Modules/_ctypes/_ctypes.c (part 2)

cpython 3.14 @ ab2d84fe1023/Modules/_ctypes/_ctypes.c ↗

This annotation covers Structure layout, POINTER, byref, cast, and library loading in _ctypes.c. For the basic Simple_Type scalar types and Array see the modules_ctypes_c annotation.

Map

Lines	Symbol	Role
1-600	Structure._fields_ processing	StructField descriptor per field, offset and size computation
601-1100	Bit-field packing	Sub-byte field allocation following C ABI rules
1101-1600	POINTER type factory	LP_* classes, contents property, byref()
1601-2200	cast()	Reinterpret a ctypes object as a different type
2201-5200	CDLL, LibraryLoader, calling convention dispatch	dlopen, function pointer, argtypes/restype

Reading

Structure._fields_ layout

When _fields_ is assigned, StructField_setter iterates the fields list, accumulates byte offsets according to the target platform's ABI alignment rules, and creates one StructFieldDescriptor per field. The Structure.__init__ then uses these descriptors for __getattr__/__setattr__.

// CPython: Modules/_ctypes/_ctypes.c:2480 StructField_setter
static int
StructField_setter(PyObject *self, PyObject *value, void *data)
{
    ...
    offset = AlignUp(current_offset, stginfo->align);
    ...
}

byref(obj)

byref(obj) is a faster equivalent of POINTER(type)(obj) that creates a temporary pointer without allocating a full POINTER instance. It is only valid as a function argument, not storable.

CDLL and FuncPtr

CDLL(name) calls dlopen(name, RTLD_LOCAL|RTLD_NOW). Attribute access on the returned object creates a FuncPtr that wraps the symbol. Calling the FuncPtr invokes libffi ffi_call with the declared argtypes and restype.

gopy notes

Not yet ported in gopy. ctypes requires libffi integration and platform ABI knowledge. It is not currently on the gopy roadmap since gopy targets pure-Python workloads. The objects/ layer does not expose C struct layout APIs.