version 6.1 - fix CStruct.pack() padding

Author: andreax79

Makefile

@@ -27,7 +27,7 @@ black: isort
 clean:
 	-rm -rf build dist
 	-rm -rf *.egg-info
-	-rm -rf bin lib share pyvenv.cfg
+	-rm -rf bin lib lib64 share include pyvenv.cfg
 
 coverage:
 	@pytest --cov --cov-report=term-missing

changelog.txt

@@ -192,3 +192,9 @@
 ### Improved
 
 - Python 3.13 support
+
+## [6.1] - 2025-03-21
+
+### Fix
+
+- fix CStruct.pack() padding

cstruct/__init__.py

@@ -24,7 +24,7 @@
 
 __author__ = "Andrea Bonomi <andrea.bonomi@gmail.com>"
 __license__ = "MIT"
-__version__ = "6.0"
+__version__ = "6.1"
 __date__ = "15 August 2013"
 
 from typing import Any, Dict, Optional, Type, Union

cstruct/cstruct.py

@@ -66,6 +66,10 @@ def pack(self) -> bytes:
         """
         result: List[bytes] = []
         for field, field_type in self.__fields_types__.items():
+            # Add padding if needed
+            if field_type.padding:
+                result.append(CHAR_ZERO * field_type.padding)
+
             if field_type.is_struct or field_type.is_union:
                 if field_type.vlen == 1:  # single struct
                     v = getattr(self, field, field_type.ref())

tests/test_padding.py

@@ -0,0 +1,71 @@
+#!/usr/bin/env python
+
+import cstruct
+from cstruct import LITTLE_ENDIAN, NATIVE_ORDER, sizeof
+
+DEFINITION = """
+    struct struct1 {
+        char      a[2];
+        uint32_t  b;
+    }
+"""
+
+
+class CStructNative(cstruct.CStruct):
+    __byte_order__ = NATIVE_ORDER
+    __def__ = DEFINITION
+
+
+class MemCStructNative(cstruct.MemCStruct):
+    __byte_order__ = NATIVE_ORDER
+    __def__ = DEFINITION
+
+
+class CStructLittleEndian(cstruct.CStruct):
+    __byte_order__ = LITTLE_ENDIAN
+    __def__ = DEFINITION
+
+
+class MemCStructLittleEndian(cstruct.MemCStruct):
+    __byte_order__ = LITTLE_ENDIAN
+    __def__ = DEFINITION
+
+
+def test_memcstruct_padding():
+    for struct in (CStructNative, MemCStructNative):
+        data = b"\x41\x42\x00\x00\x01\x00\x00\x00"
+        assert sizeof(struct) == 8
+        t = struct()
+        t.unpack(data)
+        assert t.a == b"AB"
+        assert t.b == 1
+        buf = t.pack()
+        assert len(buf) == sizeof(struct)
+        assert buf == data
+
+        t2 = struct()
+        t2.unpack(buf)
+        assert t2.a == b"AB"
+        assert t2.b == 1
+        buf = t.pack()
+        assert len(buf) == sizeof(struct)
+
+
+def test_no_padding():
+    for struct in (CStructLittleEndian, MemCStructLittleEndian):
+        data = b"\x41\x42\x01\x00\x00\x00"
+        assert sizeof(struct) == 6
+        t = struct()
+        t.unpack(data)
+        assert t.a == b"AB"
+        assert t.b == 1
+        buf = t.pack()
+        assert len(buf) == sizeof(struct)
+        assert buf == data
+
+        t2 = struct()
+        t2.unpack(buf)
+        assert t2.a == b"AB"
+        assert t2.b == 1
+        buf = t.pack()
+        assert len(buf) == sizeof(struct)