From 242a6a02457ed298e3273f001bd76cca10dc2f08 Mon Sep 17 00:00:00 2001
From: Matthew Gregan <kinetik@flim.org>
Date: Tue, 20 Jun 2017 15:30:48 +1200
Subject: [PATCH] Fix sendmsg on macOS when passing a zero entry cmsgs array.

---
 CHANGELOG.md            |  2 ++
 src/sys/socket/mod.rs   |  8 +++++++-
 test/sys/test_socket.rs | 33 +++++++++++++++++++++++++++++++++
 3 files changed, 42 insertions(+), 1 deletion(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 9eb9b913a4..7499566f48 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -44,6 +44,8 @@ This project adheres to [Semantic Versioning](http://semver.org/).
   `MacOS/x86_64,i686` ([#553](https://github.com/nix-rust/nix/pull/553)),
   `NetBSD/x64_64` ([#538](https://github.com/nix-rust/nix/pull/538)), and
   `FreeBSD/x86_64,i686` ([#536](https://github.com/nix-rust/nix/pull/536)).
+- Fixed `sys::socket::sendmsg` with zero entry `cmsgs` parameter.
+  ([#623](https://github.com/nix-rust/nix/pull/623))
 
 ## [0.8.1] 2017-04-16
 
diff --git a/src/sys/socket/mod.rs b/src/sys/socket/mod.rs
index 3a7e5eba7c..042f9a0060 100644
--- a/src/sys/socket/mod.rs
+++ b/src/sys/socket/mod.rs
@@ -277,12 +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,
     };
diff --git a/test/sys/test_socket.rs b/test/sys/test_socket.rs
index b5465aa0b6..8ba195addb 100644
--- a/test/sys/test_socket.rs
+++ b/test/sys/test_socket.rs
@@ -145,6 +145,39 @@ pub fn test_scm_rights() {
     close(w).unwrap();
 }
 
+#[test]
+pub fn test_sendmsg_empty_cmsg() {
+    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_eq!(msg.flags & (MSG_TRUNC | MSG_CTRUNC), MsgFlags::empty());
+        close(fd2).unwrap();
+    }
+}
+
 // Test creating and using named unix domain sockets
 #[test]
 pub fn test_unixdomain() {