diff --git a/ompi/mca/common/sm/common_sm_mmap.c b/ompi/mca/common/sm/common_sm_mmap.c
index 675ee6a049..4315323183 100644
--- a/ompi/mca/common/sm/common_sm_mmap.c
+++ b/ompi/mca/common/sm/common_sm_mmap.c
@@ -9,6 +9,7 @@
  *                         University of Stuttgart.  All rights reserved.
  * Copyright (c) 2004-2005 The Regents of the University of California.
  *                         All rights reserved.
+ * Copyright (c) 2007      Sun Microsystems, Inc.  All rights reserved.
  * $COPYRIGHT$
  * 
  * Additional copyrights may follow
@@ -165,7 +166,8 @@ mca_common_sm_mmap_t* mca_common_sm_mmap_init(size_t size, char *file_name,
     
         /* initialize the segment - only the first process to open the file */
         seg->seg_offset = mem_offset;
-        seg->seg_size = size;
+	/* initialize size after subtracting out space used by the header */
+        seg->seg_size = size - mem_offset;
         opal_atomic_unlock(&seg->seg_lock);
         seg->seg_inited = false;
 
@@ -370,7 +372,8 @@ mca_common_sm_mmap_t* mca_common_sm_mmap_init(size_t size, char *file_name,
         opal_atomic_unlock(&seg->seg_lock);
         seg->seg_inited = false;
         seg->seg_offset = mem_offset;
-        seg->seg_size = size;
+	/* initialize size after subtracting out space used by the header */
+        seg->seg_size = size - mem_offset;
     }
 
     map->hMappedObject = hMapObject;
@@ -432,7 +435,7 @@ void* mca_common_sm_mmap_seg_alloc(
     void* addr;
 
     opal_atomic_lock(&seg->seg_lock);
-    if(seg->seg_offset + *size > map->map_size) {
+    if(seg->seg_offset + *size > seg->seg_size) {
         addr = NULL;
     } else {
         size_t fixup;