dd: add a flag to discard cached data

* src/dd.c (FFS_MASK): A new macro (Find First Set) refactored
from the following enum as it's now used twice.
(usage): Mention the new 'nocache' flag.
(cache_round): A new function to help ignore requests
to drop cache, that are less than page_size.
(invalidate_cache): A new function to call posix_fadvise()
with the appropriate offset and length.  Note we don't
use fdadvise() so we can detect errors when count=0.
(dd_copy): Call invalidate_cache() for the portions read.
(iwrite): Likewise for the portions written.
(main): Call invalidate_cache for page_size slop or
for full file when count=0.
* cfg.mk (sc_dd_O_FLAGS): Adjust to pass.
* doc/coreutils.texi (dd invocation): Describe the 'nocache' flag,
and give some examples of how it can be used.
* tests/dd/nocache: A new test.
* tests/Makefile.am: Reference the new test.
* NEWS: Mention the new feature.

1 files changed, 6 insertions, 0 deletions

diff --git a/NEWS b/NEWS
index 658a89af7..3157ef201 100644
--- a/NEWS
+++ b/NEWS
@@ -14,6 +14,12 @@ GNU coreutils NEWS                                    -*- outline -*-
   wc would dereference a NULL pointer upon an early out-of-memory error
   [bug introduced in coreutils-7.1]
 
+** New features
+
+  dd now accepts the 'nocache' flag to the iflag and oflag options,
+  which will discard any cache associated with the files, or
+  processed portion thereof.
+
 
 * Noteworthy changes in release 8.10 (2011-02-04) [stable]