Interconvert a ctypes.Structure to/from a binary string?

[Andrew Lentvorski]

Basically, I'd like to use the ctypes module as a much more descriptive
"struct" module.

Is there a way to take a ctypes.Structur e-based class and convert it
to/from a binary string?

Thanks,
-a

[Uwe Schmitt]

Re: Interconvert a ctypes.Structur e to/from a binary string?

On 2 Aug., 08:35, Andrew Lentvorski <bs...@allcaps. orgwrote:

Basically, I'd like to use the ctypes module as a much more descriptive
"struct" module.
>
Is there a way to take a ctypes.Structur e-based class and convert it
to/from a binary string?
>
Thanks,
-a

My first idea was :

from ctypes import *
from pickle import *

class T(Structure): pass

print dumps(T())

which raises an TypeError, "abstract class".

And then I found

http://osdir.com/ml/python.ctypes/2006-03/msg00009.html

Greetings, Uwe

[Andrew P. Lentvorski, Jr.]

Re: Interconvert a ctypes.Structur e to/from a binary string?

On Aug 1, 11:35 pm, Andrew Lentvorski <bs...@allcaps. orgwrote:

Basically, I'd like to use the ctypes module as a much more descriptive
"struct" module.
>
Is there a way to take a ctypes.Structur e-based class and convert it
to/from a binary string?
>
Thanks,
-a

After chugging through the ctypes source code, I found that I can
abuse ctypes into doing what I want.

Here is how I did it. I can abuse
string_at(addre ssof(SomeCtypes Class), length) to get a binary string
out of ctypes while I use:

def analyze_elf_hea der(binaryData) :
headerSize = ctypes.sizeof(E lf32_Ehdr)
header = Elf32_Ehdr()

# Abuse ctypes to initialize from a string
bb = ctypes.create_s tring_buffer(bi naryData[0:headerSize])
ctypes.memmove( ctypes.addresso f(header), ctypes.addresso f(bb),
headerSize)

To jam stuff into a ctypes class. This seems like an oversight in the
module though. It would really be better if the class itself had
methods to init from/produce to a binary string.

However, I would prefer that somebody who actually knows ctypes to
weigh in here with comments about what I did.

Thanks,
-a

[Nick Craig-Wood]

Re: Interconvert a ctypes.Structur e to/from a binary string?

Andrew P. Lentvorski, Jr. <bsdder@gmail.c omwrote:

On Aug 1, 11:35 pm, Andrew Lentvorski <bs...@allcaps. orgwrote:

Basically, I'd like to use the ctypes module as a much more descriptive
"struct" module.

Is there a way to take a ctypes.Structur e-based class and convert it
to/from a binary string?

Thanks,
-a

>
After chugging through the ctypes source code, I found that I can
abuse ctypes into doing what I want.
>
Here is how I did it. I can abuse
string_at(addre ssof(SomeCtypes Class), length) to get a binary string
out of ctypes while I use:
>
def analyze_elf_hea der(binaryData) :
headerSize = ctypes.sizeof(E lf32_Ehdr)
header = Elf32_Ehdr()
>
# Abuse ctypes to initialize from a string
bb = ctypes.create_s tring_buffer(bi naryData[0:headerSize])
ctypes.memmove( ctypes.addresso f(header), ctypes.addresso f(bb),
headerSize)
>
To jam stuff into a ctypes class. This seems like an oversight in the
module though. It would really be better if the class itself had
methods to init from/produce to a binary string.
>
However, I would prefer that somebody who actually knows ctypes to
weigh in here with comments about what I did.

I have found myself doing this quite a bit with ctypes. It is common
to get a block of data in which represents a stream of structures
which have to be unpicked and made into ctypes structs before use.
Making that stream is an equal challenge.

I've found a couple of ways of doing it.

Setup

>>from ctypes import *
>>class A(Structure):

... _fields_ = [("x", c_int)]
...

>>packet="\x02\ x01\x00\x00"

Eg to make a struct from a string

>>a = cast(packet, POINTER(A)).con tents
>>a.x

258

>>>

Or (this is identical to your method)

>>a = A()
>>a.x

0

>>memmove(addre ssof(a), packet, sizeof(a))

3083811008L

>>a.x

258

I think the second of those methods is promoted by the ctypes
documentation. I'm not sure about the lifetimes of the .contents in
the first method!

And the reverse

>>string_at(add ressof(a), sizeof(a))

'\x02\x01\x00\x 00'

>>>

Which I think is probably acceptable...

Some to/from binary methods would be nice, or failing that an explicit
section in the docs!

--
Nick Craig-Wood <nick@craig-wood.com-- http://www.craig-wood.com/nick