| Messages (3) | |||
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| msg191213 - (view) | Author: Tomer Levi (Tomer.Levi) | Date: 2013-06-15 16:42 | |
Whenever you create a ctypes.Structure with mixed ctypes, the size of all components is calculated by the size of largest one. This is especially irritating when trying to use Structure on bytearray.
The problem repeated for me in Python2.6 and 2.7 (both 32bit and 64bit versions) [My computer is 64bit]
Example:
#Creating a Structure that should take up 5 bytes
class Test(ctypes.Structure):
_fields_ = [("test", ctypes.c_byte),
("test2", ctypes.c_uint32),]
#Initiating the Structure
Test.from_buffer(bytearray(5))
--- OUTPUT ----
Traceback (most recent call last):
File "<ipython-input-45-cd4b7501baee>", line 1, in <module>
Test.from_buffer(bytearray(5))
ValueError: Buffer size too small (5 instead of at least 8 bytes)
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| msg191214 - (view) | Author: Alex Gaynor (alex) *  | Date: 2013-06-15 16:45 | |
This is how padding works in the C ABI, not a bug. |
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| msg191340 - (view) | Author: Amaury Forgeot d'Arc (amaury.forgeotdarc) * | Date: 2013-06-17 14:30 | |
You can also use "_pack_ = 1" to override the default alignment rules: http://docs.python.org/3/library/ctypes.html#ctypes.Structure._pack_ |
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- class
ctypes.Structure(*args, **kw) -
Abstract base class for structures in native byte order.
Concrete structure and union types must be created by subclassing one of these types, and at least define a
_fields_class variable.ctypeswill create descriptors which allow reading and writing the fields by direct attribute accesses. These are the_fields_-
A sequence defining the structure fields. The items must be 2-tuples or 3-tuples. The first item is the name of the field, the second item specifies the type of the field; it can be any ctypes data type.
For integer type fields like
c_int, a third optional item can be given. It must be a small positive integer defining the bit width of the field.Field names must be unique within one structure or union. This is not checked, only one field can be accessed when names are repeated.
It is possible to define the
_fields_class variable after the class statement that defines the Structure subclass, this allows creating data types that directly or indirectly reference themselves:class List(Structure): pass List._fields_ = [("pnext", POINTER(List)), ... ]The
_fields_class variable must, however, be defined before the type is first used (an instance is created,sizeof()is called on it, and so on). Later assignments to the_fields_class variable will raise an AttributeError.It is possible to defined sub-subclasses of structure types, they inherit the fields of the base class plus the
_fields_defined in the sub-subclass, if any.