diff --git a/nsprpub/lib/ds/plarena.c b/nsprpub/lib/ds/plarena.c
index c41d6b0a..80ff263a 100644
--- a/nsprpub/lib/ds/plarena.c
+++ b/nsprpub/lib/ds/plarena.c
@@ -260,6 +260,21 @@ PR_IMPLEMENT(void *) PL_ArenaGrow(
     return newp;
 }
 
+static void ClearArenaList(PLArena *a, PRInt32 pattern)
+{
+
+    for (; a; a = a->next) {
+        PR_ASSERT(a->base <= a->avail && a->avail <= a->limit);
+        a->avail = a->base;
+	PL_CLEAR_UNUSED_PATTERN(a, pattern);
+    }
+}
+
+PR_IMPLEMENT(void) PL_ClearArenaPool(PLArenaPool *pool, PRInt32 pattern)
+{
+    ClearArenaList(pool->first.next, pattern);
+}
+
 /*
  * Free tail arenas linked after head, which may not be the true list head.
  * Reset pool->current to point to head in case it pointed at a tail arena.
@@ -274,12 +289,7 @@ static void FreeArenaList(PLArenaPool *pool, PLArena *head, PRBool reallyFree)
         return;
 
 #ifdef DEBUG
-    do {
-        PR_ASSERT(a->base <= a->avail && a->avail <= a->limit);
-        a->avail = a->base;
-        PL_CLEAR_UNUSED(a);
-    } while ((a = a->next) != 0);
-    a = *ap;
+    ClearArenaList(a, PL_FREE_PATTERN);
 #endif
 
     if (reallyFree) {
diff --git a/nsprpub/lib/ds/plarena.h b/nsprpub/lib/ds/plarena.h
index 38f3e399..d6094046 100644
--- a/nsprpub/lib/ds/plarena.h
+++ b/nsprpub/lib/ds/plarena.h
@@ -138,11 +138,12 @@ struct PLArenaPool {
 #define PL_ARENA_MARK(pool) ((void *) (pool)->current->avail)
 #define PR_UPTRDIFF(p,q) ((PRUword)(p) - (PRUword)(q))
 
+#define PL_CLEAR_UNUSED_PATTERN(a, pattern) \
+	   (PR_ASSERT((a)->avail <= (a)->limit), \
+	   memset((void*)(a)->avail, (pattern), (a)->limit - (a)->avail))
 #ifdef DEBUG
 #define PL_FREE_PATTERN 0xDA
-#define PL_CLEAR_UNUSED(a) (PR_ASSERT((a)->avail <= (a)->limit), \
-                           memset((void*)(a)->avail, PL_FREE_PATTERN, \
-                           (a)->limit - (a)->avail))
+#define PL_CLEAR_UNUSED(a) PL_CLEAR_UNUSED_PATTERN((a), PL_FREE_PATTERN)
 #define PL_CLEAR_ARENA(a)  memset((void*)(a), PL_FREE_PATTERN, \
                            (a)->limit - (PRUword)(a))
 #else
diff --git a/nsprpub/lib/ds/plarenas.h b/nsprpub/lib/ds/plarenas.h
index e3eca358..778d3af5 100644
--- a/nsprpub/lib/ds/plarenas.h
+++ b/nsprpub/lib/ds/plarenas.h
@@ -108,6 +108,11 @@ PR_EXTERN(void *) PL_ArenaGrow(
 
 PR_EXTERN(void) PL_ArenaRelease(PLArenaPool *pool, char *mark);
 
+/*
+** memset contents of all arenas in pool to pattern
+*/
+PR_EXTERN(void) PL_ClearArenaPool(PLArenaPool *pool, PRInt32 pattern);
+
 PR_END_EXTERN_C
 
 #endif /* defined(PLARENAS_H) */
diff --git a/nsprpub/lib/ds/plds.def b/nsprpub/lib/ds/plds.def
index d72796a4..a80e2bca 100644
--- a/nsprpub/lib/ds/plds.def
+++ b/nsprpub/lib/ds/plds.def
@@ -81,3 +81,8 @@ libVersionPoint;
 PL_HashTableLookupConst;
 PL_HashTableRawLookupConst;
 ;+} NSPR_4.0;
+;+
+;+NSPR_4.8.5 {
+;+    global:
+PL_ClearArenaPool;
+;+} NSPR_4.1;
diff --git a/nsprpub/pr/include/prtypes.h b/nsprpub/pr/include/prtypes.h
index 67ee6082..948f59c6 100644
--- a/nsprpub/pr/include/prtypes.h
+++ b/nsprpub/pr/include/prtypes.h
@@ -277,6 +277,13 @@
 #define PR_MAX(x,y)     ((x)>(y)?(x):(y))
 #define PR_ABS(x)       ((x)<0?-(x):(x))
 
+/***********************************************************************
+** MACROS:      PR_ARRAY_SIZE
+** DESCRIPTION:
+**  The number of elements in an array.
+***********************************************************************/
+#define PR_ARRAY_SIZE(a) (sizeof(a)/sizeof((a)[0]))
+
 PR_BEGIN_EXTERN_C
 
 /************************************************************************
diff --git a/nsprpub/pr/tests/sprintf.c b/nsprpub/pr/tests/sprintf.c
index 7fe2ac19..659bcfec 100644
--- a/nsprpub/pr/tests/sprintf.c
+++ b/nsprpub/pr/tests/sprintf.c
@@ -56,8 +56,6 @@
 #include <stdio.h>
 #include <stdlib.h>
 
-#define countof(a) (sizeof(a)/sizeof(a[0]))
-
 static char sbuf[20000];
 
 
@@ -121,15 +119,15 @@ static void TestI(void)
     int f, s, n, p;
     char fmt[20];
 
-    for (f = 0; f < countof(formats); f++) {
-	for (s = 0; s < countof(signs); s++) {
-	    for (p = 0; p < countof(precs); p++) {
+    for (f = 0; f < PR_ARRAY_SIZE(formats); f++) {
+	for (s = 0; s < PR_ARRAY_SIZE(signs); s++) {
+	    for (p = 0; p < PR_ARRAY_SIZE(precs); p++) {
 		fmt[0] = '%';
 		fmt[1] = 0;
 		if (signs[s]) strcat(fmt, signs[s]);
 		if (precs[p]) strcat(fmt, precs[p]);
 		if (formats[f]) strcat(fmt, formats[f]);
-		for (n = 0; n < countof(nums); n++) {
+		for (n = 0; n < PR_ARRAY_SIZE(nums); n++) {
 		    test_i(fmt, nums[n]);
 		}
 	    }
@@ -213,9 +211,9 @@ static void TestL(void)
     int f, s, n, p;
     char fmt[40], sfmt[40];
 
-    for (f = 0; f < countof(formats); f++) {
-	for (s = 0; s < countof(signs); s++) {
-	    for (p = 0; p < countof(precs); p++) {
+    for (f = 0; f < PR_ARRAY_SIZE(formats); f++) {
+	for (s = 0; s < PR_ARRAY_SIZE(signs); s++) {
+	    for (p = 0; p < PR_ARRAY_SIZE(precs); p++) {
 		fmt[0] = '%';
 		fmt[1] = 0;
 		if (signs[s]) strcat(fmt, signs[s]);
@@ -223,7 +221,7 @@ static void TestL(void)
 		strcpy(sfmt, fmt);
 		if (formats[f]) strcat(fmt, formats[f]);
 		if (sformats[f]) strcat(sfmt, sformats[f]);
-		for (n = 0; n < countof(nums); n++) {
+		for (n = 0; n < PR_ARRAY_SIZE(nums); n++) {
 		    test_l(fmt, sfmt, nums[n]);
 		}
 	    }
@@ -336,9 +334,9 @@ static void TestLL(void)
     int f, s, n, p;
     char fmt[40], sfmt[40];
 
-    for (f = 0; f < countof(formats); f++) {
-	for (s = 0; s < countof(signs); s++) {
-	    for (p = 0; p < countof(precs); p++) {
+    for (f = 0; f < PR_ARRAY_SIZE(formats); f++) {
+	for (s = 0; s < PR_ARRAY_SIZE(signs); s++) {
+	    for (p = 0; p < PR_ARRAY_SIZE(precs); p++) {
 		fmt[0] = '%';
 		fmt[1] = 0;
 		if (signs[s]) strcat(fmt, signs[s]);
@@ -346,7 +344,7 @@ static void TestLL(void)
 		strcpy(sfmt, fmt);
 		if (formats[f]) strcat(fmt, formats[f]);
 		if (sformats[f]) strcat(sfmt, sformats[f]);
-		for (n = 0; n < countof(nums); n++) {
+		for (n = 0; n < PR_ARRAY_SIZE(nums); n++) {
 		    test_ll(fmt, sfmt, nums[n]);
 		}
 	    }
@@ -424,15 +422,15 @@ static void TestS(void)
     int f, s, n, p;
     char fmt[40];
 
-    for (f = 0; f < countof(formats); f++) {
-	for (s = 0; s < countof(signs); s++) {
-	    for (p = 0; p < countof(precs); p++) {
+    for (f = 0; f < PR_ARRAY_SIZE(formats); f++) {
+	for (s = 0; s < PR_ARRAY_SIZE(signs); s++) {
+	    for (p = 0; p < PR_ARRAY_SIZE(precs); p++) {
 		fmt[0] = '%';
 		fmt[1] = 0;
 		if (signs[s]) strcat(fmt+strlen(fmt), signs[s]);
 		if (precs[p]) strcat(fmt+strlen(fmt), precs[p]);
 		if (formats[f]) strcat(fmt+strlen(fmt), formats[f]);
-		for (n = 0; n < countof(strs); n++) {
+		for (n = 0; n < PR_ARRAY_SIZE(strs); n++) {
 		    test_s(fmt, strs[n]);
 		}
 	    }