1 files changed, 4 insertions, 0 deletions
diff --git a/Doc/library/mmap.rst b/Doc/library/mmap.rst
index 125b34f3c7..f036a60ce0 100644
--- a/Doc/library/mmap.rst
+++ b/Doc/library/mmap.rst
@@ -94,6 +94,10 @@ memory but does not update the underlying file.
    defaults to 0.  *offset* must be a multiple of the PAGESIZE or
    ALLOCATIONGRANULARITY.
 
+   To ensure validity of the created memory mapping the file specified
+   by the descriptor *fileno* is internally automatically synchronized
+   with physical backing store on Mac OS X and OpenVMS.
+
    This example shows a simple way of using :class:`mmap`::
 
       import mmap