diff --git a/kernel/atom.h b/kernel/atom.h
index 64c5ce8..152f98e 100755
--- a/kernel/atom.h
+++ b/kernel/atom.h
@@ -42,6 +42,15 @@ extern "C" {
 /* Forward declaration */
 struct atom_tcb;
 
+/*
+ *  Define THREAD_PORT_PRIV to be empty if the used atomport.h does not define
+ *  a port specific entry for the atom_tcb struct. This way we do not have an
+ *  unused element.
+ */  
+#if !defined(THREAD_PORT_PRIV)
+#define THREAD_PORT_PRIV
+#endif
+
 typedef struct atom_tcb
 {
     /*
@@ -50,6 +59,12 @@ typedef struct atom_tcb
      */
     POINTER sp_save_ptr;
 
+    /* Thread's port specific private data. Do not move, some poorly written
+     * thread switching code (*cough* Cortex-M *cough*) might depend on a 
+     * known offset into the atom_tcb struct
+     */
+    THREAD_PORT_PRIV;
+
     /* Thread priority (0-255) */
     uint8_t priority;
 
diff --git a/ports/cortex-m/Makefile b/ports/cortex-m/Makefile
index 2a1e14f..20bc7e7 100644
--- a/ports/cortex-m/Makefile
+++ b/ports/cortex-m/Makefile
@@ -113,6 +113,10 @@ CPPFLAGS	+= -Wall -Wundef -Werror
 CPPFLAGS	+= -I$(INCLUDE_DIR) $(DEFS)
 CPPFLAGS    += -I$(board_dir) -I$(common_dir) -I$(src_dir) -I$(kernel_dir) -I$(tests_dir)
 
+# Assembler flags
+ASFLAGS     += -D__ASSEMBLY__ 
+ASFLAGS     += -D__NEWLIB__
+
 # Linker flags
 LDFLAGS		+= --static -nostartfiles
 LDFLAGS		+= -L$(LIB_DIR)
@@ -254,7 +258,7 @@ $(build_dir)/%.elf $(build_dir)/%.map: $(LIB_DIR)/lib$(LIBNAME).a $(build_dir)/%
 $(build_dir)/%.o: $(src_dir)/%.S Makefile $(board_dir)/Makefile.include
 	$(Q)mkdir -p `dirname $@`
 	$(if $(Q), @echo " (AS)        $(subst $(build_dir)/,,$@)")
-	$(Q)$(CC) $(CFLAGS) $(CPPFLAGS) $(ARCH_FLAGS) -D__ASSEMBLY__ -I`dirname $<` -c $< -o $@
+	$(Q)$(CC) $(CFLAGS) $(CPPFLAGS) $(ARCH_FLAGS) $(ASFLAGS) -I`dirname $<` -c $< -o $@
 
 $(build_dir)/%.o: $(src_dir)/%.c Makefile $(board_dir)/Makefile.include
 	$(Q)mkdir -p `dirname $@`
@@ -264,7 +268,7 @@ $(build_dir)/%.o: $(src_dir)/%.c Makefile $(board_dir)/Makefile.include
 $(build_dir)/%.o: $(board_dir)/%.S Makefile $(board_dir)/Makefile.include
 	$(Q)mkdir -p `dirname $@`
 	$(if $(Q), @echo " (AS)        $(subst $(build_dir)/,,$@)")
-	$(Q)$(CC) $(CFLAGS) $(CPPFLAGS) $(ARCH_FLAGS) -D__ASSEMBLY__ -I`dirname $<` -c $< -o $@
+	$(Q)$(CC) $(CFLAGS) $(CPPFLAGS) $(ARCH_FLAGS) $(ASFLAGS) -I`dirname $<` -c $< -o $@
 
 $(build_dir)/%.o: $(board_dir)/%.c Makefile $(board_dir)/Makefile.include
 	$(Q)mkdir -p `dirname $@`
@@ -274,7 +278,7 @@ $(build_dir)/%.o: $(board_dir)/%.c Makefile $(board_dir)/Makefile.include
 $(build_dir)/%.o: $(common_dir)/%.S Makefile $(board_dir)/Makefile.include
 	$(Q)mkdir -p `dirname $@`
 	$(if $(Q), @echo " (AS)        $(subst $(build_dir)/,,$@)")
-	$(Q)$(CC) $(CFLAGS) $(CPPFLAGS) $(ARCH_FLAGS) -D__ASSEMBLY__ -I`dirname $<` -c $< -o $@
+	$(Q)$(CC) $(CFLAGS) $(CPPFLAGS) $(ARCH_FLAGS) $(ASFLAGS) -I`dirname $<` -c $< -o $@
 
 $(build_dir)/%.o: $(common_dir)/%.c Makefile $(board_dir)/Makefile.include
 	$(Q)mkdir -p `dirname $@`
@@ -363,4 +367,4 @@ else
 endif
 
 # Include auto-generated dependencies
--include $(all_objs:.o=.d)
\ No newline at end of file
+-include $(all_objs:.o=.d)
diff --git a/ports/cortex-m/atomport-asm.S b/ports/cortex-m/atomport-asm.S
index 6098ba3..24f1060 100644
--- a/ports/cortex-m/atomport-asm.S
+++ b/ports/cortex-m/atomport-asm.S
@@ -49,6 +49,13 @@
 .extern CTX_SW_NFO
 .extern vector_table
 
+#if defined(__NEWLIB__)
+/**
+ * When using newlib, reentry context needs to be updated on task switch
+ */
+.extern _impure_ptr
+#endif
+
 /**
  * Some bit masks and registers used
  */
@@ -87,6 +94,17 @@ archFirstThreadRestore:
     ldr     r1,         [r0, #0]
     msr     PSP,        r1
 
+#if defined(__NEWLIB__)
+    /**
+     * Store the thread's reentry context address in _impure_ptr. This
+     * should be the TCB's second element
+     */ 
+    ldr     r1,         = _impure_ptr
+    mov     r2,         r0
+    add     r2,         #4
+    str     r2,         [r1, #0]
+#endif
+
     /**
      * Set bit #1 in CONTROL. Causes switch to PSP, so we can work directly
      * with SP now and use pop/push.
@@ -223,7 +241,8 @@ pend_sv_handler:
 
     /**
      * Copy running thread's process stack pointer to r3 and use it to push
-     * */
+     * the thread's register context on its stack
+     */
     mrs     r3,         PSP
 
 #if defined(THUMB_2)
@@ -281,7 +300,7 @@ pend_sv_handler:
     subs    r3,         r3, #32
 #endif // !THUMB_2
     /**
-     * Address of running TCB still in r1. Store threads current stack top
+     * Address of running TCB still in r1. Store thread's current stack top
      * into its sp_save_ptr, which is the struct's first element
      */
     str     r3,         [r1, #0]
@@ -292,6 +311,17 @@ pend_sv_handler:
      */
     str     r2,         [r0, #0]
     
+#if defined(__NEWLIB__)
+    /**
+     * Store the thread's reentry context address in _impure_ptr. This
+     * should be the TCB's second element
+     */ 
+    mov     r4,         r2
+    add     r4,         #4
+    ldr     r3,         = _impure_ptr
+    str     r4,         [r3, #0]
+#endif
+
     /**
      * Fetch next thread's stack pointer from its TCB's sp_save_ptr and restore
      * the thread's register context.
diff --git a/ports/cortex-m/atomport.c b/ports/cortex-m/atomport.c
index 49ea73d..67767cf 100644
--- a/ports/cortex-m/atomport.c
+++ b/ports/cortex-m/atomport.c
@@ -27,6 +27,8 @@
  * POSSIBILITY OF SUCH DAMAGE.
  */
 
+#include <string.h>
+
 #include <libopencm3/cm3/nvic.h>
 #include <libopencm3/cm3/systick.h>
 #include <libopencm3/cm3/cortex.h>
@@ -201,5 +203,12 @@ void archThreadContextInit(ATOM_TCB *tcb_ptr, void *stack_top,
     tcb_ptr->sp_save_ptr = tsk_ctx;
     tcb_ptr->entry_point = entry_point;
     tcb_ptr->entry_param = entry_param;
+
+#if defined(__NEWLIB__)
+    /**
+     * Initialise thread's reentry context for newlib
+     */ 
+    _REENT_INIT_PTR(&(tcb_ptr->port_priv.reent));
+#endif
 }
 
diff --git a/ports/cortex-m/atomport.h b/ports/cortex-m/atomport.h
index 562b84e..1f662f9 100644
--- a/ports/cortex-m/atomport.h
+++ b/ports/cortex-m/atomport.h
@@ -33,6 +33,7 @@
 #include <stdint.h>
 #include <stddef.h>
 #include <libopencm3/cm3/cortex.h>
+#include <stdlib.h>
 
 /* Required number of system ticks per second (normally 100 for 10ms tick) */
 #define SYSTEM_TICKS_PER_SEC            100
@@ -61,6 +62,18 @@
 #define CRITICAL_START()    __irq_flags = cm_mask_interrupts(true)
 #define CRITICAL_END()      (void) cm_mask_interrupts(__irq_flags)
 
+/**
+ * When using newlib, define port private field in atom_tcb to be a
+ * struct _reent.
+ */
+#if defined(__NEWLIB__)
+struct cortex_port_priv {
+    struct _reent reent;
+};
+
+#define THREAD_PORT_PRIV    struct cortex_port_priv port_priv
+#endif
+
 /* Uncomment to enable stack-checking */
 /* #define ATOM_STACK_CHECKING */
 
diff --git a/ports/cortex-m/common/stubs.c b/ports/cortex-m/common/stubs.c
index 4bcf302..d12a17c 100644
--- a/ports/cortex-m/common/stubs.c
+++ b/ports/cortex-m/common/stubs.c
@@ -64,19 +64,23 @@ static char *heap_end = 0;
 caddr_t _sbrk(int incr)
 {
     char *prev_end;
+    CRITICAL_STORE;
 
-    if(heap_end == 0){
+    prev_end = NULL;
+
+    CRITICAL_START();
+
+    if(unlikely(heap_end == 0)){
         heap_end = &end;
     }
 
-    prev_end = heap_end;
-
-    if(heap_end + incr + MST_SIZE > (char *) vector_table.initial_sp_value){
-        /* new heap size would collide with main stack area*/
-        return (caddr_t) 0;
+    /* make sure new heap size does not collide with main stack area*/
+    if(heap_end + incr + MST_SIZE <= (char *) vector_table.initial_sp_value){
+        prev_end = heap_end;
+        heap_end += incr;
     }
 
-    heap_end += incr;
+    CRITICAL_END();
 
     return (caddr_t) prev_end;
 }