Fix sendmsg on macOS when passing a zero entry cmsgs array.

CHANGELOG.md

@@ -60,6 +60,8 @@ This project adheres to [Semantic Versioning](http://semver.org/).
 - Added `nix::ptrace` on all Linux-kernel-based platforms
   [#624](https://github.com/nix-rust/nix/pull/624). Previously it was
   only available on x86, x86-64, and ARM, and also not on Android.
+- Fixed `sys::socket::sendmsg` with zero entry `cmsgs` parameter.
+  ([#623](https://github.com/nix-rust/nix/pull/623))
 
 ## [0.8.1] 2017-04-16
 

src/sys/socket/mod.rs

@@ -258,14 +258,12 @@ pub fn sendmsg<'a>(fd: RawFd, iov: &[IoVec<&'a [u8]>], cmsgs: &[ControlMessage<'
         len += cmsg.len();
         capacity += cmsg.space();
     }
-    // Alignment hackery. Note that capacity is guaranteed to be a
-    // multiple of size_t. Note also that the resulting vector claims
-    // to have length == capacity, so it's presently uninitialized.
+    // Note that the resulting vector claims to have length == capacity,
+    // so it's presently uninitialized.
     let mut cmsg_buffer = unsafe {
         let mut vec = Vec::<u8>::with_capacity(len);
-        let ptr = vec.as_mut_ptr();
-        mem::forget(vec);
-        Vec::<u8>::from_raw_parts(ptr as *mut _, len, len)
+        vec.set_len(len);
+        vec
     };
     {
         let mut ptr = &mut cmsg_buffer[..];
@@ -279,12 +277,18 @@ pub fn sendmsg<'a>(fd: RawFd, iov: &[IoVec<&'a [u8]>], cmsgs: &[ControlMessage<'
         None => (0 as *const _, 0),
     };
 
+    let cmsg_ptr = if capacity > 0 {
+        cmsg_buffer.as_ptr() as *const c_void
+    } else {
+        ptr::null()
+    };
+
     let mhdr = msghdr {
         msg_name: name as *const c_void,
         msg_namelen: namelen,
         msg_iov: iov.as_ptr(),
         msg_iovlen: iov.len() as size_t,
-        msg_control: cmsg_buffer.as_ptr() as *const c_void,
+        msg_control: cmsg_ptr,
         msg_controllen: capacity as size_t,
         msg_flags: 0,
     };

test/sys/test_socket.rs

@@ -131,7 +131,7 @@ pub fn test_scm_rights() {
                 panic!("unexpected cmsg");
             }
         }
-        assert_eq!(msg.flags & (MSG_TRUNC | MSG_CTRUNC), MsgFlags::empty());
+        assert!(!msg.flags.intersects(MSG_TRUNC | MSG_CTRUNC));
         close(fd2).unwrap();
     }
 
@@ -145,6 +145,43 @@ pub fn test_scm_rights() {
     close(w).unwrap();
 }
 
+// Verify `sendmsg` builds a valid `msghdr` when passing an empty
+// `cmsgs` argument.  This should result in a msghdr with a nullptr
+// msg_control field and a msg_controllen of 0 when calling into the
+// raw `sendmsg`.
+#[test]
+pub fn test_sendmsg_empty_cmsgs() {
+    use nix::sys::uio::IoVec;
+    use nix::unistd::close;
+    use nix::sys::socket::{socketpair, sendmsg, recvmsg,
+                           AddressFamily, SockType, SockFlag,
+                           CmsgSpace, MsgFlags,
+                           MSG_TRUNC, MSG_CTRUNC};
+
+    let (fd1, fd2) = socketpair(AddressFamily::Unix, SockType::Stream, 0,
+                                SockFlag::empty())
+                     .unwrap();
+
+    {
+        let iov = [IoVec::from_slice(b"hello")];
+        assert_eq!(sendmsg(fd1, &iov, &[], MsgFlags::empty(), None).unwrap(), 5);
+        close(fd1).unwrap();
+    }
+
+    {
+        let mut buf = [0u8; 5];
+        let iov = [IoVec::from_mut_slice(&mut buf[..])];
+        let mut cmsgspace: CmsgSpace<[RawFd; 1]> = CmsgSpace::new();
+        let msg = recvmsg(fd2, &iov, Some(&mut cmsgspace), MsgFlags::empty()).unwrap();
+
+        for _ in msg.cmsgs() {
+            panic!("unexpected cmsg");
+        }
+        assert!(!msg.flags.intersects(MSG_TRUNC | MSG_CTRUNC));
+        close(fd2).unwrap();
+    }
+}
+
 // Test creating and using named unix domain sockets
 #[test]
 pub fn test_unixdomain() {