dd: warn about counts specified with confusing 0x prefix

* src/dd.c (parse_integer): Suggest to use "00x" instead of "0x",
which is significant for the "count", "seek", and "skip" operands.
* tests/dd/misc.sh: Add a test case.
Fixes http://bugs.gnu.org/24874

src/dd.c

@@ -1342,6 +1342,12 @@ parse_integer (const char *str, strtol_error *invalid)
           return 0;
         }
 
+      if (n == 0 && STRPREFIX (str, "0x"))
+        error (0, 0,
+               _("warning: %s is a zero multiplier; "
+                 "use %s if that is intended"),
+               quote_n (0, "0x"), quote_n (1, "00x"));
+
       n *= multiplier;
     }
   else if (e != LONGINT_OK)

tests/dd/misc.sh

@@ -107,4 +107,14 @@ compare err_ok err || fail=1
 
 test $fail -eq 0 && fail=$warn
 
+# Check a warning is issued for ambiguous 0x... numbers
+dd if=/dev/null count=0x1 seek=0x1 skip=0x1 status=none 2>err || fail=1
+cat <<\EOF >exp
+dd: warning: '0x' is a zero multiplier; use '00x' if that is intended
+dd: warning: '0x' is a zero multiplier; use '00x' if that is intended
+dd: warning: '0x' is a zero multiplier; use '00x' if that is intended
+EOF
+compare exp err || fail=1
+
+
 Exit $fail