unistd: add {get,set}domainname

This implements `getdomainname(2)` and `setdomainname(2)`, which are
common UNIX extensions not covered by POSIX. References:
 * http://man7.org/linux/man-pages/man2/getdomainname.2.html
 * http://man7.org/linux/man-pages/man2/setdomainname.2.html

Author: lucab

CHANGELOG.md

@@ -6,6 +6,8 @@ This project adheres to [Semantic Versioning](http://semver.org/).
 ## [Unreleased]
 
 ### Added
+- Added `nix::unistd::{getdomainname, setdomainname}`.
+  ([#816](https://github.com/nix-rust/nix/pull/816))
 
 ### Changed
 - Display and Debug for SysControlAddr now includes all fields.

src/unistd.rs

@@ -760,6 +760,56 @@ pub fn gethostname() -> Result<CString> {
     })
 }
 
+cfg_if!{
+    if #[cfg(any(target_os = "dragonfly", target_os = "freebsd",
+                 target_os = "ios", target_os = "macos"))] {
+        type namelen_t = c_int;
+    } else {
+        type namelen_t = size_t;
+    }
+}
+
+/// Set the NIS domain name (see
+/// [setdomainname(2)](http://man7.org/linux/man-pages/man2/setdomainname.2.html)).
+#[cfg(not(target_os = "android"))]
+pub fn setdomainname<S: AsRef<OsStr>>(name: S) -> Result<()> {
+    let ptr = name.as_ref().as_bytes().as_ptr() as *const c_char;
+    let len = name.as_ref().len() as namelen_t;
+    let res = unsafe { libc::setdomainname(ptr, len) };
+    Errno::result(res).map(drop)
+}
+
+/// Get the NIS domain name, returning a `CString` on success (see
+/// [getdomainname(2)](http://man7.org/linux/man-pages/man2/getdomainname.2.html)).
+///
+/// This function call attempts to get the NIS domain name for the running system
+/// and return it as a NUL-terminated string.
+///
+/// # Examples
+///
+/// ```no_run
+/// use nix::unistd;
+///
+/// let domainname_cstr = unistd::getdomainname().expect("Failed getting domain name");
+/// let domainname = domainname_cstr.into_string().expect("Domain name wasn't valid UTF-8");
+/// println!("Domain name: {}", domainname);
+/// ```
+#[cfg(not(target_os = "android"))]
+pub fn getdomainname() -> Result<CString> {
+    const MAXDOMAINNAMELEN: usize = 256;
+    let buf = vec![0; MAXDOMAINNAMELEN];
+    let ptr = unsafe { CString::from_vec_unchecked(buf).into_raw() };
+
+    let res = unsafe { libc::getdomainname(ptr, MAXDOMAINNAMELEN as namelen_t) };
+    Errno::result(res).map(|_| {
+        unsafe {
+            // Ensure returned string is always null-terminated
+            *ptr.offset((MAXDOMAINNAMELEN - 1) as isize) = 0;
+            CString::from_raw(ptr)
+        }
+    })
+}
+
 /// Close a raw file descriptor
 ///
 /// Be aware that many Rust types implicitly close-on-drop, including

test/test_unistd.rs

@@ -408,3 +408,12 @@ fn test_gethostname() {
     let hn_string = hn.unwrap().into_string();
     assert!(hn_string.is_ok(), "Error: {:?}", hn_string.unwrap_err());
 }
+
+#[test]
+#[cfg(not(target_os = "android"))]
+fn test_getdomainname() {
+    let dn = getdomainname();
+    assert!(dn.is_ok(), "Error: {:?}", dn.unwrap_err());
+    let dn_string = dn.unwrap().into_string();
+    assert!(dn_string.is_ok(), "Error: {:?}", dn_string.unwrap_err());
+}