writeable buffer and struct.pack_into and struct.unpck_from

[Tzury Bar Yochay]

Hi,

I can't find in the documentation the way to use these two functions.

can someone share a simple code that utilize these two functions?

[Gabriel Genellina]

En Sat, 20 Sep 2008 15:45:48 -0300, Tzury Bar Yochay
<Af**********@gmail.comescribió:

I can't find in the documentation the way to use these two functions.

can someone share a simple code that utilize these two functions?

struct.pack_into is intended to "fill" a buffer you got from somewhere,
probably other language or process. ctypes.create_string_buffer may be
used to create a writable buffer in python code.

pyfrom ctypes import create_string_buffer
pyb = create_string_buffer(10)
pyb.raw
'\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00'
pyfrom struct import *
pypack_into("hhh", b, 0, 1, 2, -1)
pyb.raw
'\x01\x00\x02\x00\xff\xff\x00\x00\x00\x00'

unpack_from does the opposite.
Before Python 2.5, you had to use pack to create a string object, and then
copy its contents into the destination buffer; using pack_into avoids the
memory copy.

--
Gabriel Genellina

[Aaron \Castironpi\ Brady]

On Sep 20, 6:42*pm, "Gabriel Genellina" <[e-mail]>
wrote:

En Sat, 20 Sep 2008 15:45:48 -0300, Tzury Bar Yochay *
<[e-mail]ó:

I can't find in the documentation the way to use these two functions.

can someone share a simple code that utilize these two functions?

struct.pack_into is intended to "fill" a buffer you got from somewhere, *
probably other language or process. ctypes.create_string_buffer may be *
used to create a writable buffer in python code.

pyfrom ctypes import create_string_buffer
pyb = create_string_buffer(10)
pyb.raw
'\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00'
pyfrom struct import *
pypack_into("hhh", b, 0, 1, 2, -1)
pyb.raw
'\x01\x00\x02\x00\xff\xff\x00\x00\x00\x00'

unpack_from does the opposite.
Before Python 2.5,

That was when Python broke string immutability with the ctypes module.

you had to use pack to create a string object, and then *
copy its contents into the destination buffer; using pack_into avoids the*
memory copy.

--
Gabriel Genellina

'mmap' seems to have always offered random-access writes. A search
through the implementation for "tp_as_buffer" returns a lot of zeros
however.

[Tzury Bar Yochay]

Thanks Gabriel,
I was missing the information how to create a writable buffer.

[John Machin]

On Sep 21, 3:16*pm, Tzury Bar Yochay <[e-mail]:

Thanks Gabriel,
I was missing the information how to create a writable buffer.

array.array objects also have the writable buffer nature:

>>import array
b = array.array('c', '\0' * 10)
b

array('c', '\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00')

>>import struct
struct.pack_into('<hhhh', b, 0, 1, 2, -1, -32768)
b

array('c', '\x01\x00\x02\x00\xff\xff\x00\x80\x00\x00')

HTH,
John