diff --git a/Cargo.toml b/Cargo.toml
index 2b813cc1..c67beac3 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -22,7 +22,7 @@ log = { version = "0.4", optional = true }
 cfg-if = "0.1"
 
 [target.'cfg(any(unix, target_os = "redox", target_os = "wasi"))'.dependencies]
-libc = "0.2.54"
+libc = "0.2.60"
 
 [target.wasm32-unknown-unknown.dependencies]
 wasm-bindgen = { version = "0.2.29", optional = true }
diff --git a/src/cloudabi.rs b/src/cloudabi.rs
index e87359ee..d3d09289 100644
--- a/src/cloudabi.rs
+++ b/src/cloudabi.rs
@@ -17,14 +17,9 @@ extern "C" {
 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 code {}", code);
+        error!("cloudabi_sys_random_get: failed with {}", errno);
         Err(Error::from(code))
     } else {
         Ok(()) // Zero means success for CloudABI
     }
 }
-
-#[inline(always)]
-pub fn error_msg_inner(_: NonZeroU32) -> Option<&'static str> {
-    None
-}
diff --git a/src/dummy.rs b/src/dummy.rs
index b53f66c1..0c24ba0f 100644
--- a/src/dummy.rs
+++ b/src/dummy.rs
@@ -6,17 +6,9 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-//! A dummy implementation for unsupported targets which always returns
-//! `Err(Error::UNAVAILABLE)`
-use crate::Error;
-use core::num::NonZeroU32;
+//! A dummy implementation for unsupported targets which always fails
+use crate::{error::UNSUPPORTED, Error};
 
 pub fn getrandom_inner(_: &mut [u8]) -> Result<(), Error> {
-    error!("no support for this platform");
-    Err(Error::UNAVAILABLE)
-}
-
-#[inline(always)]
-pub fn error_msg_inner(_: NonZeroU32) -> Option<&'static str> {
-    None
+    Err(UNSUPPORTED)
 }
diff --git a/src/error.rs b/src/error.rs
index a83ea111..a61f2e4d 100644
--- a/src/error.rs
+++ b/src/error.rs
@@ -5,78 +5,151 @@
 // <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::convert::From;
 use core::fmt;
 use core::num::NonZeroU32;
 
-// A randomly-chosen 24-bit prefix for our codes
-pub(crate) const CODE_PREFIX: u32 = 0x57f4c500;
-const CODE_UNKNOWN: u32 = CODE_PREFIX | 0x00;
-const CODE_UNAVAILABLE: u32 = CODE_PREFIX | 0x01;
-
-/// The error type.
+/// 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.
 ///
-/// This type is small and no-std compatible.
+/// 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(pub(crate) NonZeroU32);
+pub struct Error(NonZeroU32);
 
 impl Error {
-    /// An unknown error.
-    pub const UNKNOWN: Error = Error(unsafe { NonZeroU32::new_unchecked(CODE_UNKNOWN) });
+    #[deprecated(since = "0.1.7")]
+    pub const UNKNOWN: Error = UNSUPPORTED;
+    #[deprecated(since = "0.1.7")]
+    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;
 
-    /// No generator is available.
-    pub const UNAVAILABLE: Error = Error(unsafe { NonZeroU32::new_unchecked(CODE_UNAVAILABLE) });
+    /// 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 error code.
+    /// Extract the raw OS error code (if this error came from the OS)
     ///
-    /// This may equal one of the codes defined in this library or may be a
-    /// system error code.
+    /// 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 `Diplay` 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.
     ///
-    /// One may attempt to format this error via the `Display` implementation.
+    /// 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
     }
+}
 
-    pub(crate) fn msg(&self) -> Option<&'static str> {
-        if let Some(msg) = crate::imp::error_msg_inner(self.0) {
-            Some(msg)
-        } else {
-            match *self {
-                Error::UNKNOWN => Some("getrandom: unknown error"),
-                Error::UNAVAILABLE => Some("getrandom: unavailable"),
-                _ => None,
-            }
-        }
+#[cfg(any(unix, target_os = "redox"))]
+fn os_err_desc(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 idx = buf.iter().position(|&b| b == 0).unwrap_or(buf.len());
+    core::str::from_utf8(&buf[..idx]).ok()
+}
+
+#[cfg(not(any(unix, target_os = "redox")))]
+fn os_err_desc(_errno: i32, _buf: &mut [u8]) -> Option<&str> {
+    None
 }
 
 impl fmt::Debug for Error {
-    fn fmt(&self, f: &mut fmt::Formatter) -> Result<(), fmt::Error> {
-        match self.msg() {
-            Some(msg) => write!(f, "Error(\"{}\")", msg),
-            None => write!(f, "Error(0x{:08X})", self.0),
+    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(desc) = os_err_desc(errno, &mut buf) {
+                dbg.field("description", &desc);
+            }
+        } 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) -> Result<(), fmt::Error> {
-        match self.msg() {
-            Some(msg) => write!(f, "{}", msg),
-            None => write!(f, "getrandom: unknown code 0x{:08X}", self.0),
+    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_desc(errno, &mut buf) {
+                Some(desc) => f.write_str(desc),
+                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 {
-        Error(code)
+        Self(code)
     }
 }
 
-impl From<&Error> for Error {
-    fn from(error: &Error) -> Self {
-        *error
+// 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);
+
+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"),
+        _ => None,
     }
 }
 
diff --git a/src/error_impls.rs b/src/error_impls.rs
index 6bd2c69a..007472e4 100644
--- a/src/error_impls.rs
+++ b/src/error_impls.rs
@@ -7,28 +7,29 @@
 // except according to those terms.
 extern crate std;
 
-use crate::error::Error;
+use crate::{error::UNKNOWN_IO_ERROR, Error};
 use core::convert::From;
 use core::num::NonZeroU32;
-use std::{error, io};
+use std::io;
 
 impl From<io::Error> for Error {
     fn from(err: io::Error) -> Self {
-        err.raw_os_error()
-            .and_then(|code| NonZeroU32::new(code as u32))
-            .map(|code| Error(code))
-            // in practice this should never happen
-            .unwrap_or(Error::UNKNOWN)
+        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.msg() {
-            Some(msg) => io::Error::new(io::ErrorKind::Other, msg),
-            None => io::Error::from_raw_os_error(err.0.get() as i32),
+        match err.raw_os_error() {
+            Some(errno) => io::Error::from_raw_os_error(errno),
+            None => io::Error::new(io::ErrorKind::Other, err),
         }
     }
 }
 
-impl error::Error for Error {}
+impl std::error::Error for Error {}
diff --git a/src/freebsd.rs b/src/freebsd.rs
index aa77b221..61feaf73 100644
--- a/src/freebsd.rs
+++ b/src/freebsd.rs
@@ -9,7 +9,6 @@
 //! Implementation for FreeBSD
 use crate::util_libc::{sys_fill_exact, Weak};
 use crate::Error;
-use core::num::NonZeroU32;
 use core::{mem, ptr};
 
 type GetRandomFn = unsafe extern "C" fn(*mut u8, libc::size_t, libc::c_uint) -> libc::ssize_t;
@@ -44,8 +43,3 @@ pub fn getrandom_inner(dest: &mut [u8]) -> Result<(), Error> {
         sys_fill_exact(dest, kern_arnd)
     }
 }
-
-#[inline(always)]
-pub fn error_msg_inner(_: NonZeroU32) -> Option<&'static str> {
-    None
-}
diff --git a/src/fuchsia.rs b/src/fuchsia.rs
index a9f53888..572ff534 100644
--- a/src/fuchsia.rs
+++ b/src/fuchsia.rs
@@ -8,7 +8,6 @@
 
 //! Implementation for Fuchsia Zircon
 use crate::Error;
-use core::num::NonZeroU32;
 
 #[link(name = "zircon")]
 extern "C" {
@@ -19,8 +18,3 @@ pub fn getrandom_inner(dest: &mut [u8]) -> Result<(), Error> {
     unsafe { zx_cprng_draw(dest.as_mut_ptr(), dest.len()) }
     Ok(())
 }
-
-#[inline(always)]
-pub fn error_msg_inner(_: NonZeroU32) -> Option<&'static str> {
-    None
-}
diff --git a/src/ios.rs b/src/ios.rs
index 97d68979..30c008c2 100644
--- a/src/ios.rs
+++ b/src/ios.rs
@@ -7,10 +7,7 @@
 // except according to those terms.
 
 //! Implementation for iOS
-extern crate std;
-
-use crate::Error;
-use core::num::NonZeroU32;
+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
@@ -27,14 +24,8 @@ extern "C" {
 pub fn getrandom_inner(dest: &mut [u8]) -> Result<(), Error> {
     let ret = unsafe { SecRandomCopyBytes(kSecRandomDefault, dest.len(), dest.as_mut_ptr()) };
     if ret == -1 {
-        error!("SecRandomCopyBytes call failed");
-        Err(Error::UNKNOWN)
+        Err(SEC_RANDOM_FAILED)
     } else {
         Ok(())
     }
 }
-
-#[inline(always)]
-pub fn error_msg_inner(_: NonZeroU32) -> Option<&'static str> {
-    None
-}
diff --git a/src/lib.rs b/src/lib.rs
index d98d8584..03e1cc48 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -79,25 +79,8 @@
 //! `getrandom`, hence after the first successful call one can be reasonably
 //! confident that no errors will occur.
 //!
-//! On unsupported platforms, `getrandom` always fails with [`Error::UNAVAILABLE`].
-//!
-//! ## Error codes
-//! The crate uses the following custom error codes:
-//! - `0x57f4c500` (dec: 1475659008) - an unknown error. Constant:
-//! [`Error::UNKNOWN`]
-//! - `0x57f4c501` (dec: 1475659009) - no generator is available. Constant:
-//! [`Error::UNAVAILABLE`]
-//! - `0x57f4c580` (dec: 1475659136) - `self.crypto` is undefined,
-//! `wasm-bindgen` specific error.
-//! - `0x57f4c581` (dec: 1475659137) - `crypto.getRandomValues` is undefined,
-//! `wasm-bindgen` specific error.
-//!
-//! These codes are provided for reference only and should not be matched upon
-//! (but you can match on `Error` constants). The codes may change in future and
-//! such change will not be considered a breaking one.
-//!
-//! Other error codes will originate from an underlying system. In case if such
-//! error is encountered, please consult with your system documentation.
+//! 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
@@ -146,14 +129,12 @@ cfg_if! {
     }
 }
 
-#[cfg(feature = "std")]
-extern crate std;
-
 mod error;
 pub use crate::error::Error;
 
 #[allow(dead_code)]
 mod util;
+// Unlike the other Unix, Fuchsia and iOS don't use the libc to make any calls.
 #[cfg(any(
     target_os = "android",
     target_os = "dragonfly",
@@ -197,7 +178,6 @@ mod error_impls;
     target_os = "solaris",
     target_os = "illumos",
 ))]
-#[allow(dead_code)]
 mod use_file;
 
 // System-specific implementations.
diff --git a/src/linux_android.rs b/src/linux_android.rs
index 8aa1d58d..0f10e08f 100644
--- a/src/linux_android.rs
+++ b/src/linux_android.rs
@@ -7,13 +7,9 @@
 // except according to those terms.
 
 //! Implementation for Linux / Android
-extern crate std;
-
 use crate::util::LazyBool;
-use crate::util_libc::sys_fill_exact;
+use crate::util_libc::{last_os_error, sys_fill_exact};
 use crate::{use_file, Error};
-use core::num::NonZeroU32;
-use std::io;
 
 pub fn getrandom_inner(dest: &mut [u8]) -> Result<(), Error> {
     static HAS_GETRANDOM: LazyBool = LazyBool::new();
@@ -29,7 +25,7 @@ pub fn getrandom_inner(dest: &mut [u8]) -> Result<(), Error> {
 fn is_getrandom_available() -> bool {
     let res = unsafe { libc::syscall(libc::SYS_getrandom, 0, 0, libc::GRND_NONBLOCK) };
     if res < 0 {
-        match io::Error::last_os_error().raw_os_error() {
+        match last_os_error().raw_os_error() {
             Some(libc::ENOSYS) => false, // No kernel support
             Some(libc::EPERM) => false,  // Blocked by seccomp
             _ => true,
@@ -38,8 +34,3 @@ fn is_getrandom_available() -> bool {
         true
     }
 }
-
-#[inline(always)]
-pub fn error_msg_inner(_: NonZeroU32) -> Option<&'static str> {
-    None
-}
diff --git a/src/macos.rs b/src/macos.rs
index 84d95565..c3bc5334 100644
--- a/src/macos.rs
+++ b/src/macos.rs
@@ -7,13 +7,9 @@
 // except according to those terms.
 
 //! Implementation for macOS
-extern crate std;
-
-use crate::util_libc::Weak;
+use crate::util_libc::{last_os_error, Weak};
 use crate::{use_file, Error};
 use core::mem;
-use core::num::NonZeroU32;
-use std::io;
 
 type GetEntropyFn = unsafe extern "C" fn(*mut u8, libc::size_t) -> libc::c_int;
 
@@ -24,8 +20,9 @@ pub fn getrandom_inner(dest: &mut [u8]) -> Result<(), Error> {
         for chunk in dest.chunks_mut(256) {
             let ret = unsafe { func(chunk.as_mut_ptr(), chunk.len()) };
             if ret != 0 {
-                error!("getentropy syscall failed with ret={}", ret);
-                return Err(io::Error::last_os_error().into());
+                let err = last_os_error();
+                error!("getentropy syscall failed");
+                return Err(err);
             }
         }
         Ok(())
@@ -35,8 +32,3 @@ pub fn getrandom_inner(dest: &mut [u8]) -> Result<(), Error> {
         use_file::getrandom_inner(dest)
     }
 }
-
-#[inline(always)]
-pub fn error_msg_inner(_: NonZeroU32) -> Option<&'static str> {
-    None
-}
diff --git a/src/openbsd.rs b/src/openbsd.rs
index e213e5f5..e1ac179f 100644
--- a/src/openbsd.rs
+++ b/src/openbsd.rs
@@ -7,24 +7,17 @@
 // except according to those terms.
 
 //! Implementation for OpenBSD
-extern crate std;
-
+use crate::util_libc::last_os_error;
 use crate::Error;
-use core::num::NonZeroU32;
-use std::io;
 
 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(io::Error::last_os_error().into());
+            return Err(err);
         }
     }
     Ok(())
 }
-
-#[inline(always)]
-pub fn error_msg_inner(_: NonZeroU32) -> Option<&'static str> {
-    None
-}
diff --git a/src/rdrand.rs b/src/rdrand.rs
index c55d2074..96229673 100644
--- a/src/rdrand.rs
+++ b/src/rdrand.rs
@@ -7,12 +7,12 @@
 // 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;
-use core::num::NonZeroU32;
 
 // Recommendation from "Intel® Digital Random Number Generator (DRNG) Software
 // Implementation Guide" - Section 5.2.1 and "Intel® 64 and IA-32 Architectures
@@ -37,8 +37,7 @@ unsafe fn rdrand() -> Result<[u8; WORD_SIZE], Error> {
             // Keep looping in case this was a false positive.
         }
     }
-    error!("RDRAND failed, CPU issue likely");
-    Err(Error::UNKNOWN)
+    Err(FAILED_RDRAND)
 }
 
 // "rdrand" target feature requires "+rdrnd" flag, see https://github.com/rust-lang/rust/issues/49653.
@@ -65,7 +64,7 @@ fn is_rdrand_supported() -> bool {
 
 pub fn getrandom_inner(dest: &mut [u8]) -> Result<(), Error> {
     if !is_rdrand_supported() {
-        return Err(Error::UNAVAILABLE);
+        return Err(NO_RDRAND);
     }
 
     // SAFETY: After this point, rdrand is supported, so calling the rdrand
@@ -89,8 +88,3 @@ unsafe fn rdrand_exact(dest: &mut [u8]) -> Result<(), Error> {
     }
     Ok(())
 }
-
-#[inline(always)]
-pub fn error_msg_inner(_: NonZeroU32) -> Option<&'static str> {
-    None
-}
diff --git a/src/solaris_illumos.rs b/src/solaris_illumos.rs
index 8539971b..7550fce9 100644
--- a/src/solaris_illumos.rs
+++ b/src/solaris_illumos.rs
@@ -20,7 +20,6 @@
 use crate::util_libc::{sys_fill_exact, Weak};
 use crate::{use_file, Error};
 use core::mem;
-use core::num::NonZeroU32;
 
 #[cfg(target_os = "illumos")]
 type GetRandomFn = unsafe extern "C" fn(*mut u8, libc::size_t, libc::c_uint) -> libc::ssize_t;
@@ -43,8 +42,3 @@ pub fn getrandom_inner(dest: &mut [u8]) -> Result<(), Error> {
         use_file::getrandom_inner(dest)
     }
 }
-
-#[inline(always)]
-pub fn error_msg_inner(_: NonZeroU32) -> Option<&'static str> {
-    None
-}
diff --git a/src/use_file.rs b/src/use_file.rs
index adc1b3cd..74a12ef7 100644
--- a/src/use_file.rs
+++ b/src/use_file.rs
@@ -9,15 +9,11 @@
 //! Implementations that just need to read from a file
 extern crate std;
 
-use crate::util_libc::LazyFd;
+use crate::util_libc::{last_os_error, LazyFd};
 use crate::Error;
 use core::mem::ManuallyDrop;
-use core::num::NonZeroU32;
 use std::os::unix::io::{FromRawFd, IntoRawFd, RawFd};
-use std::{
-    fs::File,
-    io::{self, Read},
-};
+use std::{fs::File, io::Read};
 
 #[cfg(target_os = "redox")]
 const FILE_PATH: &str = "rand:";
@@ -35,7 +31,7 @@ const FILE_PATH: &str = "/dev/random";
 
 pub fn getrandom_inner(dest: &mut [u8]) -> Result<(), Error> {
     static FD: LazyFd = LazyFd::new();
-    let fd = FD.init(init_file).ok_or(io::Error::last_os_error())?;
+    let fd = FD.init(init_file).ok_or(last_os_error())?;
     let file = ManuallyDrop::new(unsafe { File::from_raw_fd(fd) });
     let mut file_ref: &File = &file;
 
@@ -60,9 +56,3 @@ fn init_file() -> Option<RawFd> {
     }
     Some(File::open(FILE_PATH).ok()?.into_raw_fd())
 }
-
-#[inline(always)]
-#[allow(dead_code)]
-pub fn error_msg_inner(_: NonZeroU32) -> Option<&'static str> {
-    None
-}
diff --git a/src/util_libc.rs b/src/util_libc.rs
index 606bc1ff..015d1a04 100644
--- a/src/util_libc.rs
+++ b/src/util_libc.rs
@@ -5,12 +5,34 @@
 // <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::ERRNO_NOT_POSITIVE;
 use crate::util::LazyUsize;
 use crate::Error;
+use core::num::NonZeroU32;
 use core::ptr::NonNull;
-use std::io;
+
+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", target_os = "dragonfly"))] {
+        use libc::__error as errno_location;
+    } else if #[cfg(target_os = "haiku")] {
+        use libc::_errnop as errno_location;
+    }
+}
+
+pub fn last_os_error() -> Error {
+    let errno = unsafe { *errno_location() };
+    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
@@ -22,10 +44,10 @@ pub fn sys_fill_exact(
     while !buf.is_empty() {
         let res = sys_fill(buf);
         if res < 0 {
-            let err = io::Error::last_os_error();
+            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.into());
+                return Err(err);
             }
         } else {
             // We don't check for EOF (ret = 0) as the data we are reading
diff --git a/src/wasi.rs b/src/wasi.rs
index 10b91788..0920752f 100644
--- a/src/wasi.rs
+++ b/src/wasi.rs
@@ -14,14 +14,9 @@ pub fn getrandom_inner(dest: &mut [u8]) -> Result<(), Error> {
     let ret =
         unsafe { libc::__wasi_random_get(dest.as_mut_ptr() as *mut libc::c_void, dest.len()) };
     if let Some(code) = NonZeroU32::new(ret as u32) {
-        error!("WASI: __wasi_random_get failed with return value {}", code);
+        error!("WASI: __wasi_random_get: failed with {}", ret);
         Err(Error::from(code))
     } else {
         Ok(()) // Zero means success for WASI
     }
 }
-
-#[inline(always)]
-pub fn error_msg_inner(_: NonZeroU32) -> Option<&'static str> {
-    None
-}
diff --git a/src/wasm32_bindgen.rs b/src/wasm32_bindgen.rs
index 08df426b..c6eef5fd 100644
--- a/src/wasm32_bindgen.rs
+++ b/src/wasm32_bindgen.rs
@@ -11,17 +11,13 @@ extern crate std;
 
 use core::cell::RefCell;
 use core::mem;
-use core::num::NonZeroU32;
 use std::thread_local;
 
 use wasm_bindgen::prelude::*;
 
-use crate::error::CODE_PREFIX;
+use crate::error::{BINDGEN_CRYPTO_UNDEF, BINDGEN_GRV_UNDEF};
 use crate::Error;
 
-const CODE_CRYPTO_UNDEF: u32 = CODE_PREFIX | 0x80;
-const CODE_GRV_UNDEF: u32 = CODE_PREFIX | 0x81;
-
 #[derive(Clone, Debug)]
 enum RngSource {
     Node(NodeCrypto),
@@ -83,17 +79,13 @@ fn getrandom_init() -> Result<RngSource, Error> {
     // we're in an older web browser and the OS RNG isn't available.
     let crypto = this.crypto();
     if crypto.is_undefined() {
-        return Err(Error::from(unsafe {
-            NonZeroU32::new_unchecked(CODE_CRYPTO_UNDEF)
-        }));
+        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(Error::from(unsafe {
-            NonZeroU32::new_unchecked(CODE_GRV_UNDEF)
-        }));
+        return Err(BINDGEN_GRV_UNDEF);
     }
 
     // Ok! `self.crypto.getRandomValues` is a defined value, so let's
@@ -101,15 +93,6 @@ fn getrandom_init() -> Result<RngSource, Error> {
     Ok(RngSource::Browser(crypto))
 }
 
-#[inline(always)]
-pub fn error_msg_inner(n: NonZeroU32) -> Option<&'static str> {
-    match n.get() {
-        CODE_CRYPTO_UNDEF => Some("getrandom: self.crypto is undefined"),
-        CODE_GRV_UNDEF => Some("crypto.getRandomValues is undefined"),
-        _ => None,
-    }
-}
-
 #[wasm_bindgen]
 extern "C" {
     type Function;
diff --git a/src/wasm32_stdweb.rs b/src/wasm32_stdweb.rs
index 32a5686c..c59c9c42 100644
--- a/src/wasm32_stdweb.rs
+++ b/src/wasm32_stdweb.rs
@@ -8,12 +8,12 @@
 
 //! Implementation for WASM via stdweb
 use core::mem;
-use core::num::NonZeroU32;
 
 use stdweb::unstable::TryInto;
 use stdweb::web::error::Error as WebError;
 use stdweb::{_js_impl, js};
 
+use crate::error::{STDWEB_NO_RNG, STDWEB_RNG_FAILED};
 use crate::Error;
 use std::sync::Once;
 
@@ -26,7 +26,7 @@ enum RngSource {
 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> = Err(Error::UNAVAILABLE);
+    static mut RNG_SOURCE: Result<RngSource, Error> = Ok(RngSource::Node);
 
     // SAFETY: RNG_SOURCE is only written once, before being read.
     ONCE.call_once(|| unsafe {
@@ -69,7 +69,7 @@ fn getrandom_init() -> Result<RngSource, Error> {
     } else {
         let err: WebError = js! { return @{ result }.error }.try_into().unwrap();
         error!("getrandom unavailable: {}", err);
-        Err(Error::UNAVAILABLE)
+        Err(STDWEB_NO_RNG)
     }
 }
 
@@ -105,13 +105,8 @@ fn getrandom_fill(source: RngSource, dest: &mut [u8]) -> Result<(), Error> {
         if js! { return @{ result.as_ref() }.success } != true {
             let err: WebError = js! { return @{ result }.error }.try_into().unwrap();
             error!("getrandom failed: {}", err);
-            return Err(Error::UNKNOWN);
+            return Err(STDWEB_RNG_FAILED);
         }
     }
     Ok(())
 }
-
-#[inline(always)]
-pub fn error_msg_inner(_: NonZeroU32) -> Option<&'static str> {
-    None
-}
diff --git a/src/windows.rs b/src/windows.rs
index c52e9f10..e1b8df66 100644
--- a/src/windows.rs
+++ b/src/windows.rs
@@ -7,10 +7,7 @@
 // except according to those terms.
 
 //! Implementation for Windows
-extern crate std;
-
-use crate::Error;
-use core::num::NonZeroU32;
+use crate::{error::RTL_GEN_RANDOM_FAILED, Error};
 
 extern "system" {
     #[link_name = "SystemFunction036"]
@@ -22,14 +19,8 @@ pub fn getrandom_inner(dest: &mut [u8]) -> Result<(), Error> {
     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 {
-            error!("RtlGenRandom call failed");
-            return Err(Error::UNKNOWN);
+            return Err(RTL_GEN_RANDOM_FAILED);
         }
     }
     Ok(())
 }
-
-#[inline(always)]
-pub fn error_msg_inner(_: NonZeroU32) -> Option<&'static str> {
-    None
-}