ctypes: Get full contents of character array

[overdrigzed@gmail.com]

Hello!

I wanted to get the full contents of a character array stored in a
struct, i.e.
_fields_ = [...("array", c_char * 12)...]
however, ctypes seems to try to return struct.array as a Python string
rather than a character array, and stops as soon as it encounters a
null within the character array.

I ended up having to define a dummy struct
class dummystruct(Str ucture):
_fields_ = []

and declare array as:
("array", dummystruct)

then use string_at(byref (struct.array), 12).

Is this really the best way of doing it? Is there no better way to
work around ctypes 'guess what you want' behaviour?

Thanks in advance,
Rodrigo

[Aaron \Castironpi\ Brady]

Re: ctypes: Get full contents of character array

On Sep 12, 6:38 pm, overdrig...@gma il.com wrote:

Hello!
>
I wanted to get the full contents of a character array stored in a
struct, i.e.
_fields_ = [...("array", c_char * 12)...]
however, ctypes seems to try to return struct.array as a Python string
rather than a character array, and stops as soon as it encounters a
null within the character array.
>
I ended up having to define a dummy struct
class dummystruct(Str ucture):
    _fields_ = []
>
and declare array as:
("array", dummystruct)
>
then use string_at(byref (struct.array), 12).
>
Is this really the best way of doing it? Is there no better way to
work around ctypes 'guess what you want' behaviour?
>
Thanks in advance,
Rodrigo

Rodrigo,

If you have the option to change your declaration to c_byte* 12, you
have more options. This example prints the null character you wanted.

from ctypes import *
import struct
class S( Structure ):
_fields_= [
( 'array', c_byte* 12 )
]
s= S()

#initialize
struct.pack_int o( '7s', s.array, 0, 'abc\x00def' )

#prototype and call PyString_FromSt ringAndSize
prototype= PYFUNCTYPE( py_object, POINTER( c_byte ), c_size_t )
PyString_FromSt ringAndSize= prototype( ( "PyString_FromS tringAndSize",
pythonapi ) )
x= PyString_FromSt ringAndSize( s.array, 12 )
print repr( x )

#prototype and call PyString_FromSt ring for contrast
prototype= PYFUNCTYPE( py_object, POINTER( c_byte ) )
PyString_FromSt ring= prototype( ( "PyString_FromS tring", pythonapi ) )
x= PyString_FromSt ring( s.array )
print repr( x )

/Output:

'abc\x00def\x00 \x00\x00\x00\x0 0'
'abc'

[overdrigzed@gmail.com]

Re: ctypes: Get full contents of character array

On Sep 13, 6:45 am, "Aaron \"Castironpi \" Brady"
<castiro...@gma il.comwrote:

On Sep 12, 6:38 pm, overdrig...@gma il.com wrote:
>
>
>
>
>
>
>

Hello!

>

I wanted to get the full contents of a character array stored in a
struct, i.e.
_fields_ = [...("array", c_char * 12)...]
however,ctypess eems to try to return struct.array as a Python string
rather than a character array, and stops as soon as it encounters a
null within the character array.

>

I ended up having to define a dummy struct
class dummystruct(Str ucture):
    _fields_ = []

>

and declare array as:
("array", dummystruct)

>

then use string_at(byref (struct.array), 12).

>

Is this really the best way of doing it? Is there no better way to
work aroundctypes'gu ess what you want' behaviour?

>

Thanks in advance,
Rodrigo

>
Rodrigo,
>
If you have the option to change your declaration to c_byte* 12, you
have more options.  This example prints the null character you wanted.
>
fromctypesimpor t *
import struct
class S( Structure ):
    _fields_= [
        ( 'array', c_byte* 12 )
    ]
s= S()
>
#initialize
struct.pack_int o( '7s', s.array, 0, 'abc\x00def' )
>
#prototype and call PyString_FromSt ringAndSize
prototype= PYFUNCTYPE( py_object, POINTER( c_byte ), c_size_t )
PyString_FromSt ringAndSize= prototype( ( "PyString_FromS tringAndSize",
pythonapi ) )
x= PyString_FromSt ringAndSize( s.array, 12 )
print repr( x )
>
#prototype and call PyString_FromSt ring for contrast
prototype= PYFUNCTYPE( py_object, POINTER( c_byte ) )
PyString_FromSt ring= prototype( ( "PyString_FromS tring", pythonapi ) )
x= PyString_FromSt ring( s.array )
print repr( x )
>
/Output:
>
'abc\x00def\x00 \x00\x00\x00\x0 0'
'abc'

Great - and thank you for replying so fast! I found that [chr(x) for x
in struct.array] with array as a c_byte * 12 did what I needed.