Use libc function declarations for errno getters

Author: Bryant Mairs

src/errno.rs

@@ -1,40 +1,35 @@
-use libc::c_int;
+use libc::{self, c_int};
 use std::{fmt, io, error};
 use {Error, Result};
 
 pub use self::consts::*;
 pub use self::consts::Errno::*;
 
-#[cfg(any(target_os = "macos",
-          target_os = "ios",
-          target_os = "freebsd"))]
-unsafe fn errno_location() -> *mut c_int {
-    extern { fn __error() -> *mut c_int; }
-    __error()
-}
-
-#[cfg(target_os = "dragonfly")]
-unsafe fn errno_location() -> *mut c_int {
-    extern { fn __dfly_error() -> *mut c_int; }
-    __dfly_error()
-}
-
-#[cfg(any(target_os = "openbsd", target_os = "netbsd"))]
-unsafe fn errno_location() -> *mut c_int {
-    extern { fn __errno() -> *mut c_int; }
-    __errno()
-}
-
-#[cfg(target_os = "linux")]
-unsafe fn errno_location() -> *mut c_int {
-    extern { fn __errno_location() -> *mut c_int; }
-    __errno_location()
-}
-
-#[cfg(target_os = "android")]
-unsafe fn errno_location() -> *mut c_int {
-    extern { fn __errno() -> *mut c_int; }
-    __errno()
+cfg_if! {
+    if #[cfg(any(target_os = "freebsd",
+                 target_os = "ios",
+                 target_os = "macos"))] {
+        unsafe fn errno_location() -> *mut c_int {
+            libc::__error()
+        }
+    } else if #[cfg(target_os = "dragonfly")] {
+        unsafe fn errno_location() -> *mut c_int {
+            // FIXME: Replace with errno-dragonfly crate as this is no longer the correct
+            //        implementation.
+            extern { fn __dfly_error() -> *mut c_int; }
+            __dfly_error()
+        }
+    } else if #[cfg(any(target_os = "android",
+                        target_os = "netbsd",
+                        target_os = "openbsd"))] {
+        unsafe fn errno_location() -> *mut c_int {
+            libc::__errno()
+        }
+    } else if #[cfg(target_os = "linux")] {
+        unsafe fn errno_location() -> *mut c_int {
+            libc::__errno_location()
+        }
+    }
 }
 
 /// Sets the platform-specific errno to no-error