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dissect.cstruct

A Dissect module implementing a parser for C-like structures. Structure parsing in Python made easy. With cstruct, you can write C-like structures and use them to parse binary data, either as file-like objects or bytestrings.

Parsing binary data with cstruct feels familiar and easy. No need to learn a new syntax or the quirks of a new parsing library before you can start parsing data. The syntax isn't strict C but it's compatible with most common structure definitions. You can often use structure definitions from open-source C projects and use them out of the box with little to no changes. Need to parse an EXT4 super block? Just copy the structure definition from the Linux kernel source code. Need to parse some custom file format? Write up a simple structure and immediately start parsing data, tweaking the structure as you go.

By design cstruct is incredibly simple. No complex syntax, filters, pre- or post-processing steps. Just structure parsing. For more information, please see the documentation.

Requirements

This project is part of the Dissect framework and requires Python.

Information on the supported Python versions can be found in the Getting Started section of the documentation.

Installation

dissect.cstruct is available on PyPI.

pip install dissect.cstruct

This module is also automatically installed if you install the dissect package.

Build and test instructions

This project uses tox to build source and wheel distributions. Run the following command from the root folder to build these:

tox -e build

The build artifacts can be found in the dist/ directory.

tox is also used to run linting and unit tests in a self-contained environment. To run both linting and unit tests using the default installed Python version, run:

tox

For a more elaborate explanation on how to build and test the project, please see the documentation.

Contributing

The Dissect project encourages any contribution to the codebase. To make your contribution fit into the project, please refer to the development guide.

Usage

All you need to do is instantiate a new cstruct instance and load some structure definitions in there. After that you can start using them from your Python code.

from dissect.cstruct import cstruct

# Default endianness is LE, but can be configured using a kwarg or setting the 'endian' attribute
# e.g. cstruct(endian='>') or c_parser.endian = '>'
parser_def = """
#define SOME_CONSTANT   5

enum Example : uint16 {
    A, B = 0x5, C
};

struct some_struct {
    uint8   field_1;
    char    field_2[SOME_CONSTANT];
    char    field_3[(field_1 & 1) * 5];  // Some random expression to calculate array length
    Example field_4[2];
};
"""
c_parser = cstruct().load(parser_def)

data = b"\x01helloworld\x00\x00\x06\x00"
result = c_parser.some_struct(data)  # Also accepts file-like objects
assert result.field_1 == 0x01
assert result.field_2 == b"hello"
assert result.field_3 == b"world"
assert result.field_4 == [c_parser.Example.A, c_parser.Example.C]

assert c_parser.Example.A == 0
assert c_parser.Example.C == 6
assert c_parser.Example(5) == c_parser.Example.B

assert result.dumps() == data

# You can also instantiate structures from Python by using kwargs
# Note that array sizes are not enforced
instance = c_parser.some_struct(
    field_1=5, field_2="lorem", field_3="ipsum", field_4=[c_parser.Example.B, c_parser.Example.A]
)
assert instance.dumps() == b"\x05loremipsum\x05\x00\x00\x00"

By default, all structures are compiled into classes that provide optimised performance. You can disable this by passing a compiled=False keyword argument to the .load() call. You can also inspect the resulting source code by accessing the source attribute of the structure: print(c_parser.some_struct.source).

More examples can be found in the examples directory.

Features

Structure parsing

Write simple C-like structures and use them to parse binary data, as can be seen in the examples.

Type parsing

Aside from loading structure definitions, any of the supported types can be used individually for parsing data. For example, the following is all supported:

from dissect.cstruct import cstruct

cs = cstruct()
# Default endianness is LE, but can be configured using a kwarg or setting the attribute
# e.g. cstruct(endian='>') or cs.endian = '>'
assert cs.uint32(b"\x05\x00\x00\x00") == 5
assert cs.uint24[2](b"\x01\x00\x00\x02\x00\x00") == [1, 2]  # You can also parse arrays using list indexing
assert cs.char[None](b"hello world!\x00") == b"hello world!"  # A list index of None means null terminated

Unions and nested structures

Unions and nested structures are supported, both anonymous and named.

from dissect.cstruct import cstruct

c_def = """
struct test_union {
    char magic[4];
    union {
        struct {
            uint32 a;
            uint32 b;
        } a;
        struct {
            char   b[8];
        } b;
    } c;
};

struct test_anonymous {
    char magic[4];
    struct {
        uint32 a;
        uint32 b;
    };
    struct {
        char   c[8];
    };
};
"""
# Default endianness is LE, but can be configured using a kwarg or setting the attribute
# e.g. cstruct(endian='>') or cs.endian = '>'
c = cstruct().load(c_def)

assert len(c.test_union) == 12

a = c.test_union(b"ohaideadbeef")
assert a.magic == b"ohai"
assert a.c.a.a == 0x64616564
assert a.c.a.b == 0x66656562
assert a.c.b.b == b"deadbeef"

assert a.dumps() == b"ohaideadbeef"

b = c.test_anonymous(b"ohai\x39\x05\x00\x00\x28\x23\x00\x00deadbeef")
assert b.magic == b"ohai"
assert b.a == 1337
assert b.b == 9000
assert b.c == b"deadbeef"

Parse bit fields

Bit fields are supported as part of structures. They are properly aligned to their boundaries.

from dissect.cstruct import cstruct

bit_def = """
struct test {
    uint16  a:1;
    uint16  b:1;  // Read 2 bits from an uint16
    uint32  c;    // The next field is properly aligned
    uint16  d:2;
    uint16  e:3;
};
"""
bitfields = cstruct().load(bit_def)

d = b"\x03\x00\xff\x00\x00\x00\x1f\x00"
a = bitfields.test(d)

assert a.a == 0b1
assert a.b == 0b1
assert a.c == 0xFF
assert a.d == 0b11
assert a.e == 0b111
assert a.dumps() == d

Enums and Flags

The API to access enum and flag members and their values in the same way as the native Enum types in Python 3. Functionally, it's best comparable to the IntEnum or IntFlag type.

Custom types

You can implement your own types by subclassing BaseType, and adding them to your cstruct instance with add_custom_type(name, type, size, alignment, ...)

Custom definition parsers

Don't like the C-like definition syntax? Write your own syntax parser!

Copyright and license

Dissect is released as open source by Fox-IT (https://www.fox-it.com) part of NCC Group Plc (https://www.nccgroup.com).

Developed by the Dissect Team ([email protected]) and made available at https://github.com/fox-it/dissect.

License terms: Apache License 2.0 (https://www.apache.org/licenses/LICENSE-2.0). For more information, see the LICENSE file.