.canonicalize();
+    }
+    // Search PWD to infer current_exe.
+    if let Some(pstr) = path.to_str()
+        && pstr.contains("/")
+    {
+        return getcwd().map(|cwd| cwd.join(path))?.canonicalize();
+    }
+    // Search PATH to infer current_exe.
+    if let Some(p) = getenv(OsStr::from_bytes("PATH".as_bytes())) {
+        for search_path in split_paths(&p) {
+            let pb = search_path.join(&path);
+            if pb.is_file()
+                && let Ok(metadata) = crate::fs::metadata(&pb)
+                && metadata.permissions().mode() & 0o111 != 0
+            {
+                return pb.canonicalize();
+            }
+        }
+    }
+    Err(io::const_io_error!(ErrorKind::NotFound, "an executable path was not found"))
+}
+
+#[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::const_io_error!(
+                    io::ErrorKind::Uncategorized,
+                    "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 crate::fs::read_link(curproc_exe);
+        }
+        Err(io::const_io_error!(
+            io::ErrorKind::Uncategorized,
+            "/proc/curproc/exe doesn't point to regular file.",
+        ))
+    }
+    sysctl().or_else(|_| procfs())
+}
+
+#[cfg(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::const_io_error!(
+                io::ErrorKind::Uncategorized,
+                "no current exe available",
+            ));
+        }
+        let argv0 = CStr::from_ptr(argv[0]).to_bytes();
+        if argv0[0] == b'.' || argv0.iter().any(|b| *b == b'/') {
+            crate::fs::canonicalize(OsStr::from_bytes(argv0))
+        } else {
+            Ok(PathBuf::from(OsStr::from_bytes(argv0)))
+        }
+    }
+}
+
+#[cfg(any(
+    target_os = "linux",
+    target_os = "hurd",
+    target_os = "android",
+    target_os = "emscripten"
+))]
+pub fn current_exe() -> io::Result<PathBuf> {
+    match crate::fs::read_link("/proc/self/exe") {
+        Err(ref e) if e.kind() == io::ErrorKind::NotFound => Err(io::const_io_error!(
+            io::ErrorKind::Uncategorized,
+            "no /proc/self/exe available. Is /proc mounted?",
+        )),
+        other => other,
+    }
+}
+
+#[cfg(target_os = "nto")]
+pub fn current_exe() -> io::Result<PathBuf> {
+    let mut e = crate::fs::read("/proc/self/exefile")?;
+    // Current versions of QNX Neutrino provide a null-terminated path.
+    // Ensure the trailing null byte is not returned here.
+    if let Some(0) = e.last() {
+        e.pop();
+    }
+    Ok(PathBuf::from(OsString::from_vec(e)))
+}
+
+#[cfg(any(target_os = "macos", target_os = "ios", target_os = "tvos", target_os = "watchos"))]
+pub fn current_exe() -> io::Result<PathBuf> {
+    unsafe {
+        let mut sz: u32 = 0;
+        libc::_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 = libc::_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", target_os = "illumos"))]
+pub fn current_exe() -> io::Result<PathBuf> {
+    if let Ok(path) = crate::fs::read_link("/proc/self/path/a.out") {
+        Ok(path)
+    } else {
+        unsafe {
+            let path = libc::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> {
+    unsafe {
+        let mut info: mem::MaybeUninit<libc::image_info> = mem::MaybeUninit::uninit();
+        let mut cookie: i32 = 0;
+        // the executable can be found at team id 0
+        let result = libc::_get_next_image_info(
+            0,
+            &mut cookie,
+            info.as_mut_ptr(),
+            mem::size_of::<libc::image_info>(),
+        );
+        if result != 0 {
+            use crate::io::ErrorKind;
+            Err(io::const_io_error!(ErrorKind::Uncategorized, "Error getting executable path"))
+        } else {
+            let name = CStr::from_ptr((*info.as_ptr()).name.as_ptr()).to_bytes();
+            Ok(PathBuf::from(OsStr::from_bytes(name)))
+        }
+    }
+}
+
+#[cfg(target_os = "redox")]
+pub fn current_exe() -> io::Result<PathBuf> {
+    crate::fs::read_to_string("sys:exe").map(PathBuf::from)
+}
+
+#[cfg(target_os = "l4re")]
+pub fn current_exe() -> io::Result<PathBuf> {
+    use crate::io::ErrorKind;
+    Err(io::const_io_error!(ErrorKind::Unsupported, "Not yet implemented!"))
+}
+
+#[cfg(target_os = "vxworks")]
+pub fn current_exe() -> io::Result<PathBuf> {
+    #[cfg(test)]
+    use realstd::env;
+
+    #[cfg(not(test))]
+    use crate::env;
+
+    let exe_path = env::args().next().unwrap();
+    let path = path::Path::new(&exe_path);
+    path.canonicalize()
+}
+
+#[cfg(any(target_os = "espidf", target_os = "horizon", target_os = "vita"))]
+pub fn current_exe() -> io::Result<PathBuf> {
+    super::unsupported::unsupported()
+}
+
+#[cfg(target_os = "fuchsia")]
+pub fn current_exe() -> io::Result<PathBuf> {
+    use crate::io::ErrorKind;
+
+    #[cfg(test)]
+    use realstd::env;
+
+    #[cfg(not(test))]
+    use crate::env;
+
+    let exe_path = env::args().next().ok_or(io::const_io_error!(
+        ErrorKind::Uncategorized,
+        "an executable path was not found because no arguments were provided through argv"
+    ))?;
+    let path = PathBuf::from(exe_path);
+
+    // Prepend the current working directory to the path if it's not absolute.
+    if !path.is_absolute() { getcwd().map(|cwd| cwd.join(path)) } else { Ok(path) }
+}
+
+pub struct Env {
+    iter: vec::IntoIter<(OsString, OsString)>,
+}
+
+// FIXME(https://github.com/rust-lang/rust/issues/114583): Remove this when <OsStr as Debug>::fmt matches <str as Debug>::fmt.
+pub struct EnvStrDebug<'a> {
+    slice: &'a [(OsString, OsString)],
+}
+
+impl fmt::Debug for EnvStrDebug<'_> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        let Self { slice } = self;
+        f.debug_list()
+            .entries(slice.iter().map(|(a, b)| (a.to_str().unwrap(), b.to_str().unwrap())))
+            .finish()
+    }
+}
+
+impl Env {
+    pub fn str_debug(&self) -> impl fmt::Debug + '_ {
+        let Self { iter } = self;
+        EnvStrDebug { slice: iter.as_slice() }
+    }
+}
+
+impl fmt::Debug for Env {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        let Self { iter } = self;
+        f.debug_list().entries(iter.as_slice()).finish()
+    }
+}
+
+impl !Send for Env {}
+impl !Sync for Env {}
+
+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 {
+    libc::_NSGetEnviron() as *mut *const *const c_char
+}
+
+#[cfg(not(target_os = "macos"))]
+pub unsafe fn environ() -> *mut *const *const c_char {
+    extern "C" {
+        static mut environ: *const *const c_char;
+    }
+    ptr::addr_of_mut!(environ)
+}
+
+static ENV_LOCK: RwLock<()> = RwLock::new(());
+
+pub fn env_read_lock() -> impl Drop {
+    ENV_LOCK.read().unwrap_or_else(PoisonError::into_inner)
+}
+
+/// 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_read_lock();
+        let mut environ = *environ();
+        let mut result = Vec::new();
+        if !environ.is_null() {
+            while !(*environ).is_null() {
+                if let Some(key_value) = parse(CStr::from_ptr(*environ).to_bytes()) {
+                    result.push(key_value);
+                }
+                environ = environ.add(1);
+            }
+        }
+        return Env { iter: result.into_iter() };
+    }
+
+    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) -> Option<OsString> {
+    // environment variables with a nul byte can't be set, so their value is
+    // always None as well
+    run_with_cstr(k.as_bytes(), |k| {
+        let _guard = env_read_lock();
+        let v = unsafe { libc::getenv(k.as_ptr()) } as *const libc::c_char;
+
+        if v.is_null() {
+            Ok(None)
+        } else {
+            // SAFETY: `v` cannot be mutated while executing this line since we've a read lock
+            let bytes = unsafe { CStr::from_ptr(v) }.to_bytes().to_vec();
+
+            Ok(Some(OsStringExt::from_vec(bytes)))
+        }
+    })
+    .ok()
+    .flatten()
+}
+
+pub fn setenv(k: &OsStr, v: &OsStr) -> io::Result<()> {
+    run_with_cstr(k.as_bytes(), |k| {
+        run_with_cstr(v.as_bytes(), |v| {
+            let _guard = ENV_LOCK.write();
+            cvt(unsafe { libc::setenv(k.as_ptr(), v.as_ptr(), 1) }).map(drop)
+        })
+    })
+}
+
+pub fn unsetenv(n: &OsStr) -> io::Result<()> {
+    run_with_cstr(n.as_bytes(), |nbuf| {
+        let _guard = ENV_LOCK.write();
+        cvt(unsafe { libc::unsetenv(nbuf.as_ptr()) }).map(drop)
+    })
+}
+
+#[cfg(not(target_os = "espidf"))]
+pub fn page_size() -> usize {
+    unsafe { libc::sysconf(libc::_SC_PAGESIZE) as usize }
+}
+
+pub fn temp_dir() -> PathBuf {
+    crate::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 crate::env::var_os("HOME").or_else(|| unsafe { fallback() }).map(PathBuf::from);
+
+    #[cfg(any(
+        target_os = "android",
+        target_os = "ios",
+        target_os = "tvos",
+        target_os = "watchos",
+        target_os = "emscripten",
+        target_os = "redox",
+        target_os = "vxworks",
+        target_os = "espidf",
+        target_os = "horizon",
+        target_os = "vita",
+    ))]
+    unsafe fn fallback() -> Option<OsString> {
+        None
+    }
+    #[cfg(not(any(
+        target_os = "android",
+        target_os = "ios",
+        target_os = "tvos",
+        target_os = "watchos",
+        target_os = "emscripten",
+        target_os = "redox",
+        target_os = "vxworks",
+        target_os = "espidf",
+        target_os = "horizon",
+        target_os = "vita",
+    )))]
+    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(all(target_os = "linux", target_env = "gnu"))]
+pub fn glibc_version() -> Option<(usize, usize)> {
+    extern "C" {
+        fn gnu_get_libc_version() -> *const libc::c_char;
+    }
+    let version_cstr = unsafe { CStr::from_ptr(gnu_get_libc_version()) };
+    if let Ok(version_str) = version_cstr.to_str() {
+        parse_glibc_version(version_str)
+    } 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(all(target_os = "linux", 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,
+    }
+}