Add socketpair

Author: sfackler

src/unix/mod.rs

@@ -132,6 +132,8 @@ extern {
     pub fn setsockopt(socket: ::c_int, level: ::c_int, name: ::c_int,
                       value: *const ::c_void,
                       option_len: socklen_t) -> ::c_int;
+    pub fn socketpair(domain: ::c_int, type_: ::c_int, protocol: ::c_int,
+                      socket_vector: *mut ::c_int) -> ::c_int;
     #[cfg_attr(all(target_os = "macos", target_arch = "x86"),
                link_name = "sendto$UNIX2003")]
     pub fn sendto(socket: ::c_int, buf: *const ::c_void, len: ::size_t,