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Subjectdd PATCH: add conv=direct
Linux-kernel:  is this patch horribly wrong?

On modern Linux, apparently the correct way to bypass the buffer cache
when writing to a block device is to open the block device with
O_DIRECT. This enables, for example, the user to more easily force a
reallocation of a single sector of an IDE disk with a media error
(without overwriting anything but the 1k "sector pair" containing the
error). dd(1) is convenient for this purpose, but is lacking a method
to force O_DIRECT. The enclosed patch adds a "conv=direct" flag to
enable this usage.

Alas, I don't do autoconf, so I hope someone else can add the
appropriate stanza to autoconfiscate HAS_O_DIRECT.

-andy

diff -ur coreutils-5.0.91/doc/coreutils.texi coreutils-5.0.91-adi/doc/coreutils.texi
--- coreutils-5.0.91/doc/coreutils.texi 2003-09-04 16:26:51.000000000 -0500
+++ coreutils-5.0.91-adi/doc/coreutils.texi 2004-04-06 13:48:54.000000000 -0500
@@ -6373,6 +6373,11 @@
Pad every input block to size of @samp{ibs} with trailing zero bytes.
When used with @samp{block} or @samp{unblock}, pad with spaces instead of
zero bytes.
+
+@item direct
+@opindex direct
+Open the output file with O_DIRECT, avoiding (on Linux) using the buffer
+cache.
@end table

@end table
diff -ur coreutils-5.0.91/src/dd.c coreutils-5.0.91-adi/src/dd.c
--- coreutils-5.0.91/src/dd.c 2003-07-25 02:43:09.000000000 -0500
+++ coreutils-5.0.91-adi/src/dd.c 2004-04-06 13:44:09.000000000 -0500
@@ -66,6 +66,9 @@
/* Default input and output blocksize. */
#define DEFAULT_BLOCKSIZE 512

+/* XXX hack */
+#define HAVE_O_DIRECT 1
+
/* Conversions bit masks. */
#define C_ASCII 01
#define C_EBCDIC 02
@@ -78,6 +81,7 @@
#define C_NOERROR 0400
#define C_NOTRUNC 01000
#define C_SYNC 02000
+#define C_DIRECT 010000
/* Use separate input and output buffers, and combine partial input blocks. */
#define C_TWOBUFS 04000

@@ -162,6 +166,9 @@
{"noerror", C_NOERROR}, /* Ignore i/o errors. */
{"notrunc", C_NOTRUNC}, /* Do not truncate output file. */
{"sync", C_SYNC}, /* Pad input records to ibs with NULs. */
+#ifdef HAVE_O_DIRECT
+ {"direct", C_DIRECT}, /* open files with O_DIRECT */
+#endif
{NULL, 0}
};

@@ -1190,6 +1197,11 @@
= (O_CREAT
| (seek_records || (conversions_mask & C_NOTRUNC) ? 0 : O_TRUNC));

+#if HAVE_O_DIRECT
+ if (conversions_mask & C_DIRECT)
+ opts |= O_DIRECT;
+#endif
+
/* Open the output file with *read* access only if we might
need to read to satisfy a `seek=' request. If we can't read
the file, go ahead with write-only access; it might work. */
-
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