diff -urN otp_src_R11B-5.orig/erts/emulator/beam/erl_bif_port.c otp_src_R11B-5.portfix/erts/emulator/beam/erl_bif_port.c
--- otp_src_R11B-5.orig/erts/emulator/beam/erl_bif_port.c	2007-09-28 18:55:12.000000000 +0900
+++ otp_src_R11B-5.portfix/erts/emulator/beam/erl_bif_port.c	2007-09-28 20:04:53.000000000 +0900
@@ -39,6 +39,8 @@
 #include "register.h"
 #include "external.h"
 
+#include <pthread.h>
+
 extern ErlDrvEntry fd_driver_entry;
 extern ErlDrvEntry vanilla_driver_entry;
 extern ErlDrvEntry spawn_driver_entry;
@@ -153,25 +155,44 @@
     return res;
 }
 
-static byte *erts_port_call_buff;
-static Uint erts_port_call_buff_size;
 /* Reversed logic to make VxWorks happy */
-static int erts_port_call_need_init = 1; 
+typedef struct
+{
+  byte *buff;
+  Uint  buff_size;
+} erts_port_call_cache_t;
+
+static pthread_once_t erts_port_call_once = PTHREAD_ONCE_INIT;
+static pthread_key_t erts_port_call_key;
+static void ensure_buff_init(void)
+{
+  int r;
+  r = pthread_key_create(&erts_port_call_key, NULL);
+  ASSERT( r==0 );
+  return;
+}
+
 
 static byte *ensure_buff(Uint size)
 {
-    if (erts_port_call_need_init) {
-	erts_port_call_buff = erts_alloc(ERTS_ALC_T_PORT_CALL_BUF,
-					 (size_t) size);
-	erts_port_call_buff_size = size;
-	erts_port_call_need_init = 0;
-    } else if (erts_port_call_buff_size < size) {
-	erts_port_call_buff_size = size;
-	erts_port_call_buff = erts_realloc(ERTS_ALC_T_PORT_CALL_BUF,
-					   (void *) erts_port_call_buff,
-					   (size_t) size);
-    }
-    return erts_port_call_buff;
+  erts_port_call_cache_t* cache;
+  pthread_once(&erts_port_call_once, &ensure_buff_init);
+
+  cache = pthread_getspecific(erts_port_call_key);
+  if( cache==NULL )
+  {
+    cache = erts_alloc(ERTS_ALC_T_PORT_CALL_BUF, sizeof(*cache));
+    cache->buff = erts_alloc(ERTS_ALC_T_PORT_CALL_BUF, (size_t)size);
+    cache->buff_size = size;
+    pthread_setspecific(erts_port_call_key, cache);
+  } else if ( cache->buff_size < size )
+  {
+    cache->buff_size = size;
+    cache->buff = erts_realloc(ERTS_ALC_T_PORT_CALL_BUF,
+                               cache->buff,
+                               (size_t) size);
+  }
+  return cache->buff;
 }
 BIF_RETTYPE port_call_2(BIF_ALIST_2)
 {