diff --git a/src/bsd_arandom.rs b/src/bsd_arandom.rs
new file mode 100644
index 0000000..eb564ff
--- /dev/null
+++ b/src/bsd_arandom.rs
@@ -0,0 +1,49 @@
+// Copyright 2018 Developers of the Rand project.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// https://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or https://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+//! Implementation for FreeBSD and NetBSD
+use crate::util_libc::sys_fill_exact;
+use crate::Error;
+use core::ptr;
+
+fn kern_arnd(buf: &mut [u8]) -> libc::ssize_t {
+    static MIB: [libc::c_int; 2] = [libc::CTL_KERN, libc::KERN_ARND];
+    let mut len = buf.len();
+    let ret = unsafe {
+        libc::sysctl(
+            MIB.as_ptr(),
+            MIB.len() as libc::c_uint,
+            buf.as_mut_ptr() as *mut _,
+            &mut len,
+            ptr::null(),
+            0,
+        )
+    };
+    if ret == -1 {
+        error!("sysctl kern.arandom: syscall failed");
+        -1
+    } else {
+        len as libc::ssize_t
+    }
+}
+
+pub fn getrandom_inner(dest: &mut [u8]) -> Result<(), Error> {
+    #[cfg(target_os = "freebsd")]
+    {
+        use crate::util_libc::Weak;
+        static GETRANDOM: Weak = unsafe { Weak::new("getrandom\0") };
+        type GetRandomFn =
+            unsafe extern "C" fn(*mut u8, libc::size_t, libc::c_uint) -> libc::ssize_t;
+
+        if let Some(fptr) = GETRANDOM.ptr() {
+            let func: GetRandomFn = unsafe { core::mem::transmute(fptr) };
+            return sys_fill_exact(dest, |buf| unsafe { func(buf.as_mut_ptr(), buf.len(), 0) });
+        }
+    }
+    sys_fill_exact(dest, kern_arnd)
+}
diff --git a/src/cloudabi.rs b/src/cloudabi.rs
new file mode 100644
index 0000000..d3d0928
--- /dev/null
+++ b/src/cloudabi.rs
@@ -0,0 +1,25 @@
+// Copyright 2018 Developers of the Rand project.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// https://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or https://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+//! Implementation for CloudABI
+use crate::Error;
+use core::num::NonZeroU32;
+
+extern "C" {
+    fn cloudabi_sys_random_get(buf: *mut u8, buf_len: usize) -> u16;
+}
+
+pub fn getrandom_inner(dest: &mut [u8]) -> Result<(), Error> {
+    let errno = unsafe { cloudabi_sys_random_get(dest.as_mut_ptr(), dest.len()) };
+    if let Some(code) = NonZeroU32::new(errno as u32) {
+        error!("cloudabi_sys_random_get: failed with {}", errno);
+        Err(Error::from(code))
+    } else {
+        Ok(()) // Zero means success for CloudABI
+    }
+}
diff --git a/src/dummy.rs b/src/dummy.rs
new file mode 100644
index 0000000..0c24ba0
--- /dev/null
+++ b/src/dummy.rs
@@ -0,0 +1,14 @@
+// Copyright 2018 Developers of the Rand project.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// https://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or https://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+//! A dummy implementation for unsupported targets which always fails
+use crate::{error::UNSUPPORTED, Error};
+
+pub fn getrandom_inner(_: &mut [u8]) -> Result<(), Error> {
+    Err(UNSUPPORTED)
+}
diff --git a/src/error.rs b/src/error.rs
new file mode 100644
index 0000000..31ae24d
--- /dev/null
+++ b/src/error.rs
@@ -0,0 +1,177 @@
+// Copyright 2018 Developers of the Rand project.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// https://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or https://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+use core::fmt;
+use core::num::NonZeroU32;
+
+/// A small and `no_std` compatible error type.
+///
+/// The [`Error::raw_os_error()`] will indicate if the error is from the OS, and
+/// if so, which error code the OS gave the application. If such an error is
+/// encountered, please consult with your system documentation.
+///
+/// Internally this type is a NonZeroU32, with certain values reserved for
+/// certain purposes, see [`Error::INTERNAL_START`] and [`Error::CUSTOM_START`].
+#[derive(Copy, Clone, Eq, PartialEq)]
+pub struct Error(NonZeroU32);
+
+impl Error {
+    #[deprecated(since = "0.1.7")]
+    /// Unknown error.
+    pub const UNKNOWN: Error = UNSUPPORTED;
+    #[deprecated(since = "0.1.7")]
+    /// System entropy source is unavailable.
+    pub const UNAVAILABLE: Error = UNSUPPORTED;
+
+    /// Codes below this point represent OS Errors (i.e. positive i32 values).
+    /// Codes at or above this point, but below [`Error::CUSTOM_START`] are
+    /// reserved for use by the `rand` and `getrandom` crates.
+    pub const INTERNAL_START: u32 = 1 << 31;
+
+    /// Codes at or above this point can be used by users to define their own
+    /// custom errors.
+    pub const CUSTOM_START: u32 = (1 << 31) + (1 << 30);
+
+    /// Extract the raw OS error code (if this error came from the OS)
+    ///
+    /// This method is identical to `std::io::Error::raw_os_error()`, except
+    /// that it works in `no_std` contexts. If this method returns `None`, the
+    /// error value can still be formatted via the `Display` implementation.
+    #[inline]
+    pub fn raw_os_error(self) -> Option<i32> {
+        if self.0.get() < Self::INTERNAL_START {
+            Some(self.0.get() as i32)
+        } else {
+            None
+        }
+    }
+
+    /// Extract the bare error code.
+    ///
+    /// This code can either come from the underlying OS, or be a custom error.
+    /// Use [`Error::raw_os_error()`] to disambiguate.
+    #[inline]
+    pub fn code(self) -> NonZeroU32 {
+        self.0
+    }
+}
+
+cfg_if! {
+    if #[cfg(unix)] {
+        fn os_err(errno: i32, buf: &mut [u8]) -> Option<&str> {
+            let buf_ptr = buf.as_mut_ptr() as *mut libc::c_char;
+            if unsafe { libc::strerror_r(errno, buf_ptr, buf.len()) } != 0 {
+                return None;
+            }
+
+            // Take up to trailing null byte
+            let n = buf.len();
+            let idx = buf.iter().position(|&b| b == 0).unwrap_or(n);
+            core::str::from_utf8(&buf[..idx]).ok()
+        }
+    } else if #[cfg(target_os = "wasi")] {
+        fn os_err(errno: i32, _buf: &mut [u8]) -> Option<wasi::Error> {
+            wasi::Error::from_raw_error(errno as _)
+        }
+    } else {
+        fn os_err(_errno: i32, _buf: &mut [u8]) -> Option<&str> {
+            None
+        }
+    }
+}
+
+impl fmt::Debug for Error {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        let mut dbg = f.debug_struct("Error");
+        if let Some(errno) = self.raw_os_error() {
+            dbg.field("os_error", &errno);
+            let mut buf = [0u8; 128];
+            if let Some(err) = os_err(errno, &mut buf) {
+                dbg.field("description", &err);
+            }
+        } else if let Some(desc) = internal_desc(*self) {
+            dbg.field("internal_code", &self.0.get());
+            dbg.field("description", &desc);
+        } else {
+            dbg.field("unknown_code", &self.0.get());
+        }
+        dbg.finish()
+    }
+}
+
+impl fmt::Display for Error {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        if let Some(errno) = self.raw_os_error() {
+            let mut buf = [0u8; 128];
+            match os_err(errno, &mut buf) {
+                Some(err) => err.fmt(f),
+                None => write!(f, "OS Error: {}", errno),
+            }
+        } else if let Some(desc) = internal_desc(*self) {
+            f.write_str(desc)
+        } else {
+            write!(f, "Unknown Error: {}", self.0.get())
+        }
+    }
+}
+
+impl From<NonZeroU32> for Error {
+    fn from(code: NonZeroU32) -> Self {
+        Self(code)
+    }
+}
+
+// TODO: Convert to a function when min_version >= 1.33
+macro_rules! internal_error {
+    ($n:expr) => {
+        Error(unsafe { NonZeroU32::new_unchecked(Error::INTERNAL_START + $n as u16 as u32) })
+    };
+}
+
+/// Internal Error constants
+pub(crate) const UNSUPPORTED: Error = internal_error!(0);
+pub(crate) const ERRNO_NOT_POSITIVE: Error = internal_error!(1);
+pub(crate) const UNKNOWN_IO_ERROR: Error = internal_error!(2);
+pub(crate) const SEC_RANDOM_FAILED: Error = internal_error!(3);
+pub(crate) const RTL_GEN_RANDOM_FAILED: Error = internal_error!(4);
+pub(crate) const FAILED_RDRAND: Error = internal_error!(5);
+pub(crate) const NO_RDRAND: Error = internal_error!(6);
+pub(crate) const BINDGEN_CRYPTO_UNDEF: Error = internal_error!(7);
+pub(crate) const BINDGEN_GRV_UNDEF: Error = internal_error!(8);
+pub(crate) const STDWEB_NO_RNG: Error = internal_error!(9);
+pub(crate) const STDWEB_RNG_FAILED: Error = internal_error!(10);
+pub(crate) const RAND_SECURE_FATAL: Error = internal_error!(11);
+
+fn internal_desc(error: Error) -> Option<&'static str> {
+    match error {
+        UNSUPPORTED => Some("getrandom: this target is not supported"),
+        ERRNO_NOT_POSITIVE => Some("errno: did not return a positive value"),
+        UNKNOWN_IO_ERROR => Some("Unknown std::io::Error"),
+        SEC_RANDOM_FAILED => Some("SecRandomCopyBytes: call failed"),
+        RTL_GEN_RANDOM_FAILED => Some("RtlGenRandom: call failed"),
+        FAILED_RDRAND => Some("RDRAND: failed multiple times: CPU issue likely"),
+        NO_RDRAND => Some("RDRAND: instruction not supported"),
+        BINDGEN_CRYPTO_UNDEF => Some("wasm-bindgen: self.crypto is undefined"),
+        BINDGEN_GRV_UNDEF => Some("wasm-bindgen: crypto.getRandomValues is undefined"),
+        STDWEB_NO_RNG => Some("stdweb: no randomness source available"),
+        STDWEB_RNG_FAILED => Some("stdweb: failed to get randomness"),
+        RAND_SECURE_FATAL => Some("randSecure: random number generator module is not initialized"),
+        _ => None,
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::Error;
+    use core::mem::size_of;
+
+    #[test]
+    fn test_size() {
+        assert_eq!(size_of::<Error>(), 4);
+        assert_eq!(size_of::<Result<(), Error>>(), 4);
+    }
+}
diff --git a/src/error_impls.rs b/src/error_impls.rs
new file mode 100644
index 0000000..007472e
--- /dev/null
+++ b/src/error_impls.rs
@@ -0,0 +1,35 @@
+// Copyright 2018 Developers of the Rand project.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// https://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or https://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+extern crate std;
+
+use crate::{error::UNKNOWN_IO_ERROR, Error};
+use core::convert::From;
+use core::num::NonZeroU32;
+use std::io;
+
+impl From<io::Error> for Error {
+    fn from(err: io::Error) -> Self {
+        if let Some(errno) = err.raw_os_error() {
+            if let Some(code) = NonZeroU32::new(errno as u32) {
+                return Error::from(code);
+            }
+        }
+        UNKNOWN_IO_ERROR
+    }
+}
+
+impl From<Error> for io::Error {
+    fn from(err: Error) -> Self {
+        match err.raw_os_error() {
+            Some(errno) => io::Error::from_raw_os_error(errno),
+            None => io::Error::new(io::ErrorKind::Other, err),
+        }
+    }
+}
+
+impl std::error::Error for Error {}
diff --git a/src/fuchsia.rs b/src/fuchsia.rs
new file mode 100644
index 0000000..572ff53
--- /dev/null
+++ b/src/fuchsia.rs
@@ -0,0 +1,20 @@
+// Copyright 2018 Developers of the Rand project.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// https://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or https://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+//! Implementation for Fuchsia Zircon
+use crate::Error;
+
+#[link(name = "zircon")]
+extern "C" {
+    fn zx_cprng_draw(buffer: *mut u8, length: usize);
+}
+
+pub fn getrandom_inner(dest: &mut [u8]) -> Result<(), Error> {
+    unsafe { zx_cprng_draw(dest.as_mut_ptr(), dest.len()) }
+    Ok(())
+}
diff --git a/src/ios.rs b/src/ios.rs
new file mode 100644
index 0000000..30c008c
--- /dev/null
+++ b/src/ios.rs
@@ -0,0 +1,31 @@
+// Copyright 2018 Developers of the Rand project.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// https://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or https://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+//! Implementation for iOS
+use crate::{error::SEC_RANDOM_FAILED, Error};
+
+// TODO: Make extern once extern_types feature is stabilized. See:
+//   https://github.com/rust-lang/rust/issues/43467
+#[repr(C)]
+struct SecRandom([u8; 0]);
+
+#[link(name = "Security", kind = "framework")]
+extern "C" {
+    static kSecRandomDefault: *const SecRandom;
+
+    fn SecRandomCopyBytes(rnd: *const SecRandom, count: usize, bytes: *mut u8) -> i32;
+}
+
+pub fn getrandom_inner(dest: &mut [u8]) -> Result<(), Error> {
+    let ret = unsafe { SecRandomCopyBytes(kSecRandomDefault, dest.len(), dest.as_mut_ptr()) };
+    if ret == -1 {
+        Err(SEC_RANDOM_FAILED)
+    } else {
+        Ok(())
+    }
+}
diff --git a/src/lib.rs b/src/lib.rs
new file mode 100644
index 0000000..c305406
--- /dev/null
+++ b/src/lib.rs
@@ -0,0 +1,282 @@
+// Copyright 2019 Developers of the Rand project.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// https://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or https://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+//! Interface to the random number generator of the operating system.
+//!
+//! # Platform sources
+//!
+//! | OS               | interface
+//! |------------------|---------------------------------------------------------
+//! | Linux, Android   | [`getrandom`][1] system call if available, otherwise [`/dev/urandom`][2] after successfully polling `/dev/random`
+//! | Windows          | [`RtlGenRandom`][3]
+//! | macOS            | [`getentropy()`][19] if available, otherwise [`/dev/random`][20] (identical to `/dev/urandom`)
+//! | iOS              | [`SecRandomCopyBytes`][4]
+//! | FreeBSD          | [`getrandom()`][21] if available, otherwise [`kern.arandom`][5]
+//! | OpenBSD          | [`getentropy`][6]
+//! | NetBSD           | [`kern.arandom`][7]
+//! | Dragonfly BSD    | [`/dev/random`][8]
+//! | Solaris, illumos | [`getrandom`][9] system call if available, otherwise [`/dev/random`][10]
+//! | Fuchsia OS       | [`cprng_draw`][11]
+//! | Redox            | [`rand:`][12]
+//! | CloudABI         | [`cloudabi_sys_random_get`][13]
+//! | Haiku            | `/dev/random` (identical to `/dev/urandom`)
+//! | L4RE, SGX, UEFI  | [RDRAND][18]
+//! | Hermit           | [RDRAND][18] as [`sys_rand`][22] is currently broken.
+//! | VxWorks          | `randABytes` after checking entropy pool initialization with `randSecure`
+//! | Web browsers     | [`Crypto.getRandomValues`][14] (see [Support for WebAssembly and asm.js][16])
+//! | Node.js          | [`crypto.randomBytes`][15] (see [Support for WebAssembly and asm.js][16])
+//! | WASI             | [`__wasi_random_get`][17]
+//!
+//! Getrandom doesn't have a blanket implementation for all Unix-like operating
+//! systems that reads from `/dev/urandom`. This ensures all supported operating
+//! systems are using the recommended interface and respect maximum buffer
+//! sizes.
+//!
+//! ## Unsupported targets
+//!
+//! By default, compiling `getrandom` for an unsupported target will result in
+//! a compilation error. If you want to build an application which uses `getrandom`
+//! for such target, you can either:
+//! - Use [`[replace]`][replace] or [`[patch]`][patch] section in your `Cargo.toml`
+//! to switch to a custom implementation with a support of your target.
+//! - Enable the `dummy` feature to have getrandom use an implementation that always
+//! fails at run-time on unsupported targets.
+//!
+//! [replace]: https://doc.rust-lang.org/cargo/reference/manifest.html#the-replace-section
+//! [patch]: https://doc.rust-lang.org/cargo/reference/manifest.html#the-patch-section
+//!
+//! ## Support for WebAssembly and asm.js
+//!
+//! Getrandom supports all of Rust's current `wasm32` targets, and it works with
+//! both Node.js and web browsers. The three Emscripten targets
+//! `asmjs-unknown-emscripten`, `wasm32-unknown-emscripten`, and
+//! `wasm32-experimental-emscripten` use Emscripten's `/dev/random` emulation.
+//! The WASI target `wasm32-wasi` uses the [`__wasi_random_get`][17] function
+//! defined by the WASI standard.
+//!
+//! Getrandom also supports `wasm32-unknown-unknown` by directly calling
+//! JavaScript methods. Rust currently has two ways to do this: [bindgen] and
+//! [stdweb]. Getrandom supports using either one by enabling the
+//! `wasm-bindgen` or `stdweb` crate features. Note that if both features are
+//! enabled, `wasm-bindgen` will be used. If neither feature is enabled, calls
+//! to `getrandom` will always fail at runtime.
+//!
+//! [bindgen]: https://github.com/rust-lang/rust-bindgen
+//! [stdweb]: https://github.com/koute/stdweb
+//!
+//! ## Early boot
+//!
+//! It is possible that early in the boot process the OS hasn't had enough time
+//! yet to collect entropy to securely seed its RNG, especially on virtual
+//! machines.
+//!
+//! Some operating systems always block the thread until the RNG is securely
+//! seeded. This can take anywhere from a few seconds to more than a minute.
+//! Others make a best effort to use a seed from before the shutdown and don't
+//! document much.
+//!
+//! A few, Linux, NetBSD and Solaris, offer a choice between blocking and
+//! getting an error; in these cases we always choose to block.
+//!
+//! On Linux (when the `getrandom` system call is not available) and on NetBSD
+//! reading from `/dev/urandom` never blocks, even when the OS hasn't collected
+//! enough entropy yet. To avoid returning low-entropy bytes, we first read from
+//! `/dev/random` and only switch to `/dev/urandom` once this has succeeded.
+//!
+//! # Error handling
+//!
+//! We always choose failure over returning insecure "random" bytes. In general,
+//! on supported platforms, failure is highly unlikely, though not impossible.
+//! If an error does occur, then it is likely that it will occur on every call to
+//! `getrandom`, hence after the first successful call one can be reasonably
+//! confident that no errors will occur.
+//!
+//! On unsupported platforms, `getrandom` always fails. See the [`Error`] type
+//! for more information on what data is returned on failure.
+//!
+//! [1]: http://man7.org/linux/man-pages/man2/getrandom.2.html
+//! [2]: http://man7.org/linux/man-pages/man4/urandom.4.html
+//! [3]: https://docs.microsoft.com/en-us/windows/desktop/api/ntsecapi/nf-ntsecapi-rtlgenrandom
+//! [4]: https://developer.apple.com/documentation/security/1399291-secrandomcopybytes?language=objc
+//! [5]: https://www.freebsd.org/cgi/man.cgi?query=random&sektion=4
+//! [6]: https://man.openbsd.org/getentropy.2
+//! [7]: https://netbsd.gw.com/cgi-bin/man-cgi?sysctl+7+NetBSD-8.0
+//! [8]: https://leaf.dragonflybsd.org/cgi/web-man?command=random&section=4
+//! [9]: https://docs.oracle.com/cd/E88353_01/html/E37841/getrandom-2.html
+//! [10]: https://docs.oracle.com/cd/E86824_01/html/E54777/random-7d.html
+//! [11]: https://fuchsia.dev/fuchsia-src/zircon/syscalls/cprng_draw
+//! [12]: https://github.com/redox-os/randd/blob/master/src/main.rs
+//! [13]: https://github.com/nuxinl/cloudabi#random_get
+//! [14]: https://www.w3.org/TR/WebCryptoAPI/#Crypto-method-getRandomValues
+//! [15]: https://nodejs.org/api/crypto.html#crypto_crypto_randombytes_size_callback
+//! [16]: #support-for-webassembly-and-asmjs
+//! [17]: https://github.com/WebAssembly/WASI/blob/master/design/WASI-core.md#__wasi_random_get
+//! [18]: https://software.intel.com/en-us/articles/intel-digital-random-number-generator-drng-software-implementation-guide
+//! [19]: https://www.unix.com/man-page/mojave/2/getentropy/
+//! [20]: https://www.unix.com/man-page/mojave/4/random/
+//! [21]: https://www.freebsd.org/cgi/man.cgi?query=getrandom&manpath=FreeBSD+12.0-stable
+//! [22]: https://github.com/hermitcore/libhermit-rs/blob/09c38b0371cee6f56a541400ba453e319e43db53/src/syscalls/random.rs#L21
+
+#![doc(
+    html_logo_url = "https://www.rust-lang.org/logos/rust-logo-128x128-blk.png",
+    html_favicon_url = "https://www.rust-lang.org/favicon.ico",
+    html_root_url = "https://rust-random.github.io/rand/"
+)]
+#![no_std]
+#![cfg_attr(feature = "stdweb", recursion_limit = "128")]
+#![warn(rust_2018_idioms, unused_lifetimes, missing_docs)]
+
+#[macro_use]
+extern crate cfg_if;
+
+cfg_if! {
+    if #[cfg(feature = "log")] {
+        #[allow(unused)]
+        #[macro_use]
+        extern crate log;
+    } else {
+        #[allow(unused)]
+        macro_rules! error {
+            ($($x:tt)*) => {};
+        }
+        #[allow(unused)]
+        macro_rules! warn {
+            ($($x:tt)*) => {};
+        }
+        #[allow(unused)]
+        macro_rules! info {
+            ($($x:tt)*) => {};
+        }
+    }
+}
+
+mod error;
+pub use crate::error::Error;
+
+#[allow(dead_code)]
+mod util;
+
+#[cfg(target_os = "vxworks")]
+#[allow(dead_code)]
+mod util_libc;
+
+cfg_if! {
+    // Unlike the other Unix, Fuchsia and iOS don't use the libc to make any calls.
+    if #[cfg(any(target_os = "android", target_os = "dragonfly", target_os = "emscripten",
+                 target_os = "freebsd", target_os = "haiku",     target_os = "illumos",
+                 target_os = "linux",   target_os = "macos",     target_os = "netbsd",
+                 target_os = "openbsd", target_os = "redox",     target_os = "solaris"))] {
+        #[allow(dead_code)]
+        mod util_libc;
+        // Keep std-only trait definitions for backwards compatibility
+        mod error_impls;
+    } else if #[cfg(feature = "std")] {
+        mod error_impls;
+    }
+}
+
+// These targets read from a file as a fallback method.
+#[cfg(any(
+    target_os = "android",
+    target_os = "linux",
+    target_os = "macos",
+    target_os = "solaris",
+    target_os = "illumos",
+))]
+mod use_file;
+
+// System-specific implementations.
+//
+// These should all provide getrandom_inner with the same signature as getrandom.
+cfg_if! {
+    if #[cfg(target_os = "android")] {
+        #[path = "linux_android.rs"] mod imp;
+    } else if #[cfg(target_os = "cloudabi")] {
+        #[path = "cloudabi.rs"] mod imp;
+    } else if #[cfg(target_os = "dragonfly")] {
+        #[path = "use_file.rs"] mod imp;
+    } else if #[cfg(target_os = "emscripten")] {
+        #[path = "use_file.rs"] mod imp;
+    } else if #[cfg(target_os = "freebsd")] {
+        #[path = "bsd_arandom.rs"] mod imp;
+    } else if #[cfg(target_os = "fuchsia")] {
+        #[path = "fuchsia.rs"] mod imp;
+    } else if #[cfg(target_os = "haiku")] {
+        #[path = "use_file.rs"] mod imp;
+    } else if #[cfg(target_os = "illumos")] {
+        #[path = "solaris_illumos.rs"] mod imp;
+    } else if #[cfg(target_os = "ios")] {
+        #[path = "ios.rs"] mod imp;
+    } else if #[cfg(target_os = "linux")] {
+        #[path = "linux_android.rs"] mod imp;
+    } else if #[cfg(target_os = "macos")] {
+        #[path = "macos.rs"] mod imp;
+    } else if #[cfg(target_os = "netbsd")] {
+        #[path = "bsd_arandom.rs"] mod imp;
+    } else if #[cfg(target_os = "openbsd")] {
+        #[path = "openbsd.rs"] mod imp;
+    } else if #[cfg(target_os = "redox")] {
+        #[path = "use_file.rs"] mod imp;
+    } else if #[cfg(target_os = "solaris")] {
+        #[path = "solaris_illumos.rs"] mod imp;
+    } else if #[cfg(target_os = "wasi")] {
+        #[path = "wasi.rs"] mod imp;
+    } else if #[cfg(target_os = "vxworks")] {
+        #[path = "vxworks.rs"] mod imp;
+    } else if #[cfg(all(windows, getrandom_uwp))] {
+        #[path = "windows_uwp.rs"] mod imp;
+    } else if #[cfg(windows)] {
+        #[path = "windows.rs"] mod imp;
+    } else if #[cfg(all(target_arch = "x86_64", any(
+                  target_os = "hermit",
+                  target_os = "l4re",
+                  target_os = "uefi",
+                  target_env = "sgx",
+              )))] {
+        #[path = "rdrand.rs"] mod imp;
+    } else if #[cfg(all(target_arch = "wasm32", target_os = "unknown"))] {
+        cfg_if! {
+            if #[cfg(feature = "wasm-bindgen")] {
+                #[path = "wasm32_bindgen.rs"] mod imp;
+            } else if #[cfg(feature = "stdweb")] {
+                #[path = "wasm32_stdweb.rs"] mod imp;
+            } else {
+                // Always have an implementation for wasm32-unknown-unknown.
+                // See https://github.com/rust-random/getrandom/issues/87
+                #[path = "dummy.rs"] mod imp;
+            }
+        }
+    } else if #[cfg(feature = "dummy")] {
+        #[path = "dummy.rs"] mod imp;
+    } else {
+        compile_error!("\
+            target is not supported, for more information see: \
+            https://docs.rs/getrandom/#unsupported-targets\
+        ");
+    }
+}
+
+/// Fill `dest` with random bytes from the system's preferred random number
+/// source.
+///
+/// This function returns an error on any failure, including partial reads. We
+/// make no guarantees regarding the contents of `dest` on error. If `dest` is
+/// empty, `getrandom` immediately returns success, making no calls to the
+/// underlying operating system.
+///
+/// Blocking is possible, at least during early boot; see module documentation.
+///
+/// In general, `getrandom` will be fast enough for interactive usage, though
+/// significantly slower than a user-space CSPRNG; for the latter consider
+/// [`rand::thread_rng`](https://docs.rs/rand/*/rand/fn.thread_rng.html).
+pub fn getrandom(dest: &mut [u8]) -> Result<(), error::Error> {
+    if dest.is_empty() {
+        return Ok(());
+    }
+    imp::getrandom_inner(dest)
+}
diff --git a/src/linux_android.rs b/src/linux_android.rs
new file mode 100644
index 0000000..a29feb5
--- /dev/null
+++ b/src/linux_android.rs
@@ -0,0 +1,44 @@
+// Copyright 2018 Developers of the Rand project.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// https://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or https://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+//! Implementation for Linux / Android
+use crate::util::LazyBool;
+use crate::util_libc::{last_os_error, sys_fill_exact};
+use crate::{use_file, Error};
+
+pub fn getrandom_inner(dest: &mut [u8]) -> Result<(), Error> {
+    static HAS_GETRANDOM: LazyBool = LazyBool::new();
+    if HAS_GETRANDOM.unsync_init(is_getrandom_available) {
+        sys_fill_exact(dest, |buf| unsafe {
+            getrandom(buf.as_mut_ptr() as *mut libc::c_void, buf.len(), 0)
+        })
+    } else {
+        use_file::getrandom_inner(dest)
+    }
+}
+
+fn is_getrandom_available() -> bool {
+    let res = unsafe { getrandom(core::ptr::null_mut(), 0, libc::GRND_NONBLOCK) };
+    if res < 0 {
+        match last_os_error().raw_os_error() {
+            Some(libc::ENOSYS) => false, // No kernel support
+            Some(libc::EPERM) => false,  // Blocked by seccomp
+            _ => true,
+        }
+    } else {
+        true
+    }
+}
+
+unsafe fn getrandom(
+    buf: *mut libc::c_void,
+    buflen: libc::size_t,
+    flags: libc::c_uint,
+) -> libc::ssize_t {
+    libc::syscall(libc::SYS_getrandom, buf, buflen, flags) as libc::ssize_t
+}
diff --git a/src/macos.rs b/src/macos.rs
new file mode 100644
index 0000000..c3bc533
--- /dev/null
+++ b/src/macos.rs
@@ -0,0 +1,34 @@
+// Copyright 2019 Developers of the Rand project.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// https://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or https://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+//! Implementation for macOS
+use crate::util_libc::{last_os_error, Weak};
+use crate::{use_file, Error};
+use core::mem;
+
+type GetEntropyFn = unsafe extern "C" fn(*mut u8, libc::size_t) -> libc::c_int;
+
+pub fn getrandom_inner(dest: &mut [u8]) -> Result<(), Error> {
+    static GETENTROPY: Weak = unsafe { Weak::new("getentropy\0") };
+    if let Some(fptr) = GETENTROPY.ptr() {
+        let func: GetEntropyFn = unsafe { mem::transmute(fptr) };
+        for chunk in dest.chunks_mut(256) {
+            let ret = unsafe { func(chunk.as_mut_ptr(), chunk.len()) };
+            if ret != 0 {
+                let err = last_os_error();
+                error!("getentropy syscall failed");
+                return Err(err);
+            }
+        }
+        Ok(())
+    } else {
+        // We fallback to reading from /dev/random instead of SecRandomCopyBytes
+        // to avoid high startup costs and linking the Security framework.
+        use_file::getrandom_inner(dest)
+    }
+}
diff --git a/src/openbsd.rs b/src/openbsd.rs
new file mode 100644
index 0000000..e1ac179
--- /dev/null
+++ b/src/openbsd.rs
@@ -0,0 +1,23 @@
+// Copyright 2018 Developers of the Rand project.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// https://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or https://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+//! Implementation for OpenBSD
+use crate::util_libc::last_os_error;
+use crate::Error;
+
+pub fn getrandom_inner(dest: &mut [u8]) -> Result<(), Error> {
+    for chunk in dest.chunks_mut(256) {
+        let ret = unsafe { libc::getentropy(chunk.as_mut_ptr() as *mut libc::c_void, chunk.len()) };
+        if ret == -1 {
+            let err = last_os_error();
+            error!("libc::getentropy call failed");
+            return Err(err);
+        }
+    }
+    Ok(())
+}
diff --git a/src/rdrand.rs b/src/rdrand.rs
new file mode 100644
index 0000000..e441682
--- /dev/null
+++ b/src/rdrand.rs
@@ -0,0 +1,90 @@
+// Copyright 2018 Developers of the Rand project.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// https://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or https://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+//! Implementation for SGX using RDRAND instruction
+use crate::error::{FAILED_RDRAND, NO_RDRAND};
+#[cfg(not(target_feature = "rdrand"))]
+use crate::util::LazyBool;
+use crate::Error;
+use core::arch::x86_64::_rdrand64_step;
+use core::mem;
+
+// Recommendation from "Intel® Digital Random Number Generator (DRNG) Software
+// Implementation Guide" - Section 5.2.1 and "Intel® 64 and IA-32 Architectures
+// Software Developer’s Manual" - Volume 1 - Section 7.3.17.1.
+const RETRY_LIMIT: usize = 10;
+const WORD_SIZE: usize = mem::size_of::<u64>();
+
+#[target_feature(enable = "rdrand")]
+unsafe fn rdrand() -> Result<[u8; WORD_SIZE], Error> {
+    for _ in 0..RETRY_LIMIT {
+        let mut el = mem::zeroed();
+        if _rdrand64_step(&mut el) == 1 {
+            // AMD CPUs from families 14h to 16h (pre Ryzen) sometimes fail to
+            // set CF on bogus random data, so we check these values explicitly.
+            // See https://github.com/systemd/systemd/issues/11810#issuecomment-489727505
+            // We perform this check regardless of target to guard against
+            // any implementation that incorrectly fails to set CF.
+            if el != 0 && el != !0 {
+                return Ok(el.to_ne_bytes());
+            }
+            error!("RDRAND returned {:X}, CPU RNG may be broken", el);
+            // Keep looping in case this was a false positive.
+        }
+    }
+    Err(FAILED_RDRAND)
+}
+
+// "rdrand" target feature requires "+rdrnd" flag, see https://github.com/rust-lang/rust/issues/49653.
+#[cfg(all(target_env = "sgx", not(target_feature = "rdrand")))]
+compile_error!(
+    "SGX targets require 'rdrand' target feature. Enable by using -C target-feature=+rdrnd."
+);
+
+#[cfg(target_feature = "rdrand")]
+fn is_rdrand_supported() -> bool {
+    true
+}
+
+// TODO use is_x86_feature_detected!("rdrand") when that works in core. See:
+// https://github.com/rust-lang-nursery/stdsimd/issues/464
+#[cfg(not(target_feature = "rdrand"))]
+fn is_rdrand_supported() -> bool {
+    use core::arch::x86_64::__cpuid;
+    // SAFETY: All x86_64 CPUs support CPUID leaf 1
+    const FLAG: u32 = 1 << 30;
+    static HAS_RDRAND: LazyBool = LazyBool::new();
+    HAS_RDRAND.unsync_init(|| unsafe { (__cpuid(1).ecx & FLAG) != 0 })
+}
+
+pub fn getrandom_inner(dest: &mut [u8]) -> Result<(), Error> {
+    if !is_rdrand_supported() {
+        return Err(NO_RDRAND);
+    }
+
+    // SAFETY: After this point, rdrand is supported, so calling the rdrand
+    // functions is not undefined behavior.
+    unsafe { rdrand_exact(dest) }
+}
+
+#[target_feature(enable = "rdrand")]
+unsafe fn rdrand_exact(dest: &mut [u8]) -> Result<(), Error> {
+    // We use chunks_exact_mut instead of chunks_mut as it allows almost all
+    // calls to memcpy to be elided by the compiler.
+    let mut chunks = dest.chunks_exact_mut(WORD_SIZE);
+    for chunk in chunks.by_ref() {
+        chunk.copy_from_slice(&rdrand()?);
+    }
+
+    let tail = chunks.into_remainder();
+    let n = tail.len();
+    if n > 0 {
+        tail.copy_from_slice(&rdrand()?[..n]);
+    }
+    Ok(())
+}
diff --git a/src/solaris_illumos.rs b/src/solaris_illumos.rs
new file mode 100644
index 0000000..9473123
--- /dev/null
+++ b/src/solaris_illumos.rs
@@ -0,0 +1,44 @@
+// Copyright 2018 Developers of the Rand project.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// https://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or https://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+//! Implementation for the Solaris family
+//!
+//! Read from `/dev/random`, with chunks of limited size (256 bytes).
+//! `/dev/random` uses the Hash_DRBG with SHA512 algorithm from NIST SP 800-90A.
+//! `/dev/urandom` uses the FIPS 186-2 algorithm, which is considered less
+//! secure. We choose to read from `/dev/random`.
+//!
+//! Since Solaris 11.3 and mid-2015 illumos, the `getrandom` syscall is available.
+//! To make sure we can compile on both Solaris and its derivatives, as well as
+//! function, we check for the existence of getrandom(2) in libc by calling
+//! libc::dlsym.
+use crate::util_libc::{sys_fill_exact, Weak};
+use crate::{use_file, Error};
+use core::mem;
+
+#[cfg(target_os = "illumos")]
+type GetRandomFn = unsafe extern "C" fn(*mut u8, libc::size_t, libc::c_uint) -> libc::ssize_t;
+#[cfg(target_os = "solaris")]
+type GetRandomFn = unsafe extern "C" fn(*mut u8, libc::size_t, libc::c_uint) -> libc::c_int;
+
+pub fn getrandom_inner(dest: &mut [u8]) -> Result<(), Error> {
+    static GETRANDOM: Weak = unsafe { Weak::new("getrandom\0") };
+    if let Some(fptr) = GETRANDOM.ptr() {
+        let func: GetRandomFn = unsafe { mem::transmute(fptr) };
+        // 256 bytes is the lowest common denominator across all the Solaris
+        // derived platforms for atomically obtaining random data.
+        for chunk in dest.chunks_mut(256) {
+            sys_fill_exact(chunk, |buf| unsafe {
+                func(buf.as_mut_ptr(), buf.len(), 0) as libc::ssize_t
+            })?
+        }
+        Ok(())
+    } else {
+        use_file::getrandom_inner(dest)
+    }
+}
diff --git a/src/use_file.rs b/src/use_file.rs
new file mode 100644
index 0000000..6e50955
--- /dev/null
+++ b/src/use_file.rs
@@ -0,0 +1,137 @@
+// Copyright 2018 Developers of the Rand project.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// https://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or https://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+//! Implementations that just need to read from a file
+use crate::util::LazyUsize;
+use crate::util_libc::{open_readonly, sys_fill_exact};
+use crate::Error;
+use core::cell::UnsafeCell;
+use core::sync::atomic::{AtomicUsize, Ordering::Relaxed};
+
+#[cfg(target_os = "redox")]
+const FILE_PATH: &str = "rand:\0";
+#[cfg(any(
+    target_os = "dragonfly",
+    target_os = "emscripten",
+    target_os = "haiku",
+    target_os = "macos",
+    target_os = "solaris",
+    target_os = "illumos"
+))]
+const FILE_PATH: &str = "/dev/random\0";
+#[cfg(any(target_os = "android", target_os = "linux"))]
+const FILE_PATH: &str = "/dev/urandom\0";
+
+pub fn getrandom_inner(dest: &mut [u8]) -> Result<(), Error> {
+    let fd = get_rng_fd()?;
+    let read = |buf: &mut [u8]| unsafe { libc::read(fd, buf.as_mut_ptr() as *mut _, buf.len()) };
+
+    if cfg!(target_os = "emscripten") {
+        // `Crypto.getRandomValues` documents `dest` should be at most 65536 bytes.
+        for chunk in dest.chunks_mut(65536) {
+            sys_fill_exact(chunk, read)?;
+        }
+    } else {
+        sys_fill_exact(dest, read)?;
+    }
+    Ok(())
+}
+
+// Returns the file descriptor for the device file used to retrieve random
+// bytes. The file will be opened exactly once. All successful calls will
+// return the same file descriptor. This file descriptor is never closed.
+fn get_rng_fd() -> Result<libc::c_int, Error> {
+    static FD: AtomicUsize = AtomicUsize::new(LazyUsize::UNINIT);
+    fn get_fd() -> Option<libc::c_int> {
+        match FD.load(Relaxed) {
+            LazyUsize::UNINIT => None,
+            val => Some(val as libc::c_int),
+        }
+    }
+
+    // Use double-checked locking to avoid acquiring the lock if possible.
+    if let Some(fd) = get_fd() {
+        return Ok(fd);
+    }
+
+    // SAFETY: We use the mutex only in this method, and we always unlock it
+    // before returning, making sure we don't violate the pthread_mutex_t API.
+    static MUTEX: Mutex = Mutex::new();
+    unsafe { MUTEX.lock() };
+    let _guard = DropGuard(|| unsafe { MUTEX.unlock() });
+
+    if let Some(fd) = get_fd() {
+        return Ok(fd);
+    }
+
+    // On Linux, /dev/urandom might return insecure values.
+    #[cfg(any(target_os = "android", target_os = "linux"))]
+    wait_until_rng_ready()?;
+
+    let fd = unsafe { open_readonly(FILE_PATH)? };
+    // The fd always fits in a usize without conflicting with UNINIT.
+    debug_assert!(fd >= 0 && (fd as usize) < LazyUsize::UNINIT);
+    FD.store(fd as usize, Relaxed);
+
+    Ok(fd)
+}
+
+// Succeeds once /dev/urandom is safe to read from
+#[cfg(any(target_os = "android", target_os = "linux"))]
+fn wait_until_rng_ready() -> Result<(), Error> {
+    // Poll /dev/random to make sure it is ok to read from /dev/urandom.
+    let fd = unsafe { open_readonly("/dev/random\0")? };
+    let mut pfd = libc::pollfd {
+        fd,
+        events: libc::POLLIN,
+        revents: 0,
+    };
+    let _guard = DropGuard(|| unsafe {
+        libc::close(fd);
+    });
+
+    loop {
+        // A negative timeout means an infinite timeout.
+        let res = unsafe { libc::poll(&mut pfd, 1, -1) };
+        if res >= 0 {
+            assert_eq!(res, 1); // We only used one fd, and cannot timeout.
+            return Ok(());
+        }
+        let err = crate::util_libc::last_os_error();
+        match err.raw_os_error() {
+            Some(libc::EINTR) | Some(libc::EAGAIN) => continue,
+            _ => return Err(err),
+        }
+    }
+}
+
+struct Mutex(UnsafeCell<libc::pthread_mutex_t>);
+
+impl Mutex {
+    const fn new() -> Self {
+        Self(UnsafeCell::new(libc::PTHREAD_MUTEX_INITIALIZER))
+    }
+    unsafe fn lock(&self) {
+        let r = libc::pthread_mutex_lock(self.0.get());
+        debug_assert_eq!(r, 0);
+    }
+    unsafe fn unlock(&self) {
+        let r = libc::pthread_mutex_unlock(self.0.get());
+        debug_assert_eq!(r, 0);
+    }
+}
+
+unsafe impl Sync for Mutex {}
+
+struct DropGuard<F: FnMut()>(F);
+
+impl<F: FnMut()> Drop for DropGuard<F> {
+    fn drop(&mut self) {
+        self.0()
+    }
+}
diff --git a/src/util.rs b/src/util.rs
new file mode 100644
index 0000000..8dbd8ae
--- /dev/null
+++ b/src/util.rs
@@ -0,0 +1,64 @@
+// Copyright 2019 Developers of the Rand project.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// https://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or https://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+use core::sync::atomic::{AtomicUsize, Ordering::Relaxed};
+
+// This structure represents a lazily initialized static usize value. Useful
+// when it is preferable to just rerun initialization instead of locking.
+// Both unsync_init and sync_init will invoke an init() function until it
+// succeeds, then return the cached value for future calls.
+//
+// Both methods support init() "failing". If the init() method returns UNINIT,
+// that value will be returned as normal, but will not be cached.
+//
+// Users should only depend on the _value_ returned by init() functions.
+// Specifically, for the following init() function:
+//      fn init() -> usize {
+//          a();
+//          let v = b();
+//          c();
+//          v
+//      }
+// the effects of c() or writes to shared memory will not necessarily be
+// observed and additional synchronization methods with be needed.
+pub struct LazyUsize(AtomicUsize);
+
+impl LazyUsize {
+    pub const fn new() -> Self {
+        Self(AtomicUsize::new(Self::UNINIT))
+    }
+
+    // The initialization is not completed.
+    pub const UNINIT: usize = usize::max_value();
+
+    // Runs the init() function at least once, returning the value of some run
+    // of init(). Multiple callers can run their init() functions in parallel.
+    // init() should always return the same value, if it succeeds.
+    pub fn unsync_init(&self, init: impl FnOnce() -> usize) -> usize {
+        // Relaxed ordering is fine, as we only have a single atomic variable.
+        let mut val = self.0.load(Relaxed);
+        if val == Self::UNINIT {
+            val = init();
+            self.0.store(val, Relaxed);
+        }
+        val
+    }
+}
+
+// Identical to LazyUsize except with bool instead of usize.
+pub struct LazyBool(LazyUsize);
+
+impl LazyBool {
+    pub const fn new() -> Self {
+        Self(LazyUsize::new())
+    }
+
+    pub fn unsync_init(&self, init: impl FnOnce() -> bool) -> bool {
+        self.0.unsync_init(|| init() as usize) != 0
+    }
+}
diff --git a/src/util_libc.rs b/src/util_libc.rs
new file mode 100644
index 0000000..1cdc13e
--- /dev/null
+++ b/src/util_libc.rs
@@ -0,0 +1,121 @@
+// Copyright 2019 Developers of the Rand project.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// https://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or https://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+use crate::error::ERRNO_NOT_POSITIVE;
+use crate::util::LazyUsize;
+use crate::Error;
+use core::num::NonZeroU32;
+use core::ptr::NonNull;
+
+cfg_if! {
+    if #[cfg(any(target_os = "netbsd", target_os = "openbsd", target_os = "android"))] {
+        use libc::__errno as errno_location;
+    } else if #[cfg(any(target_os = "linux", target_os = "emscripten", target_os = "redox"))] {
+        use libc::__errno_location as errno_location;
+    } else if #[cfg(any(target_os = "solaris", target_os = "illumos"))] {
+        use libc::___errno as errno_location;
+    } else if #[cfg(any(target_os = "macos", target_os = "freebsd"))] {
+        use libc::__error as errno_location;
+    } else if #[cfg(target_os = "haiku")] {
+        use libc::_errnop as errno_location;
+    }
+}
+
+cfg_if! {
+    if #[cfg(target_os = "vxworks")] {
+        use libc::errnoGet as get_errno;
+    } else if #[cfg(target_os = "dragonfly")] {
+        // Until rust-lang/rust#29594 is stable, we cannot get the errno value
+        // on DragonFlyBSD. So we just return an out-of-range errno.
+        unsafe fn get_errno() -> libc::c_int { -1 }
+    } else {
+        unsafe fn get_errno() -> libc::c_int { *errno_location() }
+    }
+}
+
+pub fn last_os_error() -> Error {
+    let errno = unsafe { get_errno() };
+    if errno > 0 {
+        Error::from(NonZeroU32::new(errno as u32).unwrap())
+    } else {
+        ERRNO_NOT_POSITIVE
+    }
+}
+
+// Fill a buffer by repeatedly invoking a system call. The `sys_fill` function:
+//   - should return -1 and set errno on failure
+//   - should return the number of bytes written on success
+pub fn sys_fill_exact(
+    mut buf: &mut [u8],
+    sys_fill: impl Fn(&mut [u8]) -> libc::ssize_t,
+) -> Result<(), Error> {
+    while !buf.is_empty() {
+        let res = sys_fill(buf);
+        if res < 0 {
+            let err = last_os_error();
+            // We should try again if the call was interrupted.
+            if err.raw_os_error() != Some(libc::EINTR) {
+                return Err(err);
+            }
+        } else {
+            // We don't check for EOF (ret = 0) as the data we are reading
+            // should be an infinite stream of random bytes.
+            buf = &mut buf[(res as usize)..];
+        }
+    }
+    Ok(())
+}
+
+// A "weak" binding to a C function that may or may not be present at runtime.
+// Used for supporting newer OS features while still building on older systems.
+// F must be a function pointer of type `unsafe extern "C" fn`. Based off of the
+// weak! macro in libstd.
+pub struct Weak {
+    name: &'static str,
+    addr: LazyUsize,
+}
+
+impl Weak {
+    // Construct a binding to a C function with a given name. This function is
+    // unsafe because `name` _must_ be null terminated.
+    pub const unsafe fn new(name: &'static str) -> Self {
+        Self {
+            name,
+            addr: LazyUsize::new(),
+        }
+    }
+
+    // Return a function pointer if present at runtime. Otherwise, return null.
+    pub fn ptr(&self) -> Option<NonNull<libc::c_void>> {
+        let addr = self.addr.unsync_init(|| unsafe {
+            libc::dlsym(libc::RTLD_DEFAULT, self.name.as_ptr() as *const _) as usize
+        });
+        NonNull::new(addr as *mut _)
+    }
+}
+
+cfg_if! {
+    if #[cfg(any(target_os = "linux", target_os = "emscripten"))] {
+        use libc::open64 as open;
+    } else {
+        use libc::open;
+    }
+}
+
+// SAFETY: path must be null terminated, FD must be manually closed.
+pub unsafe fn open_readonly(path: &str) -> Result<libc::c_int, Error> {
+    debug_assert!(path.as_bytes().last() == Some(&0));
+    let fd = open(path.as_ptr() as *const _, libc::O_RDONLY | libc::O_CLOEXEC);
+    if fd < 0 {
+        return Err(last_os_error());
+    }
+    // O_CLOEXEC works on all Unix targets except for older Linux kernels (pre
+    // 2.6.23), so we also use an ioctl to make sure FD_CLOEXEC is set.
+    #[cfg(target_os = "linux")]
+    libc::ioctl(fd, libc::FIOCLEX);
+    Ok(fd)
+}
diff --git a/src/vxworks.rs b/src/vxworks.rs
new file mode 100644
index 0000000..a2fe52a
--- /dev/null
+++ b/src/vxworks.rs
@@ -0,0 +1,35 @@
+// Copyright 2018 Developers of the Rand project.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// https://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or https://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+//! Implementation for VxWorks
+use crate::error::{Error, RAND_SECURE_FATAL};
+use crate::util_libc::last_os_error;
+use core::sync::atomic::{AtomicBool, Ordering::Relaxed};
+
+pub fn getrandom_inner(dest: &mut [u8]) -> Result<(), Error> {
+    static RNG_INIT: AtomicBool = AtomicBool::new(false);
+    while !RNG_INIT.load(Relaxed) {
+        let ret = unsafe { libc::randSecure() };
+        if ret < 0 {
+            return Err(RAND_SECURE_FATAL);
+        } else if ret > 0 {
+            RNG_INIT.store(true, Relaxed);
+            break;
+        }
+        unsafe { libc::usleep(10) };
+    }
+
+    // Prevent overflow of i32
+    for chunk in dest.chunks_mut(i32::max_value() as usize) {
+        let ret = unsafe { libc::randABytes(chunk.as_mut_ptr(), chunk.len() as i32) };
+        if ret != 0 {
+            return Err(last_os_error());
+        }
+    }
+    Ok(())
+}
diff --git a/src/wasi.rs b/src/wasi.rs
new file mode 100644
index 0000000..4674f43
--- /dev/null
+++ b/src/wasi.rs
@@ -0,0 +1,19 @@
+// Copyright 2018 Developers of the Rand project.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// https://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or https://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+//! Implementation for WASI
+use crate::Error;
+use core::num::NonZeroU32;
+use wasi::random_get;
+
+pub fn getrandom_inner(dest: &mut [u8]) -> Result<(), Error> {
+    unsafe { random_get(dest.as_mut_ptr(), dest.len()) }.map_err(|e: wasi::Error| {
+        // convert wasi's Error into getrandom's NonZeroU32 error
+        NonZeroU32::new(e.raw_error() as u32).unwrap().into()
+    })
+}
diff --git a/src/wasm32_bindgen.rs b/src/wasm32_bindgen.rs
new file mode 100644
index 0000000..86839a0
--- /dev/null
+++ b/src/wasm32_bindgen.rs
@@ -0,0 +1,113 @@
+// Copyright 2018 Developers of the Rand project.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// https://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or https://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+//! Implementation for WASM via wasm-bindgen
+extern crate std;
+
+use core::cell::RefCell;
+use core::mem;
+use std::thread_local;
+
+use wasm_bindgen::prelude::*;
+
+use crate::error::{BINDGEN_CRYPTO_UNDEF, BINDGEN_GRV_UNDEF};
+use crate::Error;
+
+#[derive(Clone, Debug)]
+enum RngSource {
+    Node(NodeCrypto),
+    Browser(BrowserCrypto),
+}
+
+// JsValues are always per-thread, so we initialize RngSource for each thread.
+//   See: https://github.com/rustwasm/wasm-bindgen/pull/955
+thread_local!(
+    static RNG_SOURCE: RefCell<Option<RngSource>> = RefCell::new(None);
+);
+
+pub fn getrandom_inner(dest: &mut [u8]) -> Result<(), Error> {
+    assert_eq!(mem::size_of::<usize>(), 4);
+
+    RNG_SOURCE.with(|f| {
+        let mut source = f.borrow_mut();
+        if source.is_none() {
+            *source = Some(getrandom_init()?);
+        }
+
+        match source.as_ref().unwrap() {
+            RngSource::Node(n) => n.random_fill_sync(dest),
+            RngSource::Browser(n) => {
+                // see https://developer.mozilla.org/en-US/docs/Web/API/Crypto/getRandomValues
+                //
+                // where it says:
+                //
+                // > A QuotaExceededError DOMException is thrown if the
+                // > requested length is greater than 65536 bytes.
+                for chunk in dest.chunks_mut(65536) {
+                    n.get_random_values(chunk)
+                }
+            }
+        };
+        Ok(())
+    })
+}
+
+fn getrandom_init() -> Result<RngSource, Error> {
+    if let Ok(self_) = Global::get_self() {
+        // If `self` is defined then we're in a browser somehow (main window
+        // or web worker). Here we want to try to use
+        // `crypto.getRandomValues`, but if `crypto` isn't defined we assume
+        // we're in an older web browser and the OS RNG isn't available.
+
+        let crypto = self_.crypto();
+        if crypto.is_undefined() {
+            return Err(BINDGEN_CRYPTO_UNDEF);
+        }
+
+        // Test if `crypto.getRandomValues` is undefined as well
+        let crypto: BrowserCrypto = crypto.into();
+        if crypto.get_random_values_fn().is_undefined() {
+            return Err(BINDGEN_GRV_UNDEF);
+        }
+
+        return Ok(RngSource::Browser(crypto));
+    }
+
+    return Ok(RngSource::Node(node_require("crypto")));
+}
+
+#[wasm_bindgen]
+extern "C" {
+    type Global;
+    #[wasm_bindgen(getter, catch, static_method_of = Global, js_class = self, js_name = self)]
+    fn get_self() -> Result<Self_, JsValue>;
+
+    type Self_;
+    #[wasm_bindgen(method, getter, structural)]
+    fn crypto(me: &Self_) -> JsValue;
+
+    #[derive(Clone, Debug)]
+    type BrowserCrypto;
+
+    // TODO: these `structural` annotations here ideally wouldn't be here to
+    // avoid a JS shim, but for now with feature detection they're
+    // unavoidable.
+    #[wasm_bindgen(method, js_name = getRandomValues, structural, getter)]
+    fn get_random_values_fn(me: &BrowserCrypto) -> JsValue;
+    #[wasm_bindgen(method, js_name = getRandomValues, structural)]
+    fn get_random_values(me: &BrowserCrypto, buf: &mut [u8]);
+
+    #[wasm_bindgen(js_name = require)]
+    fn node_require(s: &str) -> NodeCrypto;
+
+    #[derive(Clone, Debug)]
+    type NodeCrypto;
+
+    #[wasm_bindgen(method, js_name = randomFillSync, structural)]
+    fn random_fill_sync(me: &NodeCrypto, buf: &mut [u8]);
+}
diff --git a/src/wasm32_stdweb.rs b/src/wasm32_stdweb.rs
new file mode 100644
index 0000000..6e5e78a
--- /dev/null
+++ b/src/wasm32_stdweb.rs
@@ -0,0 +1,114 @@
+// Copyright 2018 Developers of the Rand project.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// https://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or https://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+//! Implementation for WASM via stdweb
+extern crate std;
+
+use core::mem;
+
+use stdweb::js;
+use stdweb::unstable::TryInto;
+use stdweb::web::error::Error as WebError;
+
+use crate::error::{STDWEB_NO_RNG, STDWEB_RNG_FAILED};
+use crate::Error;
+use std::sync::Once;
+
+#[derive(Clone, Copy, Debug)]
+enum RngSource {
+    Browser,
+    Node,
+}
+
+pub fn getrandom_inner(dest: &mut [u8]) -> Result<(), Error> {
+    assert_eq!(mem::size_of::<usize>(), 4);
+    static ONCE: Once = Once::new();
+    static mut RNG_SOURCE: Result<RngSource, Error> = Ok(RngSource::Node);
+
+    // SAFETY: RNG_SOURCE is only written once, before being read.
+    ONCE.call_once(|| unsafe {
+        RNG_SOURCE = getrandom_init();
+    });
+    getrandom_fill(unsafe { RNG_SOURCE }?, dest)
+}
+
+fn getrandom_init() -> Result<RngSource, Error> {
+    let result = js! {
+        try {
+            if (
+                typeof self === "object" &&
+                typeof self.crypto === "object" &&
+                typeof self.crypto.getRandomValues === "function"
+            ) {
+                return { success: true, ty: 1 };
+            }
+
+            if (typeof require("crypto").randomBytes === "function") {
+                return { success: true, ty: 2 };
+            }
+
+            return { success: false, error: new Error("not supported") };
+        } catch(err) {
+            return { success: false, error: err };
+        }
+    };
+
+    if js! { return @{ result.as_ref() }.success } == true {
+        let ty = js! { return @{ result }.ty };
+
+        if ty == 1 {
+            Ok(RngSource::Browser)
+        } else if ty == 2 {
+            Ok(RngSource::Node)
+        } else {
+            unreachable!()
+        }
+    } else {
+        let _err: WebError = js! { return @{ result }.error }.try_into().unwrap();
+        error!("getrandom unavailable: {}", _err);
+        Err(STDWEB_NO_RNG)
+    }
+}
+
+fn getrandom_fill(source: RngSource, dest: &mut [u8]) -> Result<(), Error> {
+    for chunk in dest.chunks_mut(65536) {
+        let len = chunk.len() as u32;
+        let ptr = chunk.as_mut_ptr() as i32;
+
+        let result = match source {
+            RngSource::Browser => js! {
+                try {
+                    let array = new Uint8Array(@{ len });
+                    self.crypto.getRandomValues(array);
+                    HEAPU8.set(array, @{ ptr });
+
+                    return { success: true };
+                } catch(err) {
+                    return { success: false, error: err };
+                }
+            },
+            RngSource::Node => js! {
+                try {
+                    let bytes = require("crypto").randomBytes(@{ len });
+                    HEAPU8.set(new Uint8Array(bytes), @{ ptr });
+
+                    return { success: true };
+                } catch(err) {
+                    return { success: false, error: err };
+                }
+            },
+        };
+
+        if js! { return @{ result.as_ref() }.success } != true {
+            let _err: WebError = js! { return @{ result }.error }.try_into().unwrap();
+            error!("getrandom failed: {}", _err);
+            return Err(STDWEB_RNG_FAILED);
+        }
+    }
+    Ok(())
+}
diff --git a/src/windows.rs b/src/windows.rs
new file mode 100644
index 0000000..e1b8df6
--- /dev/null
+++ b/src/windows.rs
@@ -0,0 +1,26 @@
+// Copyright 2018 Developers of the Rand project.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// https://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or https://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+//! Implementation for Windows
+use crate::{error::RTL_GEN_RANDOM_FAILED, Error};
+
+extern "system" {
+    #[link_name = "SystemFunction036"]
+    fn RtlGenRandom(RandomBuffer: *mut u8, RandomBufferLength: u32) -> u8;
+}
+
+pub fn getrandom_inner(dest: &mut [u8]) -> Result<(), Error> {
+    // Prevent overflow of u32
+    for chunk in dest.chunks_mut(u32::max_value() as usize) {
+        let ret = unsafe { RtlGenRandom(chunk.as_mut_ptr(), chunk.len() as u32) };
+        if ret == 0 {
+            return Err(RTL_GEN_RANDOM_FAILED);
+        }
+    }
+    Ok(())
+}
diff --git a/src/windows_uwp.rs b/src/windows_uwp.rs
new file mode 100644
index 0000000..586c6f6
--- /dev/null
+++ b/src/windows_uwp.rs
@@ -0,0 +1,59 @@
+// Copyright 2018 Developers of the Rand project.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// https://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or https://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+//! Implementation for Windows UWP targets. After deprecation of Windows XP
+//! and Vista, this can supersede the `RtlGenRandom`-based implementation.
+use crate::Error;
+use core::{ffi::c_void, num::NonZeroU32, ptr};
+
+const BCRYPT_USE_SYSTEM_PREFERRED_RNG: u32 = 0x00000002;
+
+extern "system" {
+    fn BCryptGenRandom(
+        hAlgorithm: *mut c_void,
+        pBuffer: *mut u8,
+        cbBuffer: u32,
+        dwFlags: u32,
+    ) -> u32;
+}
+
+pub fn getrandom_inner(dest: &mut [u8]) -> Result<(), Error> {
+    // Prevent overflow of u32
+    for chunk in dest.chunks_mut(u32::max_value() as usize) {
+        let ret = unsafe {
+            BCryptGenRandom(
+                ptr::null_mut(),
+                chunk.as_mut_ptr(),
+                chunk.len() as u32,
+                BCRYPT_USE_SYSTEM_PREFERRED_RNG,
+            )
+        };
+        // NTSTATUS codes use two highest bits for severity status
+        match ret >> 30 {
+            0b01 => {
+                info!("BCryptGenRandom: information code 0x{:08X}", ret);
+            }
+            0b10 => {
+                warn!("BCryptGenRandom: warning code 0x{:08X}", ret);
+            }
+            0b11 => {
+                error!("BCryptGenRandom: failed with 0x{:08X}", ret);
+                // We zeroize the highest bit, so the error code will reside
+                // inside the range of designated for OS codes.
+                let code = ret ^ (1 << 31);
+                // SAFETY: the second highest bit is always equal to one,
+                // so it's impossible to get zero. Unfortunately compiler
+                // is not smart enough to figure out it yet.
+                let code = unsafe { NonZeroU32::new_unchecked(code) };
+                return Err(Error::from(code));
+            }
+            _ => (),
+        }
+    }
+    Ok(())
+}