Is there a way to initialize a ctypes Structure to point to an offset
into a buffer? I don't know if the way I'm doing it is supported.
into a buffer? I don't know if the way I'm doing it is supported.
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Re: ctypes initializer
castironpi napisa³(a):There is a high probability you're abusing ctypes too much, but it'sIs there a way to initialize a ctypes Structure to point to an offset
into a buffer? I don't know if the way I'm doing it is supported.
possible. The following seems to work:
from ctypes import *
class S(Structure):
_fields_ = [('x', c_uint), ('y', c_int)]
rawdata = create_string_b uffer('\xEE\xFF \x78\x56\x34\x1 2\xFF\xFF\xFF
\xFF\xAA')
# Try to make a structure s of type S which takes its data from
rawdata
# buffer, starting at index 2
s = cast(c_void_p(a ddressof(rawdat a)+2), POINTER(S)).con tents
print hex(s.x), s.y # Should be 12345678h and -1
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Re: ctypes initializer
On Aug 23, 6:43 pm, marek.ro...@wp. pl wrote:Output is 0x12345678L -1, as you state. I understand that '\xEE\xFF'castironpi napisa³(a):
>>Is there a way to initialize a ctypes Structure to point to an offset
into a buffer? I don't know if the way I'm doing it is supported.
There is a high probability you're abusing ctypes too much, but it's
possible. The following seems to work:
>
from ctypes import *
>
class S(Structure):
_fields_ = [('x', c_uint), ('y', c_int)]
rawdata = create_string_b uffer('\xEE\xFF \x78\x56\x34\x1 2\xFF\xFF\xFF
\xFF\xAA')
>
# Try to make a structure s of type S which takes its data from
rawdata
# buffer, starting at index 2
s = cast(c_void_p(a ddressof(rawdat a)+2), POINTER(S)).con tents
>
print hex(s.x), s.y # Should be 12345678h and -1
is skipped with addressof(rawda ta)+ 2, which makes +2 an offset into
the buffer.
At this point, I'd say the use of 'cast' is dubious, but possible to
support. My problem comes in, in that the buffer I have comes from a
non-ctypes source. It is a, <drumroll pleasemmap.
My goals in exploring this are persistence and IPC, which are
certainly not abusing Python too much. 'ctypes' may not be right for
the job though. The solution I looked at got even worse than 'from
_ctypes import _cast_addr'. I want a supported way to do it.
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Re: ctypes initializer
On Aug 23, 7:11 pm, castironpi <castiro...@gma il.comwrote:The solution could be as simple as returning a ctypes pointer from anOn Aug 23, 6:43 pm, marek.ro...@wp. pl wrote:
>
>
>>castironpi napisa³(a):>Is there a way to initialize a ctypes Structure to point to an offset
into a buffer? I don't know if the way I'm doing it is supported.>There is a high probability you're abusing ctypes too much, but it's
possible. The following seems to work:>from ctypes import *>class S(Structure):
_fields_ = [('x', c_uint), ('y', c_int)]
rawdata = create_string_b uffer('\xEE\xFF \x78\x56\x34\x1 2\xFF\xFF\xFF
\xFF\xAA')># Try to make a structure s of type S which takes its data from
rawdata
# buffer, starting at index 2
s = cast(c_void_p(a ddressof(rawdat a)+2), POINTER(S)).con tents>print hex(s.x), s.y # Should be 12345678h and -1
Output is 0x12345678L -1, as you state. I understand that '\xEE\xFF'
is skipped with addressof(rawda ta)+ 2, which makes +2 an offset into
the buffer.
>
At this point, I'd say the use of 'cast' is dubious, but possible to
support. My problem comes in, in that the buffer I have comes from a
non-ctypes source. It is a, <drumroll pleasemmap.
>
My goals in exploring this are persistence and IPC, which are
certainly not abusing Python too much. 'ctypes' may not be right for
the job though. The solution I looked at got even worse than 'from
_ctypes import _cast_addr'. I want a supported way to do it.
mmap method. Does this require a PEP, or ought a patch to do it?
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