Fiemap: introduce constants

Introduce constants for various FIEMAP-related stuff like format strings for C
structures, their sizes, and the ioctl number. First of all, this is just
nicer. But more importantly, we'll soon introduce another function which will
use these constants as well.

Change-Id: Iac00bc93d8a590dc242b8ce7b20f2fe3d98a186d
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@intel.com>

diff --git a/bmaptools/Fiemap.py b/bmaptools/Fiemap.py
index a7748e4..2f79ae0 100644 (file)
--- a/bmaptools/Fiemap.py
+++ b/bmaptools/Fiemap.py
@@ -13,6 +13,17 @@ import fcntl
 import itertools
 from bmaptools import BmapHelpers
 
+# Format string for 'struct fiemap'
+_FIEMAP_FORMAT = "=QQLLLL"
+# sizeof(struct fiemap)
+_FIEMAP_SIZE = struct.calcsize(_FIEMAP_FORMAT)
+# Format string for 'struct fiemap_extent'
+_FIEMAP_EXTENT_FORMAT = "=QQQQQLLLL"
+# sizeof(struct fiemap_extent)
+_FIEMAP_EXTENT_SIZE = struct.calcsize(_FIEMAP_EXTENT_FORMAT)
+# The FIEMAP ioctl number
+_FIEMAP_IOCTL = 0xC020660B
+
 class Error(Exception):
     """ A class for exceptions generated by this module. We currently support
     only one type of exceptions, and we basically throw human-readable problem
@@ -85,19 +96,16 @@ class Fiemap:
 
         # Prepare a 'struct fiemap' buffer which contains a single
         # 'struct fiemap_extent' element.
-        struct_fiemap_format = "=QQLLLL"
-        struct_size = struct.calcsize(struct_fiemap_format)
-        buf = struct.pack(struct_fiemap_format,
-                          block * self.block_size,
+        buf = struct.pack(_FIEMAP_FORMAT, block * self.block_size,
                           self.block_size, 0, 0, 1, 0)
-        # sizeof(struct fiemap_extent) == 56
-        buf += "\0"*56
+        buf += '\0' * _FIEMAP_EXTENT_SIZE
+
         # Python strings are "immutable", meaning that python will pass a copy
         # of the string to the ioctl, unless we turn it into an array.
         buf = array.array('B', buf)
 
         try:
-            fcntl.ioctl(self._f_image, 0xC020660B, buf, 1)
+            fcntl.ioctl(self._f_image, _FIEMAP_IOCTL, buf, 1)
         except IOError as err:
             error_msg = "the FIBMAP ioctl failed for '%s': %s" \
                         % (self._image_path, err)
@@ -107,7 +115,8 @@ class Fiemap:
 
             raise Error(error_msg)
 
-        res = struct.unpack(struct_fiemap_format, buf[:struct_size])
+        res = struct.unpack(_FIEMAP_FORMAT, buf[:_FIEMAP_SIZE])
+
         # res[3] is the 'fm_mapped_extents' field of 'struct fiemap'. If it
         # contains zero, the block is not mapped, otherwise it is mapped.
         return bool(res[3])