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# Copyright 2014 The Android Open Source Project
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
# http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
"""A simple module for declaring C-like structures.
Example usage:
>>> # Declare a struct type by specifying name, field formats and field names.
... # Field formats are the same as those used in the struct module.
... import cstruct
>>> NLMsgHdr = cstruct.Struct("NLMsgHdr", "=LHHLL", "length type flags seq pid")
>>>
>>>
>>> # Create instances from tuples or raw bytes. Data past the end is ignored.
... n1 = NLMsgHdr((44, 32, 0x2, 0, 491))
>>> print n1
NLMsgHdr(length=44, type=32, flags=2, seq=0, pid=491)
>>>
>>> n2 = NLMsgHdr("\x2c\x00\x00\x00\x21\x00\x02\x00"
... "\x00\x00\x00\x00\xfe\x01\x00\x00" + "junk at end")
>>> print n2
NLMsgHdr(length=44, type=33, flags=2, seq=0, pid=510)
>>>
>>> # Serialize to raw bytes.
... print n1.Pack().encode("hex")
2c0000002000020000000000eb010000
>>>
>>> # Parse the beginning of a byte stream as a struct, and return the struct
... # and the remainder of the stream for further reading.
... data = ("\x2c\x00\x00\x00\x21\x00\x02\x00"
... "\x00\x00\x00\x00\xfe\x01\x00\x00"
... "more data")
>>> cstruct.Read(data, NLMsgHdr)
(NLMsgHdr(length=44, type=33, flags=2, seq=0, pid=510), 'more data')
>>>
"""
import ctypes
import string
import struct
def CalcNumElements(fmt):
size = struct.calcsize(fmt)
elements = struct.unpack(fmt, "\x00" * size)
return len(elements)
def Struct(name, fmt, fieldnames, substructs={}):
"""Function that returns struct classes."""
class Meta(type):
def __len__(cls):
return cls._length
def __init__(cls, unused_name, unused_bases, namespace):
# Make the class object have the name that's passed in.
type.__init__(cls, namespace["_name"], unused_bases, namespace)
class CStruct(object):
"""Class representing a C-like structure."""
__metaclass__ = Meta
# Name of the struct.
_name = name
# List of field names.
_fieldnames = fieldnames
# Dict mapping field indices to nested struct classes.
_nested = {}
if isinstance(_fieldnames, str):
_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.
_format = ""
laststructindex = 0
for i in xrange(len(fmt)):
if fmt[i] == "S":
# Nested struct. Record the index in our struct it should go into.
index = CalcNumElements(fmt[:i])
_nested[index] = substructs[laststructindex]
laststructindex += 1
_format += "%ds" % len(_nested[index])
else:
# Standard struct format character.
_format += fmt[i]
_length = struct.calcsize(_format)
def _SetValues(self, values):
super(CStruct, self).__setattr__("_values", list(values))
def _Parse(self, data):
data = data[:self._length]
values = list(struct.unpack(self._format, data))
for index, value in enumerate(values):
if isinstance(value, str) and index in self._nested:
values[index] = self._nested[index](value)
self._SetValues(values)
def __init__(self, values):
# Initializing from a string.
if isinstance(values, str):
if len(values) < self._length:
raise TypeError("%s requires string of length %d, got %d" %
(self._name, self._length, len(values)))
self._Parse(values)
else:
# Initializing from a tuple.
if len(values) != len(self._fieldnames):
raise TypeError("%s has exactly %d fieldnames (%d given)" %
(self._name, len(self._fieldnames), len(values)))
self._SetValues(values)
def _FieldIndex(self, attr):
try:
return self._fieldnames.index(attr)
except ValueError:
raise AttributeError("'%s' has no attribute '%s'" %
(self._name, attr))
def __getattr__(self, name):
return self._values[self._FieldIndex(name)]
def __setattr__(self, name, value):
self._values[self._FieldIndex(name)] = value
@classmethod
def __len__(cls):
return cls._length
def __ne__(self, other):
return not self.__eq__(other)
def __eq__(self, other):
return (isinstance(other, self.__class__) and
self._name == other._name and
self._fieldnames == other._fieldnames and
self._values == other._values)
@staticmethod
def _MaybePackStruct(value):
if hasattr(value, "__metaclass__"):# and value.__metaclass__ == Meta:
return value.Pack()
else:
return value
def Pack(self):
values = [self._MaybePackStruct(v) for v in self._values]
return struct.pack(self._format, *values)
def __str__(self):
def FieldDesc(index, name, value):
if isinstance(value, str) and any(
c not in string.printable for c in value):
value = value.encode("hex")
return "%s=%s" % (name, value)
descriptions = [
FieldDesc(i, n, v) for i, (n, v) in
enumerate(zip(self._fieldnames, self._values))]
return "%s(%s)" % (self._name, ", ".join(descriptions))
def __repr__(self):
return str(self)
def CPointer(self):
"""Returns a C pointer to the serialized structure."""
buf = ctypes.create_string_buffer(self.Pack())
# Store the C buffer in the object so it doesn't get garbage collected.
super(CStruct, self).__setattr__("_buffer", buf)
return ctypes.addressof(self._buffer)
return CStruct
def Read(data, struct_type):
length = len(struct_type)
return struct_type(data), data[length:]
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