diff --git a/lib/core/libimagrt/src/io.rs b/lib/core/libimagrt/src/io.rs
index 5f6bc9d8..8107db48 100644
--- a/lib/core/libimagrt/src/io.rs
+++ b/lib/core/libimagrt/src/io.rs
@@ -31,24 +31,24 @@ use std::io::Write;
 /// a "sink" which does not write to either.
 ///
 pub enum OutputProxy {
-    Out,
-    Err,
+    Out(::std::io::Stdout),
+    Err(::std::io::Stderr),
     Sink,
 }
 
 impl Write for OutputProxy {
     fn write(&mut self, buf: &[u8]) -> ::std::io::Result<usize> {
         match *self {
-            OutputProxy::Out  => ::std::io::stdout().write(buf),
-            OutputProxy::Err  => ::std::io::stderr().write(buf),
+            OutputProxy::Out(ref mut r) => r.write(buf),
+            OutputProxy::Err(ref mut r) => r.write(buf),
             OutputProxy::Sink => Ok(0),
         }
     }
 
     fn flush(&mut self) -> ::std::io::Result<()> {
         match *self {
-            OutputProxy::Out  => ::std::io::stdout().flush(),
-            OutputProxy::Err  => ::std::io::stderr().flush(),
+            OutputProxy::Out(ref mut r) => r.flush(),
+            OutputProxy::Err(ref mut r) => r.flush(),
             OutputProxy::Sink => Ok(()),
         }
     }
@@ -58,9 +58,59 @@ impl Write for OutputProxy {
 impl Debug for OutputProxy {
     fn fmt(&self, f: &mut ::std::fmt::Formatter) -> Result<(), ::std::fmt::Error> {
         match *self {
-            OutputProxy::Out  => write!(f, "OutputProxy(Stdout)"),
-            OutputProxy::Err  => write!(f, "OutputProxy(Stderr)"),
-            OutputProxy::Sink => write!(f, "OutputProxy(Sink)"),
+            OutputProxy::Out(..) => write!(f, "OutputProxy(Stdout)"),
+            OutputProxy::Err(..) => write!(f, "OutputProxy(Stderr)"),
+            OutputProxy::Sink    => write!(f, "OutputProxy(Sink)"),
         }
     }
 }
+
+impl OutputProxy {
+    pub fn lock(&self) -> LockedOutputProxy {
+        match *self {
+            OutputProxy::Out(ref r) => {
+                LockedOutputProxy::Out(r.lock())
+            },
+            OutputProxy::Err(ref r) => {
+                LockedOutputProxy::Err(r.lock())
+            },
+            OutputProxy::Sink => LockedOutputProxy::Sink,
+        }
+    }
+}
+
+pub enum LockedOutputProxy<'a> {
+    Out(::std::io::StdoutLock<'a>),
+    Err(::std::io::StderrLock<'a>),
+    Sink,
+}
+
+impl<'a> Write for LockedOutputProxy<'a> {
+    fn write(&mut self, buf: &[u8]) -> ::std::io::Result<usize> {
+        match *self {
+            LockedOutputProxy::Out(ref mut r) => r.write(buf),
+            LockedOutputProxy::Err(ref mut r) => r.write(buf),
+            LockedOutputProxy::Sink   => Ok(0),
+        }
+    }
+
+    fn flush(&mut self) -> ::std::io::Result<()> {
+        match *self {
+            LockedOutputProxy::Out(ref mut r) => r.flush(),
+            LockedOutputProxy::Err(ref mut r) => r.flush(),
+            LockedOutputProxy::Sink   => Ok(()),
+        }
+    }
+
+}
+
+impl<'a> Debug for LockedOutputProxy<'a> {
+    fn fmt(&self, f: &mut ::std::fmt::Formatter) -> Result<(), ::std::fmt::Error> {
+        match *self {
+            LockedOutputProxy::Out(..) => write!(f, "LockedOutputProxy(Stdout)"),
+            LockedOutputProxy::Err(..) => write!(f, "LockedOutputProxy(Stderr)"),
+            LockedOutputProxy::Sink    => write!(f, "LockedOutputProxy(Sink)"),
+        }
+    }
+}
+
diff --git a/lib/core/libimagrt/src/runtime.rs b/lib/core/libimagrt/src/runtime.rs
index e76618c7..aec26192 100644
--- a/lib/core/libimagrt/src/runtime.rs
+++ b/lib/core/libimagrt/src/runtime.rs
@@ -447,14 +447,14 @@ impl<'a> Runtime<'a> {
 
     pub fn stdout(&self) -> OutputProxy {
         if self.stdout_is_tty {
-            OutputProxy::Out
+            OutputProxy::Out(::std::io::stdout())
         } else {
-            OutputProxy::Err
+            OutputProxy::Err(::std::io::stderr())
         }
     }
 
     pub fn stderr(&self) -> OutputProxy {
-        OutputProxy::Err
+        OutputProxy::Err(::std::io::stderr())
     }
 
     pub fn stdin(&self) -> Option<Stdin> {