Fix empty MemCStruct size

Author: andreax79

cstruct/mem_cstruct.py

@@ -72,7 +72,8 @@ def unpack_from(self, buffer: Optional[bytes], offset: int = 0) -> bool:
         """
         self.__base__ = offset  # Base offset
         if buffer is None:
-            self.__mem__ = ctypes.create_string_buffer(self.__size__)
+            # the buffer is one item larger than its size and the last element is NUL
+            self.__mem__ = ctypes.create_string_buffer(self.__size__ + 1)
         elif isinstance(buffer, ctypes.Array):
             self.__mem__ = buffer
         else:

tests/test_cstruct.py

@@ -650,6 +650,20 @@ def test_len():
     assert mbr.size == 512
 
 
+def test_pack_len():
+    buffer = b'\x00' * 512
+    mbr = MBR(buffer)
+    d = mbr.pack()
+    assert len(d) == 512
+    mbr = MBR()
+    mbr.unpack(MBR_DATA)
+    d = mbr.pack()
+    assert len(d) == 512
+    mbr = MBR()
+    d = mbr.pack()
+    assert len(d) == 512
+
+
 def test_sizeof():
     assert sizeof("struct Partition") == sizeof("struct PartitionFlat")
     assert sizeof("struct Partition") == sizeof("struct PartitionNested")

tests/test_memcstruct.py

@@ -649,6 +649,20 @@ def test_len():
     assert mbr.size == 512
 
 
+def test_pack_len():
+    buffer = b'\x00' * 512
+    mbr = MBR(buffer)
+    d = mbr.pack()
+    assert len(d) == 512
+    mbr = MBR()
+    mbr.unpack(MBR_DATA)
+    d = mbr.pack()
+    assert len(d) == 512
+    mbr = MBR()
+    d = mbr.pack()
+    assert len(d) == 512
+
+
 def test_sizeof():
     assert sizeof("struct Partition") == sizeof("struct PartitionFlat")
     assert sizeof("struct Partition") == sizeof("struct PartitionNested")