add SO_MARK SetSockOpt for Linux

CHANGELOG.md

@@ -6,6 +6,8 @@ This project adheres to [Semantic Versioning](http://semver.org/).
 ## [Unreleased]
 
 ### Added
+- Added `SO_MARK` on Linux.
+- ([#873](https://github.com/nix-rust/nix/pull/873))
 - Added `getsid` in `::nix::unistd`
   ([#850](https://github.com/nix-rust/nix/pull/850))
 - Added `alarm`. ([#830](https://github.com/nix-rust/nix/pull/830))

src/sys/socket/sockopt.rs

@@ -253,6 +253,8 @@ sockopt_impl!(Both, IpTransparent, libc::SOL_IP, libc::IP_TRANSPARENT, bool);
 sockopt_impl!(Both, BindAny, libc::SOL_SOCKET, libc::SO_BINDANY, bool);
 #[cfg(target_os = "freebsd")]
 sockopt_impl!(Both, BindAny, libc::IPPROTO_IP, libc::IP_BINDANY, bool);
+#[cfg(target_os = "linux")]
+sockopt_impl!(Both, Mark, libc::SOL_SOCKET, libc::SO_MARK, u32);
 
 /*
  *
@@ -527,4 +529,24 @@ mod test {
         assert!(s_listening2);
         close(s).unwrap();
     }
+
+    #[cfg(target_os = "linux")]
+    #[test]
+    fn is_so_mark_functional() {
+        use super::super::*;
+        use ::unistd::Uid;
+        use ::std::io::{self, Write};
+
+        if !Uid::current().is_root() {
+            let stderr = io::stderr();
+            let mut handle = stderr.lock();
+            writeln!(handle, "SO_MARK requires root privileges. Skipping test.").unwrap();
+            return;
+        }
+
+        let s = socket(AddressFamily::Inet, SockType::Stream, SockFlag::empty(), None).unwrap();
+        setsockopt(s, super::Mark, &1337).unwrap();
+        let mark = getsockopt(s, super::Mark).unwrap();
+        assert_eq!(mark, 1337);
+    }
 }