Add attribute offset support to cstruct
Add offset value of each struct attribute to cstruct. It is assigned
when initialize the struct. Also a helper function is provided to return
the offset value. Currently it does not work with nested struct.
Test: The unit test in cstruct_test.py should pass
Signed-off-by: Chenbo Feng <fengc@google.com>
Change-Id: I0944ed13785d6a6c8ddd2ecbfbb0ff60a1dfcf36
diff --git a/net/test/cstruct.py b/net/test/cstruct.py
index d31979e..434552e 100644
--- a/net/test/cstruct.py
+++ b/net/test/cstruct.py
@@ -112,8 +112,7 @@
# List of string fields that are ASCII strings.
_asciiz = set()
- if isinstance(_fieldnames, str):
- _fieldnames = _fieldnames.split(" ")
+ _fieldnames = _fieldnames.split(" ")
# Parse fmt into _format, converting any S format characters to "XXs",
# where XX is the length of the struct type's packed representation.
@@ -137,6 +136,17 @@
_length = CalcSize(_format)
+ offset_list = [0]
+ last_offset = 0
+ for i in xrange(len(_format)):
+ offset = CalcSize(_format[:i])
+ if offset > last_offset:
+ last_offset = offset
+ offset_list.append(offset)
+
+ # A dictionary that maps field names to their offsets in the struct.
+ _offsets = dict(zip(_fieldnames, offset_list))
+
def _SetValues(self, values):
# Replace self._values with the given list. We can't do direct assignment
# because of the __setattr__ overload on this class.
@@ -197,6 +207,11 @@
# callers get an unhelpful exception when they call Pack().
self._values[self._FieldIndex(name)] = value
+ def offset(self, name):
+ if "." in name:
+ raise NotImplementedError("offset() on nested field")
+ return self._offsets[name]
+
@classmethod
def __len__(cls):
return cls._length
diff --git a/net/test/cstruct_test.py b/net/test/cstruct_test.py
index b69aeb7..3e46af9 100755
--- a/net/test/cstruct_test.py
+++ b/net/test/cstruct_test.py
@@ -137,6 +137,32 @@
expected = ("00" + text_bytes + "00000000" "3412").decode("hex")
self.assertEquals(expected, t1.Pack())
+ def testCstructOffset(self):
+ TestStruct = cstruct.Struct("TestStruct", "B16si16AH",
+ "byte1 string2 int3 ascii4 word5")
+ nullstr = "hello" + (16 - len("hello")) * "\x00"
+ t = TestStruct((2, nullstr, 12345, nullstr, 33210))
+ self.assertEquals(0, t.offset("byte1"))
+ self.assertEquals(1, t.offset("string2")) # sizeof(byte)
+ self.assertEquals(17, t.offset("int3")) # sizeof(byte) + 16*sizeof(char)
+ # The integer is automatically padded by the struct module
+ # to match native alignment.
+ # offset = sizeof(byte) + 16*sizeof(char) + padding + sizeof(int)
+ self.assertEquals(24, t.offset("ascii4"))
+ self.assertEquals(40, t.offset("word5"))
+ self.assertRaises(KeyError, t.offset, "random")
+
+ # TODO: Add support for nested struct offset
+ Nested = cstruct.Struct("Nested", "!HSSi", "word1 nest2 nest3 int4",
+ [TestStructA, TestStructB])
+ DoubleNested = cstruct.Struct("DoubleNested", "SSB", "nest1 nest2 byte3",
+ [TestStructA, Nested])
+ d = DoubleNested((TestStructA((1, 2)), Nested((5, TestStructA((3, 4)),
+ TestStructB((7, 8)), 9)), 6))
+ self.assertEqual(0, d.offset("nest1"))
+ self.assertEqual(len(TestStructA), d.offset("nest2"))
+ self.assertEqual(len(TestStructA) + len(Nested), d.offset("byte3"))
+ self.assertRaises(KeyError, t.offset, "word1")
if __name__ == "__main__":
unittest.main()
