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+// 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.
+
+//! An implementation which calls out to an externally defined function.
+use crate::Error;
+use core::num::NonZeroU32;
+
+/// Register a function to be invoked by `getrandom` on unsupported targets.
+///
+/// ## Writing a custom `getrandom` implementation
+///
+/// The function to register must have the same signature as
+/// [`getrandom::getrandom`](crate::getrandom). The function can be defined
+/// wherever you want, either in root crate or a dependant crate.
+///
+/// For example, if we wanted a `failure-getrandom` crate containing an
+/// implementation that always fails, we would first depend on `getrandom`
+/// (for the [`Error`] type) in `failure-getrandom/Cargo.toml`:
+/// ```toml
+/// [dependencies]
+/// getrandom = "0.2"
+/// ```
+/// Note that the crate containing this function does **not** need to enable the
+/// `"custom"` Cargo feature.
+///
+/// Next, in `failure-getrandom/src/lib.rs`, we define our function:
+/// ```rust
+/// use core::num::NonZeroU32;
+/// use getrandom::Error;
+///
+/// // Some application-specific error code
+/// const MY_CUSTOM_ERROR_CODE: u32 = Error::CUSTOM_START + 42;
+/// pub fn always_fail(buf: &mut [u8]) -> Result<(), Error> {
+///     let code = NonZeroU32::new(MY_CUSTOM_ERROR_CODE).unwrap();
+///     Err(Error::from(code))
+/// }
+/// ```
+///
+/// ## Registering a custom `getrandom` implementation
+///
+/// Functions can only be registered in the root binary crate. Attempting to
+/// register a function in a non-root crate will result in a linker error.
+/// This is similar to
+/// [`#[panic_handler]`](https://doc.rust-lang.org/nomicon/panic-handler.html) or
+/// [`#[global_allocator]`](https://doc.rust-lang.org/edition-guide/rust-2018/platform-and-target-support/global-allocators.html),
+/// where helper crates define handlers/allocators but only the binary crate
+/// actually _uses_ the functionality.
+///
+/// To register the function, we first depend on `failure-getrandom` _and_
+/// `getrandom` in `Cargo.toml`:
+/// ```toml
+/// [dependencies]
+/// failure-getrandom = "0.1"
+/// getrandom = { version = "0.2", features = ["custom"] }
+/// ```
+///
+/// Then, we register the function in `src/main.rs`:
+/// ```rust
+/// # mod failure_getrandom { pub fn always_fail(_: &mut [u8]) -> Result<(), getrandom::Error> { unimplemented!() } }
+/// use failure_getrandom::always_fail;
+/// use getrandom::register_custom_getrandom;
+///
+/// register_custom_getrandom!(always_fail);
+/// ```
+///
+/// Now any user of `getrandom` (direct or indirect) on this target will use the
+/// registered function. As noted in the
+/// [top-level documentation](index.html#custom-implementations) this
+/// registration only has an effect on unsupported targets.
+#[macro_export]
+#[cfg_attr(docsrs, doc(cfg(feature = "custom")))]
+macro_rules! register_custom_getrandom {
+    ($path:path) => {
+        // We use an extern "C" function to get the guarantees of a stable ABI.
+        #[no_mangle]
+        extern "C" fn __getrandom_custom(dest: *mut u8, len: usize) -> u32 {
+            let f: fn(&mut [u8]) -> Result<(), ::getrandom::Error> = $path;
+            let slice = unsafe { ::core::slice::from_raw_parts_mut(dest, len) };
+            match f(slice) {
+                Ok(()) => 0,
+                Err(e) => e.code().get(),
+            }
+        }
+    };
+}
+
+#[allow(dead_code)]
+pub fn getrandom_inner(dest: &mut [u8]) -> Result<(), Error> {
+    extern "C" {
+        fn __getrandom_custom(dest: *mut u8, len: usize) -> u32;
+    }
+    let ret = unsafe { __getrandom_custom(dest.as_mut_ptr(), dest.len()) };
+    match NonZeroU32::new(ret) {
+        None => Ok(()),
+        Some(code) => Err(Error::from(code)),
+    }
+}