Add renameat and AT_FDCWD

renameat is (somewhat oddly, imho) in stdio.h. I put it in
nix::fcntl because there's no nix::stdio and all its friends
(including the AT_* constants) are in nix::fcntl.

CHANGELOG.md

@@ -19,6 +19,8 @@ This project adheres to [Semantic Versioning](http://semver.org/).
   ([#1091](https://github.com/nix-rust/nix/pull/1091))
 - Add `unlinkat`
   ([#1058](https://github.com/nix-rust/nix/pull/1058))
+- Add `renameat` and `AT_FDCWD`.
+  ([#1097](https://github.com/nix-rust/nix/pull/1097))
 
 ### Changed
 - Support for `ifaddrs` now present when building for Android.

src/fcntl.rs

@@ -21,6 +21,8 @@ use sys::uio::IoVec;  // For vmsplice
           target_env = "freebsd"))]
 pub use self::posix_fadvise::*;
 
+pub const AT_FDCWD: RawFd = libc::AT_FDCWD as RawFd;
+
 libc_bitflags!{
     pub struct AtFlags: c_int {
         AT_REMOVEDIR;
@@ -165,6 +167,17 @@ pub fn openat<P: ?Sized + NixPath>(dirfd: RawFd, path: &P, oflag: OFlag, mode: M
     Errno::result(fd)
 }
 
+pub fn renameat<P1: ?Sized + NixPath, P2: ?Sized + NixPath>(old_dirfd: RawFd, old_path: &P1,
+                                                            new_dirfd: RawFd, new_path: &P2)
+                                                            -> Result<()> {
+    let res = old_path.with_nix_path(|old_cstr| {
+        new_path.with_nix_path(|new_cstr| unsafe {
+            libc::renameat(old_dirfd, old_cstr.as_ptr(), new_dirfd, new_cstr.as_ptr())
+        })
+    })??;
+    Errno::result(res).map(drop)
+}
+
 fn wrap_readlink_result(buffer: &mut[u8], res: ssize_t) -> Result<&OsStr> {
     match Errno::result(res) {
         Err(err) => Err(err),

test/test_fcntl.rs

@@ -1,7 +1,10 @@
-use nix::fcntl::{openat, open, OFlag, readlink, readlinkat};
+use nix::Error;
+use nix::errno::*;
+use nix::fcntl::{openat, open, OFlag, readlink, readlinkat, renameat};
 use nix::sys::stat::Mode;
 use nix::unistd::{close, read};
 use tempfile::{self, NamedTempFile};
+use std::fs::File;
 use std::io::prelude::*;
 use std::os::unix::fs;
 
@@ -27,6 +30,23 @@ fn test_openat() {
     close(dirfd).unwrap();
 }
 
+#[test]
+fn test_renameat() {
+    let old_dir = tempfile::tempdir().unwrap();
+    let old_dirfd = open(old_dir.path(), OFlag::empty(), Mode::empty()).unwrap();
+    let old_path = old_dir.path().join("old");
+    File::create(&old_path).unwrap();
+    let new_dir = tempfile::tempdir().unwrap();
+    let new_dirfd = open(new_dir.path(), OFlag::empty(), Mode::empty()).unwrap();
+    renameat(old_dirfd, "old", new_dirfd, "new").unwrap();
+    assert_eq!(renameat(old_dirfd, "old", new_dirfd, "new").unwrap_err(),
+               Error::Sys(Errno::ENOENT));
+    close(old_dirfd).unwrap();
+    close(new_dirfd).unwrap();
+    assert!(!old_path.exists());
+    assert!(new_dir.path().join("new").exists());
+}
+
 #[test]
 fn test_readlink() {
     let tempdir = tempfile::tempdir().unwrap();