diff --git a/ports/arm/atomport-asm.s b/ports/arm/atomport-asm.s
index 7ec981c..4ff7f14 100644
--- a/ports/arm/atomport-asm.s
+++ b/ports/arm/atomport-asm.s
@@ -43,6 +43,10 @@
 
 .extern __interrupt_dispatcher 
 
+/* When using newlib, reentrancy context needs to be updated on task switch */
+.extern _impure_ptr
+
+
 /**/
 .equ USR_MODE,            0x10
 .equ FIQ_MODE,            0x11
@@ -80,6 +84,10 @@
 archContextSwitch:
     STMFD       sp!, {r4 - r11, lr}             /* Save registers */
 
+    ADD         r4, r1, #4                      /* Add offset to get address of new_tcb_ptr->reent context (second TCB element) */
+    LDR         r5, = _impure_ptr               /* Get address of _impure_ptr into r5 */
+    STR         r4, [r5]                        /* Store new_tcb_ptr->reent context in _impure_ptr */
+
     STR         sp, [r0]                        /* Save old SP in old_tcb_ptr->sp_save_ptr (first TCB element) */
     LDR         r1, [r1]                        /* Load new SP from new_tcb_ptr->sp_save_ptr (first TCB element) */
     MOV         sp, r1                           
@@ -120,6 +128,11 @@ archContextSwitch:
  * void archFirstThreadRestore (ATOM_TCB *new_tcb_ptr)
  */
 archFirstThreadRestore:
+
+    ADD         r4, r0, #4                      /* Add offset to get address of new_tcb_ptr->reent context (second TCB element) */
+    LDR         r5, = _impure_ptr               /* Get address of _impure_ptr into r5 */
+    STR         r4, [r5]                        /* Store new_tcb_ptr->reent context in _impure_ptr */
+
     LDR         r0, [r0]                        /* Get SP (sp_save_ptr is conveniently first element of TCB) */
     MOV         sp, r0                          /* Load new stack pointer */
     LDMFD       sp!, {r4 - r11, pc}             /* Load new registers */
diff --git a/ports/arm/atomport.c b/ports/arm/atomport.c
index 4c117da..56e2137 100644
--- a/ports/arm/atomport.c
+++ b/ports/arm/atomport.c
@@ -27,6 +27,10 @@
  * POSSIBILITY OF SUCH DAMAGE.
  */
 
+#include <stdio.h>
+#include <string.h>
+#include <sys/reent.h>
+
 #include "atom.h"
 #include "atomport.h"
 
@@ -68,6 +72,13 @@ static void thread_shell (void)
     /* Get the TCB of the thread being started */
     curr_tcb = atomCurrentContext();
 
+   /**
+    * Open a stdout file descriptor so that the thread has its own stdout.
+    * In theory threads could open stdout to different output drivers
+    * if syscalls.s supported different output write functions.
+    */
+   stdout = fopen ("/debuguart", "w");
+ 
     /**
      * Enable interrupts - these will not be enabled when a thread
      * is first restored.
@@ -80,6 +91,9 @@ static void thread_shell (void)
         curr_tcb->entry_point(curr_tcb->entry_param);
     }
 
+    /* Clean up after thread completion */
+    _reclaim_reent (&(curr_tcb->port_priv.reent));
+
     /* Thread has run to completion: remove it from the ready list */
     curr_tcb->suspended = TRUE;
     atomSched (FALSE);
@@ -153,5 +167,10 @@ void archThreadContextInit (ATOM_TCB *tcb_ptr, void *stack_top, void (*entry_poi
      */
     tcb_ptr->sp_save_ptr = stack_ptr;
 
+    /**
+     * At thread startup (and every time we switch back to a thread)
+     * we set the global reentrancy pointer to this thread's reent struct
+     */
+    _REENT_INIT_PTR (&(tcb_ptr->port_priv.reent));
 }
 
diff --git a/ports/arm/atomport.h b/ports/arm/atomport.h
index 1025b18..a228615 100644
--- a/ports/arm/atomport.h
+++ b/ports/arm/atomport.h
@@ -36,6 +36,9 @@
 /* Definition of NULL is available from stddef.h on this platform */
 #include <stddef.h>
 
+/* Reentrancy structure */
+#include <sys/reent.h>
+
 /* Required number of system ticks per second (normally 100 for 10ms tick) */
 #define SYSTEM_TICKS_PER_SEC            100
 
@@ -83,6 +86,15 @@ extern void      contextExitCritical (uint32_t posture) ;
 #define CRITICAL_START()        __atom_critical = contextEnterCritical()
 #define CRITICAL_END()          contextExitCritical(__atom_critical)
 
+/**
+ * When using newlib, define port private field in atom_tcb to be a
+ * struct _reent.
+ */
+struct arm_port_priv {
+    struct _reent reent;
+};
+#define THREAD_PORT_PRIV    struct arm_port_priv port_priv
+
 /* Uncomment to enable stack-checking */
 /* #define ATOM_STACK_CHECKING */
 
diff --git a/ports/arm/syscalls.c b/ports/arm/syscalls.c
index 5933531..d89ba02 100644
--- a/ports/arm/syscalls.c
+++ b/ports/arm/syscalls.c
@@ -123,10 +123,17 @@ int _lseek(int file, int ptr, int dir)
  *
  * Simple stub implementation with no file table. All parameters ignored.
  *
+ * We only support reading/writing to the UART, so we don't bother inspecting
+ * the filename to decide which underlying device to use, _read() and _write()
+ * only access the UART driver.
+ *
+ * This is currently only called once (each thread opens its own stdout when
+ * it starts executing).
+ *
  */
 int _open(const char *name, int flags, int mode)
 {
-    return -1;
+    return 0;
 }