ctypes: Get full contents of character array

[overdrigzed]

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]

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]

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.