Add UdpSocket::is_open, similar to TcpSocket::is_open in function.

Fixes #31.

README.md

@@ -208,7 +208,8 @@ It responds to:
   * TCP packets on port 6971 (`cat /dev/urandom | socat stdio tcp4-connect:192.168.69.1:6971`),
     which will be ignored.
 
-The buffers are only 64 bytes long, for convenience of testing resource exhaustion conditions.
+Except for the socket on port 6971. the buffers are only 64 bytes long, for convenience
+of testing resource exhaustion conditions.
 
 ### examples/client.rs
 

examples/server.rs

@@ -63,7 +63,7 @@ fn main() {
         // udp:6969: respond "yo dawg"
         {
             let socket: &mut UdpSocket = sockets.get_mut(udp_handle).as_socket();
-            if !socket.endpoint().is_specified() {
+            if !socket.is_open() {
                 socket.bind(6969).unwrap()
             }
 

src/socket/tcp.rs

@@ -348,8 +348,9 @@ impl<'a> TcpSocket<'a> {
 
     /// Start listening on the given endpoint.
     ///
-    /// This function returns an error if the socket was open; see [is_open](#method.is_open).
-    /// It also returns an error if the specified port is zero.
+    /// This function returns `Err(Error::Illegal)` if the socket was already open
+    /// (see [is_open](#method.is_open)), and `Err(Error::Unaddressable)`
+    /// if the port in the given endpoint is zero.
     pub fn listen<T>(&mut self, local_endpoint: T) -> Result<()>
             where T: Into<IpEndpoint> {
         let local_endpoint = local_endpoint.into();

src/socket/udp.rs

@@ -101,18 +101,25 @@ impl<'a, 'b> UdpSocket<'a, 'b> {
 
     /// Bind the socket to the given endpoint.
     ///
-    /// Returns `Err(Error::Illegal)` if the socket is already bound,
-    /// and `Err(Error::Unaddressable)` if the port is unspecified.
+    /// This function returns `Err(Error::Illegal)` if the socket was open
+    /// (see [is_open](#method.is_open)), and `Err(Error::Unaddressable)`
+    /// if the port in the given endpoint is zero.
     pub fn bind<T: Into<IpEndpoint>>(&mut self, endpoint: T) -> Result<()> {
         let endpoint = endpoint.into();
         if endpoint.port == 0 { return Err(Error::Unaddressable) }
 
-        if self.endpoint.port != 0 { return Err(Error::Illegal) }
+        if self.is_open() { return Err(Error::Illegal) }
 
         self.endpoint = endpoint;
         Ok(())
     }
 
+    /// Check whether the socket is open.
+    #[inline]
+    pub fn is_open(&self) -> bool {
+        self.endpoint.port != 0
+    }
+
     /// Check whether the transmit buffer is full.
     #[inline]
     pub fn can_send(&self) -> bool {