add support for openat

Author: Mic92

CHANGELOG.md

@@ -6,6 +6,8 @@ This project adheres to [Semantic Versioning](http://semver.org/).
 ## [Unreleased]
 
 <!--### Added-->
+- Added `openat` in `::nix::unistd`
+  ([#497](https://github.com/nix-rust/nix/pull/551))
 
 ### Changed
 - Marked `sys::mman::{ mmap, munmap, madvise, munlock, msync }` as unsafe.

src/fcntl.rs

@@ -18,6 +18,25 @@ mod ffi {
     pub const F_GET_SEALS: c_int = 1034;
 }
 
+#[cfg(not(any(target_os = "ios", target_os = "macos")))]
+libc_bitflags!{
+    pub flags AtFlags: c_int {
+        AT_SYMLINK_NOFOLLOW,
+        #[cfg(any(target_os = "linux", target_os = "android"))]
+        AT_NO_AUTOMOUNT,
+        #[cfg(any(target_os = "linux", target_os = "android"))]
+        AT_EMPTY_PATH
+    }
+}
+
+#[cfg(any(target_os = "ios", target_os = "macos"))]
+bitflags!(
+    pub flags AtFlags: c_int {
+        // hack because bitflags require one entry
+        const EMPTY = 0x0
+    }
+);
+
 pub fn open<P: ?Sized + NixPath>(path: &P, oflag: OFlag, mode: Mode) -> Result<RawFd> {
     let fd = try!(path.with_nix_path(|cstr| {
         unsafe { libc::open(cstr.as_ptr(), oflag.bits(), mode.bits() as c_uint) }
@@ -26,6 +45,14 @@ pub fn open<P: ?Sized + NixPath>(path: &P, oflag: OFlag, mode: Mode) -> Result<R
     Errno::result(fd)
 }
 
+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() as c_uint) }
+    }));
+
+    Errno::result(fd)
+}
+
 pub enum FcntlArg<'a> {
     F_DUPFD(RawFd),
     F_DUPFD_CLOEXEC(RawFd),

test/test_fcntl.rs

@@ -1,3 +1,31 @@
+use nix::fcntl::{openat, open, O_PATH, O_RDONLY};
+use nix::sys::stat::Mode;
+use nix::unistd::{close, read};
+use tempfile::NamedTempFile;
+use std::io::prelude::*;
+
+#[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();
+}
+
 #[cfg(any(target_os = "linux", target_os = "android"))]
 mod linux_android {
     use std::io::prelude::*;