This fixes the initialization of the usable size of the shared memory.

The original code was not compensating for the space used by the header.  

When memory got tight, the allocator would return a pointer to memory that 
did not exist resulting in a SEGV for the application.  This is a partial 
fix for ticket #929.

Reviewed by Rich Graham.  

This commit was SVN r13950.

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;