Add fchmod, fchmodat functions.

CHANGELOG.md

@@ -21,6 +21,8 @@ This project adheres to [Semantic Versioning](http://semver.org/).
   ([#825](https://github.com/nix-rust/nix/pull/825))
 - Exposed `MSG_CMSG_CLOEXEC` on *BSD.
   ([#825](https://github.com/nix-rust/nix/pull/825))
+- Added `fchmod`, `fchmodat`.
+  ([#857](https://github.com/nix-rust/nix/pull/857))
 
 ### Changed
 - Display and Debug for SysControlAddr now includes all fields.

src/sys/stat.rs

@@ -122,3 +122,64 @@ pub fn fstatat<P: ?Sized + NixPath>(dirfd: RawFd, pathname: &P, f: AtFlags) -> R
     Ok(dst)
 }
 
+/// Change the file permission bits of the file specified by a file descriptor.
+///
+/// # References
+///
+/// [fchmod(2)](http://pubs.opengroup.org/onlinepubs/9699919799/functions/fchmod.html).
+pub fn fchmod(fd: RawFd, mode: Mode) -> Result<()> {
+    let res = unsafe { libc::fchmod(fd, mode.bits() as mode_t) };
+
+    Errno::result(res).map(|_| ())
+}
+
+/// Flags for `fchmodat` function.
+#[derive(Clone, Copy, Debug)]
+pub enum FchmodatFlags {
+    FollowSymlink,
+    NoFollowSymlink,
+}
+
+/// Change the file permission bits.
+///
+/// The file to be changed is determined relative to the directory associated
+/// with the file descriptor `dirfd` or the current working directory
+/// if `dirfd` is `None`.
+///
+/// If `flag` is `FchmodatFlags::NoFollowSymlink` and `path` names a symbolic link,
+/// then the mode of the symbolic link is changed.
+///
+/// `fchmod(None, path, mode, FchmodatFlags::FollowSymlink)` is identical to
+/// a call `libc::chmod(path, mode)`. That's why `chmod` is unimplemented
+/// in the `nix` crate.
+///
+/// # References
+///
+/// [fchmodat(2)](http://pubs.opengroup.org/onlinepubs/9699919799/functions/fchmodat.html).
+pub fn fchmodat<P: ?Sized + NixPath>(
+    dirfd: Option<RawFd>,
+    path: &P,
+    mode: Mode,
+    flag: FchmodatFlags,
+) -> Result<()> {
+    let actual_dirfd =
+        match dirfd {
+            None => libc::AT_FDCWD,
+            Some(fd) => fd,
+        };
+    let atflag =
+        match flag {
+            FchmodatFlags::FollowSymlink => AtFlags::empty(),
+            FchmodatFlags::NoFollowSymlink => AtFlags::AT_SYMLINK_NOFOLLOW,
+        };
+    let res = path.with_nix_path(|cstr| unsafe {
+        libc::fchmodat(
+            actual_dirfd,
+            cstr.as_ptr(),
+            mode.bits() as mode_t,
+            atflag.bits() as libc::c_int,
+        )
+    })?;
+
+    Errno::result(res).map(|_| ())
+}

test/test_stat.rs

@@ -5,8 +5,9 @@ use std::os::unix::prelude::AsRawFd;
 use libc::{S_IFMT, S_IFLNK};
 
 use nix::fcntl;
-use nix::sys::stat::{self, stat, fstat, lstat};
-use nix::sys::stat::FileStat;
+use nix::sys::stat::{self, fchmod, fchmodat, fstat, lstat, stat};
+use nix::sys::stat::{FileStat, Mode, FchmodatFlags};
+use nix::unistd::chdir;
 use nix::Result;
 use tempdir::TempDir;
 
@@ -102,3 +103,52 @@ fn test_stat_fstat_lstat() {
     let fstat_result = fstat(link.as_raw_fd());
     assert_stat_results(fstat_result);
 }
+
+#[test]
+fn test_fchmod() {
+    let tempdir = TempDir::new("nix-test_fchmod").unwrap();
+    let filename = tempdir.path().join("foo.txt");
+    let file = File::create(&filename).unwrap();
+
+    let mut mode1 = Mode::empty();
+    mode1.insert(Mode::S_IRUSR);
+    mode1.insert(Mode::S_IWUSR);
+    fchmod(file.as_raw_fd(), mode1).unwrap();
+
+    let file_stat1 = stat(&filename).unwrap();
+    assert_eq!(file_stat1.st_mode & 0o7777, mode1.bits());
+
+    let mut mode2 = Mode::empty();
+    mode2.insert(Mode::S_IROTH);
+    fchmod(file.as_raw_fd(), mode2).unwrap();
+
+    let file_stat2 = stat(&filename).unwrap();
+    assert_eq!(file_stat2.st_mode & 0o7777, mode2.bits());
+}
+
+#[test]
+fn test_fchmodat() {
+    let tempdir = TempDir::new("nix-test_fchmodat").unwrap();
+    let filename = "foo.txt";
+    let fullpath = tempdir.path().join(filename);
+    File::create(&fullpath).unwrap();
+
+    let dirfd = fcntl::open(tempdir.path(), fcntl::OFlag::empty(), stat::Mode::empty()).unwrap();
+
+    let mut mode1 = Mode::empty();
+    mode1.insert(Mode::S_IRUSR);
+    mode1.insert(Mode::S_IWUSR);
+    fchmodat(Some(dirfd), filename, mode1, FchmodatFlags::FollowSymlink).unwrap();
+
+    let file_stat1 = stat(&fullpath).unwrap();
+    assert_eq!(file_stat1.st_mode & 0o7777, mode1.bits());
+
+    chdir(tempdir.path()).unwrap();
+
+    let mut mode2 = Mode::empty();
+    mode2.insert(Mode::S_IROTH);
+    fchmodat(None, filename, mode2, FchmodatFlags::FollowSymlink).unwrap();
+
+    let file_stat2 = stat(&fullpath).unwrap();
+    assert_eq!(file_stat2.st_mode & 0o7777, mode2.bits());
+}