add support for openat

CHANGELOG.md

@@ -6,6 +6,7 @@ This project adheres to [Semantic Versioning](http://semver.org/).
 ## [Unreleased]
 
 ### Added
+- Added `openat` in `::nix::fcntl`
 - Added support for XNU system control sockets
   ([#478](https://github.com/nix-rust/nix/pull/478))
 - Added support for `ioctl` calls on BSD platforms

src/fcntl.rs

@@ -26,6 +26,15 @@ pub fn open<P: ?Sized + NixPath>(path: &P, oflag: OFlag, mode: Mode) -> Result<R
     Errno::result(fd)
 }
 
+#[cfg(any(target_os = "linux", target_os = "android"))]
+pub fn openat<P: ?Sized + NixPath>(dirfd: RawFd, path: &P, oflag: OFlag, mode: Mode) -> Result<RawFd> {
+    let fd = try!(path.with_nix_path(|cstr| {
+        unsafe { libc::openat(dirfd, cstr.as_ptr(), oflag.bits(), mode.bits()) }
+    }));
+
+    Errno::result(fd)
+}
+
 pub enum FcntlArg<'a> {
     F_DUPFD(RawFd),
     F_DUPFD_CLOEXEC(RawFd),

test/test_fcntl.rs

@@ -5,11 +5,35 @@ mod linux_android {
 
     use libc::loff_t;
 
-    use nix::fcntl::{SpliceFFlags, splice, tee, vmsplice};
+    use nix::fcntl::{SpliceFFlags, splice, tee, vmsplice, openat, open, O_PATH, O_RDONLY};
+    use nix::sys::stat::Mode;
     use nix::sys::uio::IoVec;
     use nix::unistd::{close, pipe, read, write};
 
-    use tempfile::tempfile;
+    use tempfile::{tempfile, NamedTempFile};
+
+    #[test]
+    fn test_openat() {
+        const CONTENTS: &'static [u8] = b"abcd";
+        let mut tmp = NamedTempFile::new().unwrap();
+        tmp.write(CONTENTS).unwrap();
+
+        let dirfd = open(tmp.path().parent().unwrap(),
+                         O_PATH,
+                         Mode::empty()).unwrap();
+        let fd = openat(dirfd,
+                        tmp.path().file_name().unwrap(),
+                        O_RDONLY,
+                        Mode::empty()).unwrap();
+
+        let mut buf = [0u8; 1024];
+        assert_eq!(4, read(fd, &mut buf).unwrap());
+        assert_eq!(CONTENTS, &buf[0..4]);
+
+        close(fd).unwrap();
+        close(dirfd).unwrap();
+    }
+
 
     #[test]
     fn test_splice() {