diff --git a/library/GNUmakefile.68k b/library/GNUmakefile.68k
index f1651e0..d91bb53 100644
--- a/library/GNUmakefile.68k
+++ b/library/GNUmakefile.68k
@@ -1,5 +1,5 @@
 #
-# $Id: GNUmakefile.68k,v 1.75 2005-11-20 17:00:22 obarthel Exp $
+# $Id: GNUmakefile.68k,v 1.76 2005-11-27 09:26:55 obarthel Exp $
 #
 # :ts=8
 #
@@ -283,6 +283,7 @@ C_LIB = \
 	stdlib_abort.o \
 	stdlib_abs.o \
 	stdlib_alloca.o \
+	stdlib_alloca_cleanup.o \
 	stdlib_assertion_failure.o \
 	stdlib_atexit.o \
 	stdlib_atoi.o \
diff --git a/library/GNUmakefile.os4 b/library/GNUmakefile.os4
index 1c6cdae..5aa522a 100644
--- a/library/GNUmakefile.os4
+++ b/library/GNUmakefile.os4
@@ -1,5 +1,5 @@
 #
-# $Id: GNUmakefile.os4,v 1.87 2005-11-20 17:00:22 obarthel Exp $
+# $Id: GNUmakefile.os4,v 1.88 2005-11-27 09:26:55 obarthel Exp $
 #
 # :ts=8
 #
@@ -317,6 +317,7 @@ C_LIB = \
 	stdlib_abort.o \
 	stdlib_abs.o \
 	stdlib_alloca.o \
+	stdlib_alloca_cleanup.o \
 	stdlib_assertion_failure.o \
 	stdlib_atexit.o \
 	stdlib_atoi.o \
diff --git a/library/changes b/library/changes
index e962ddb..3ce18bb 100644
--- a/library/changes
+++ b/library/changes
@@ -8,7 +8,19 @@
 
 - In libunix.a malloc(), calloc() and realloc() no longer treat a
   request to allocate 0 bytes as an error, returning NULL. They all
-  return a 1 byte memory chunk initialized to 0 instead.
+  return a pointer sized memory chunk (= four bytes) initialized to
+  NULL (= 0) instead.
+
+- The alloca() implementation which allocates memory from the system
+  rather than the local stack frame is thread-safe now. It also
+  interacts with the realloc(), calloc(), free() and malloc() functions
+  in that the alloca() cleanup routine is called once alloca() has
+  done its job. If all the memory allocated through alloca() has been
+  released no further calls to the cleanup function will be made.
+
+- In the thread-safe library, realloc() permitted two different overlapping
+  calls to succeed in trying to reallocate the same chunk of memory due to
+  a race condition. Fixed.
 
 
 c.lib 1.197 (4.11.2005)
diff --git a/library/smakefile b/library/smakefile
index 665c95c..8f7c3d5 100644
--- a/library/smakefile
+++ b/library/smakefile
@@ -1,5 +1,5 @@
 #
-# $Id: smakefile,v 1.56 2005-11-19 17:11:22 obarthel Exp $
+# $Id: smakefile,v 1.57 2005-11-27 09:26:55 obarthel Exp $
 #
 # :ts=8
 #
@@ -482,6 +482,7 @@ STDLIB_OBJ = \
 	stdlib_abort.o \
 	stdlib_abs.o \
 	stdlib_alloca.o \
+	stdlib_alloca_cleanup.o \
 	stdlib_assertion_failure.o \
 	stdlib_atexit.o \
 	stdlib_atof.o \
diff --git a/library/stdlib_alloca.c b/library/stdlib_alloca.c
index 73fb9ac..b13373f 100644
--- a/library/stdlib_alloca.c
+++ b/library/stdlib_alloca.c
@@ -1,5 +1,5 @@
 /*
- * $Id: stdlib_alloca.c,v 1.7 2005-11-20 17:00:22 obarthel Exp $
+ * $Id: stdlib_alloca.c,v 1.8 2005-11-27 09:26:55 obarthel Exp $
  *
  * :ts=4
  *
@@ -55,16 +55,17 @@ extern void * alloca(size_t size);
 
 /****************************************************************************/
 
-struct MemoryContextNode
-{
-	struct MinNode	mcn_MinNode;
-	void *			mcn_StackPointer;
-	void *			mcn_Memory;
-};
+static struct MinList alloca_memory_list;
 
 /****************************************************************************/
 
-static struct MinList alloca_memory_list;
+struct MemoryContextNode
+{
+	struct MinNode	mcn_MinNode;		/* The usual linkage. */
+	void *			mcn_StackPointer;	/* Points to stack frame this allocation
+										   is associated with. */
+	void *			mcn_Memory;			/* Points to the memory allocated. */
+};
 
 /****************************************************************************/
 
@@ -80,9 +81,14 @@ CLIB_DESTRUCTOR(alloca_exit)
 
 /****************************************************************************/
 
-void
-__alloca_cleanup(const char * file,int line)
+/* Cleans up after all alloca() allocations that have been made so far. */
+static void
+alloca_cleanup(const char * file,int line)
 {
+	void * stack_pointer = __get_sp();
+
+	__memory_lock();
+
 	/* Initialize this if it hasn't been taken care of yet. */
 	if(alloca_memory_list.mlh_Head == NULL)
 		NewList((struct List *)&alloca_memory_list);
@@ -90,7 +96,6 @@ __alloca_cleanup(const char * file,int line)
 	/* Is this worth cleaning up? */
 	if(NOT IsListEmpty((struct List *)&alloca_memory_list))
 	{
-		void * stack_pointer = __get_sp();
 		struct MemoryContextNode * mcn_prev;
 		struct MemoryContextNode * mcn;
 
@@ -109,10 +114,17 @@ __alloca_cleanup(const char * file,int line)
 
 			Remove((struct Node *)mcn);
 
-			__force_free(mcn->mcn_Memory,file,line);
-			__free(mcn,file,line);
+			__free_memory(mcn->mcn_Memory,TRUE,file,line);
+			__free_memory(mcn,FALSE,file,line);
 		}
+
+		/* Drop the cleanup callback if there's nothing to be cleaned
+		   up any more. */
+		if(IsListEmpty((struct List *)&alloca_memory_list))
+			__alloca_cleanup = NULL;
 	}
+
+	__memory_unlock();
 }
 
 /****************************************************************************/
@@ -124,9 +136,12 @@ __alloca(size_t size,const char * file,int line)
 	struct MemoryContextNode * mcn;
 	void * result = NULL;
 
-	__alloca_cleanup(file,line);
+	__memory_lock();
 
-	mcn = __malloc(sizeof(*mcn),file,line);
+	__alloca_cleanup = alloca_cleanup;
+	(*__alloca_cleanup)(file,line);
+
+	mcn = __allocate_memory(sizeof(*mcn),FALSE,file,line);
 	if(mcn == NULL)
 	{
 		SHOWMSG("not enough memory");
@@ -153,6 +168,8 @@ __alloca(size_t size,const char * file,int line)
 
  out:
 
+	__memory_unlock();
+
 	return(result);
 }
 
diff --git a/library/stdlib_free.c b/library/stdlib_free.c
index 675262b..b56c978 100644
--- a/library/stdlib_free.c
+++ b/library/stdlib_free.c
@@ -1,5 +1,5 @@
 /*
- * $Id: stdlib_free.c,v 1.10 2005-03-20 11:18:06 obarthel Exp $
+ * $Id: stdlib_free.c,v 1.11 2005-11-27 09:26:55 obarthel Exp $
  *
  * :ts=4
  *
@@ -446,16 +446,13 @@ __free_memory_node(struct MemoryNode * mn,const char * UNUSED file,int UNUSED li
 
 /****************************************************************************/
 
-STATIC VOID
-free_memory(void * ptr,BOOL force,const char * file,int line)
+VOID
+__free_memory(void * ptr,BOOL force,const char * file,int line)
 {
 	struct MemoryNode * mn;
 
 	assert(ptr != NULL);
 
-	/* Try to get rid of now unused memory. */
-	/*__alloca_cleanup(file,line);*/
-
 	#ifdef __MEM_DEBUG
 	{
 		/*if((rand() % 16) == 0)
@@ -499,20 +496,19 @@ free_memory(void * ptr,BOOL force,const char * file,int line)
 
 /****************************************************************************/
 
-void
-__force_free(void * ptr,const char * file,int line)
-{
-	if(ptr != NULL)
-		free_memory(ptr,TRUE,file,line);
-}
-
-/****************************************************************************/
-
 __static void
 __free(void * ptr,const char * file,int line)
 {
+	__memory_lock();
+
+	/* Try to get rid of now unused memory. */
+	if(__alloca_cleanup != NULL)
+		(*__alloca_cleanup)(file,line);
+
+	__memory_unlock();
+
 	if(ptr != NULL)
-		free_memory(ptr,FALSE,file,line);
+		__free_memory(ptr,FALSE,file,line);
 }
 
 /****************************************************************************/
diff --git a/library/stdlib_headers.h b/library/stdlib_headers.h
index 17e5c97..7d439cb 100644
--- a/library/stdlib_headers.h
+++ b/library/stdlib_headers.h
@@ -1,5 +1,5 @@
 /*
- * $Id: stdlib_headers.h,v 1.18 2005-07-03 10:36:47 obarthel Exp $
+ * $Id: stdlib_headers.h,v 1.19 2005-11-27 09:26:55 obarthel Exp $
  *
  * :ts=4
  *
@@ -154,6 +154,10 @@ extern BOOL	NOCOMMON	__exit_blocked;
 
 /****************************************************************************/
 
+extern void NOCOMMON (*__alloca_cleanup)(const char * file,int line);
+
+/****************************************************************************/
+
 extern unsigned int NOCOMMON (* __get_default_stack_size)(void);
 
 /****************************************************************************/
diff --git a/library/stdlib_malloc.c b/library/stdlib_malloc.c
index 613a995..f8fe21b 100644
--- a/library/stdlib_malloc.c
+++ b/library/stdlib_malloc.c
@@ -1,5 +1,5 @@
 /*
- * $Id: stdlib_malloc.c,v 1.16 2005-11-20 19:04:10 obarthel Exp $
+ * $Id: stdlib_malloc.c,v 1.17 2005-11-27 09:26:55 obarthel Exp $
  *
  * :ts=4
  *
@@ -106,9 +106,9 @@ __allocate_memory(size_t size,BOOL never_free,const char * UNUSED unused_file,in
 		original_size = size;
 
 		/* The libunix.a flavour accepts zero length memory allocations
-		   and quietly turns them into 1 byte allocations. */
+		   and quietly turns them into a pointer sized allocations. */
 		if(size == 0)
-			size++;
+			size = sizeof(char *);
 	}
 	#endif /* UNIX_PATH_SEMANTICS */
 
@@ -210,11 +210,9 @@ __allocate_memory(size_t size,BOOL never_free,const char * UNUSED unused_file,in
 
 	#if defined(UNIX_PATH_SEMANTICS)
 	{
-		/* Zero length memory allocations in libunix.a quietly
-		   return one byte of memory each, and that byte is
-		   set to '\0'. */
+		/* Set the zero length allocation contents to NULL. */
 		if(original_size == 0)
-			*(char *)result = '\0';
+			*(char **)result = NULL;
 	}
 	#endif /* UNIX_PATH_SEMANTICS */
 
@@ -245,8 +243,13 @@ __malloc(size_t size,const char * file,int line)
 {
 	void * result = NULL;
 
+	__memory_lock();
+
 	/* Try to get rid of now unused memory. */
-	/*__alloca_cleanup(file,line);*/
+	if(__alloca_cleanup != NULL)
+		(*__alloca_cleanup)(file,line);
+
+	__memory_unlock();
 
 	#ifdef __MEM_DEBUG
 	{
diff --git a/library/stdlib_memory.h b/library/stdlib_memory.h
index 9146c9d..42f0710 100644
--- a/library/stdlib_memory.h
+++ b/library/stdlib_memory.h
@@ -1,5 +1,5 @@
 /*
- * $Id: stdlib_memory.h,v 1.2 2005-03-20 11:18:06 obarthel Exp $
+ * $Id: stdlib_memory.h,v 1.3 2005-11-27 09:26:55 obarthel Exp $
  *
  * :ts=4
  *
@@ -114,11 +114,8 @@
 
 #define __static
 
-#define __alloca_cleanup __alloca_cleanup_debug
-
 #define __find_memory_node __find_memory_node_debug
 #define __free_memory_node __free_memory_node_debug
-#define __force_free __force_free_debug
 
 #define __get_allocation_size __get_allocation_size_debug
 #define __allocate_memory __allocate_memory_debug
diff --git a/library/stdlib_protos.h b/library/stdlib_protos.h
index 5f5815c..bcd9e03 100644
--- a/library/stdlib_protos.h
+++ b/library/stdlib_protos.h
@@ -1,5 +1,5 @@
 /*
- * $Id: stdlib_protos.h,v 1.15 2005-07-03 10:36:47 obarthel Exp $
+ * $Id: stdlib_protos.h,v 1.16 2005-11-27 09:26:55 obarthel Exp $
  *
  * :ts=4
  *
@@ -139,13 +139,10 @@ extern void * __allocate_memory(size_t size,BOOL never_free,const char * file,in
 
 /* stdlib_free.c */
 extern struct MemoryNode * __find_memory_node(void * address);
-extern void __force_free(void * ptr,const char * file,int line);
 extern void __check_memory_allocations(const char * file,int line);
+extern void __free_memory(void * ptr,BOOL force,const char * file,int line);
 extern void __free_memory_node(struct MemoryNode * mn,const char * file,int line);
 
-/* stdlib_alloca.c */
-extern void __alloca_cleanup(const char * file,int line);
-
 /****************************************************************************/
 
 /* signal_checkabort.c */
diff --git a/library/stdlib_realloc.c b/library/stdlib_realloc.c
index 93e470f..62fd309 100644
--- a/library/stdlib_realloc.c
+++ b/library/stdlib_realloc.c
@@ -1,5 +1,5 @@
 /*
- * $Id: stdlib_realloc.c,v 1.7 2005-11-20 17:00:22 obarthel Exp $
+ * $Id: stdlib_realloc.c,v 1.8 2005-11-27 09:26:55 obarthel Exp $
  *
  * :ts=4
  *
@@ -51,6 +51,7 @@ __static void *
 __realloc(void *ptr,size_t size,const char * file,int line)
 {
 	void * result = NULL;
+	BOOL locked = FALSE;
 
 	ENTER();
 
@@ -80,6 +81,9 @@ __realloc(void *ptr,size_t size,const char * file,int line)
 
 		assert( ptr != NULL );
 
+		__memory_lock();
+		locked = TRUE;
+
 		/* Try to find the allocation in the list. */
 		mn = __find_memory_node(ptr);
 
@@ -182,6 +186,9 @@ __realloc(void *ptr,size_t size,const char * file,int line)
 
  out:
 
+	if(locked)
+		__memory_unlock();
+
 	if(result == NULL)
 		SHOWMSG("ouch! realloc failed");