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97 lines
3.0 KiB
97 lines
3.0 KiB
"""This module defines the funtions byref_at(cobj, offset)
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and cast_field(struct, fieldname, fieldtype).
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"""
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from ctypes import *
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def _calc_offset():
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# Internal helper function that calculates where the object
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# returned by a byref() call stores the pointer.
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# The definition of PyCArgObject in C code (that is the type of
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# object that a byref() call returns):
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class PyCArgObject(Structure):
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class value(Union):
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_fields_ = [("c", c_char),
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("h", c_short),
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("i", c_int),
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("l", c_long),
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("q", c_longlong),
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("d", c_double),
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("f", c_float),
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("p", c_void_p)]
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#
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# Thanks to Lenard Lindstrom for this tip:
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# sizeof(PyObject_HEAD) is the same as object.__basicsize__.
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#
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_fields_ = [("PyObject_HEAD", c_byte * object.__basicsize__),
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("pffi_type", c_void_p),
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("tag", c_char),
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("value", value),
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("obj", c_void_p),
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("size", c_int)]
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_anonymous_ = ["value"]
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# additional checks to make sure that everything works as expected
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if sizeof(PyCArgObject) != type(byref(c_int())).__basicsize__:
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raise RuntimeError("sizeof(PyCArgObject) invalid")
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obj = c_int()
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ref = byref(obj)
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argobj = PyCArgObject.from_address(id(ref))
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if argobj.obj != id(obj) or \
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argobj.p != addressof(obj) or \
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argobj.tag != 'P':
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raise RuntimeError("PyCArgObject field definitions incorrect")
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return PyCArgObject.p.offset # offset of the pointer field
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################################################################
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#
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# byref_at
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#
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def byref_at(obj, offset,
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_byref=byref,
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_c_void_p_from_address = c_void_p.from_address,
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_byref_pointer_offset = _calc_offset()
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):
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"""byref_at(cobj, offset) behaves similar this C code:
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(((char *)&obj) + offset)
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In other words, the returned 'pointer' points to the address of
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'cobj' + 'offset'. 'offset' is in units of bytes.
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"""
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ref = _byref(obj)
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# Change the pointer field in the created byref object by adding
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# 'offset' to it:
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_c_void_p_from_address(id(ref)
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+ _byref_pointer_offset).value += offset
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return ref
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################################################################
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#
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# cast_field
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#
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def cast_field(struct, fieldname, fieldtype, offset=0,
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_POINTER=POINTER,
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_byref_at=byref_at,
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_byref=byref,
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_divmod=divmod,
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_sizeof=sizeof,
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):
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"""cast_field(struct, fieldname, fieldtype)
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Return the contents of a struct field as it it were of type
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'fieldtype'.
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"""
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fieldoffset = getattr(type(struct), fieldname).offset
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return cast(_byref_at(struct, fieldoffset),
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_POINTER(fieldtype))[0]
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__all__ = ["byref_at", "cast_field"]
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