Issue #12102: Document that buffered files must be flushed before being used
with mmap. Patch by Steffen Daode Nurpmeso.
diff --git a/Doc/library/mmap.rst b/Doc/library/mmap.rst
index f036a60..55861f9 100644
--- a/Doc/library/mmap.rst
+++ b/Doc/library/mmap.rst
@@ -23,6 +23,12 @@
:func:`os.open` function, which returns a file descriptor directly (the file
still needs to be closed when done).
+.. note::
+ If you want to create a memory-mapping for a writable, buffered file, you
+ should :func:`~io.IOBase.flush` the file first. This is necessary to ensure
+ that local modifications to the buffers are actually available to the
+ mapping.
+
For both the Unix and Windows versions of the constructor, *access* may be
specified as an optional keyword parameter. *access* accepts one of three
values: :const:`ACCESS_READ`, :const:`ACCESS_WRITE`, or :const:`ACCESS_COPY`