dd: avoid unnecessary memory copies

* src/dd.c (scanargs): When not otherwise required (e.g. for
conversion), use two-buffer mode only when the input and output
buffer sizes differ.  Before, some of the most basic invocations of
dd, e.g., dd < in > out, would unnecessarily use separate buffers
and perform memory copies between them.

diff --git a/src/dd.c b/src/dd.c
index f598e44..e1e38e9 100644 (file)
--- a/src/dd.c
+++ b/src/dd.c
@@ -998,13 +998,11 @@ scanargs (int argc, char *const *argv)
            {
              invalid |= ! (0 < n && n <= MAX_BLOCKSIZE (INPUT_BLOCK_SLOP));
              input_blocksize = n;
-             conversions_mask |= C_TWOBUFS;
            }
          else if (operand_is (name, "obs"))
            {
              invalid |= ! (0 < n && n <= MAX_BLOCKSIZE (OUTPUT_BLOCK_SLOP));
              output_blocksize = n;
-             conversions_mask |= C_TWOBUFS;
            }
          else if (operand_is (name, "bs"))
            {
@@ -1036,15 +1034,12 @@ scanargs (int argc, char *const *argv)
   if (blocksize)
     input_blocksize = output_blocksize = blocksize;
 
-  /* If bs= was given, both `input_blocksize' and `output_blocksize' will
-     have been set to positive values.  If either has not been set,
-     bs= was not given, so make sure two buffers are used. */
-  if (input_blocksize == 0 || output_blocksize == 0)
-    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);