Revert part of "dd: avoid unnecessary memory copies"

This reverts part of commit fbd87029cfc494a72bb73ade27ef46382c5bc832.
Paul Eggert noticed the problem in
http://lists.gnu.org/archive/html/bug-coreutils/2008-11/msg00153.html
* doc/coreutils.texi (dd invocation): Clarify.

2 files changed, 10 insertions, 3 deletions

diff --git a/doc/coreutils.texi b/doc/coreutils.texi
index 935129f25..c9c61cef0 100644
--- a/doc/coreutils.texi
+++ b/doc/coreutils.texi
@@ -7573,6 +7573,9 @@ This makes @command{dd} write @var{bytes} per block.
 Set both input and output block sizes to @var{bytes}.
 This makes @command{dd} read and write @var{bytes} per block,
 overriding any @samp{ibs} and @samp{obs} settings.
+In addition, if no data-transforming @option{conv} option is specified,
+each input block is copied to the output as a single block,
+without aggregating short reads.
 
 @item cbs=@var{bytes}
 @opindex cbs
diff --git a/src/dd.c b/src/dd.c
index e1e38e954..e54cc14df 100644
--- a/src/dd.c
+++ b/src/dd.c
@@ -455,7 +455,7 @@ Usage: %s [OPERAND]...\n\
       fputs (_("\
 Copy a file, converting and formatting according to the operands.\n\
 \n\
-  bs=BYTES        force ibs=BYTES and obs=BYTES\n\
+  bs=BYTES        read and write BYTES bytes at a time\n\
   cbs=BYTES       convert BYTES bytes at a time\n\
   conv=CONVS      convert the file as per the comma separated symbol list\n\
   count=BLOCKS    copy only BLOCKS input blocks\n\
@@ -1033,13 +1033,17 @@ scanargs (int argc, char *const *argv)
 
   if (blocksize)
     input_blocksize = output_blocksize = blocksize;
+  else
+    {
+      /* POSIX says dd aggregates short reads into
+	 output_blocksize if bs= is not specified.  */
+      conversions_mask |= C_TWOBUFS;
+    }
 
   if (input_blocksize == 0)
     input_blocksize = DEFAULT_BLOCKSIZE;
   if (output_blocksize == 0)
     output_blocksize = DEFAULT_BLOCKSIZE;
-  if (input_blocksize != output_blocksize)
-    conversions_mask |= C_TWOBUFS;
   if (conversion_blocksize == 0)
     conversions_mask &= ~(C_BLOCK | C_UNBLOCK);