[ctypes] convert pointer to string?

[Neal Becker]

In an earlier post, I was interested in passing a pointer to a structure to
fcntl.ioctl.

This works:

c = create_string_b uffer (...)
args = struct.pack("iP ", len(c), cast (pointer (c), c_void_p).value )
err = fcntl.ioctl(eos _fd, request, args)

Now to do the same with ctypes, I have one problem.

class eos_dl_args_t (Structure):
_fields_ = [("length", c_ulong),
("data", c_void_p)]
....
args = eos_dl_args_t()
args_p = cast(pointer(ar gs), c_void_ptr).val ue
fcntl.ioctl(fd, request, args_p) <<< May fail here

That last may fail, because .value creates a long int, and ioctl needs a
string.

How can I convert my ctypes structure to a string? It _cannot_ be a c-style
0-terminate string, because the structure may contain '0' values.

[marek.rocki@wp.pl]

Re: [ctypes] convert pointer to string?

Neal Becker napisa³(a):

In an earlier post, I was interested in passing a pointer to a structure to
fcntl.ioctl.
>
This works:
>
c = create_string_b uffer (...)
args = struct.pack("iP ", len(c), cast (pointer (c), c_void_p).value )
err = fcntl.ioctl(eos _fd, request, args)
>
Now to do the same with ctypes, I have one problem.
>
class eos_dl_args_t (Structure):
_fields_ = [("length", c_ulong),
("data", c_void_p)]
...
args = eos_dl_args_t()
args_p = cast(pointer(ar gs), c_void_ptr).val ue
fcntl.ioctl(fd, request, args_p) <<< May fail here
>
That last may fail, because .value creates a long int, and ioctl needs a
string.
>
How can I convert my ctypes structure to a string? It _cannot_ be a c-style
0-terminate string, because the structure may contain '0' values.

Hello. I'm not completely sure what your problem is ("may fail" is not
a good description. Does it fail or does it not?). If you want a
pointer to a structure as the third argument to ioctl, this would be
imho the easiest way to do it with ctypes:

class eos_dl_args_t(S tructure):
_fields_ = [("length", c_ulong), ("data", c_void_p)]
args = eos_dl_args_t()
fcntl.ioctl(fd, request, byref(args))

Regards,
Marek

[Thomas Heller]

Re: [ctypes] convert pointer to string?

Neal Becker schrieb:

In an earlier post, I was interested in passing a pointer to a structure to
fcntl.ioctl.
>
This works:
>
c = create_string_b uffer (...)
args = struct.pack("iP ", len(c), cast (pointer (c), c_void_p).value )
err = fcntl.ioctl(eos _fd, request, args)
>
Now to do the same with ctypes, I have one problem.
>
class eos_dl_args_t (Structure):
_fields_ = [("length", c_ulong),
("data", c_void_p)]
...
args = eos_dl_args_t()
args_p = cast(pointer(ar gs), c_void_ptr).val ue
fcntl.ioctl(fd, request, args_p) <<< May fail here
>
That last may fail, because .value creates a long int, and ioctl needs a
string.
>
How can I convert my ctypes structure to a string? It _cannot_ be a c-style
0-terminate string, because the structure may contain '0' values.
>

ctypes instances support the buffer interface, and you can convert the buffer
contents into a string:

>>from ctypes import *
>>c_int(42)

c_long(42)

>>buffer(c_int( 42))

<read-only buffer for 0x00AE7260, size -1, offset 0 at 0x00B06040>

>>buffer(c_int( 42))[:]

'*\x00\x00\x00'

>>>

Thomas

[Neal Becker]

Re: [ctypes] convert pointer to string?

marek.rocki@wp. pl wrote:

Neal Becker napisał(a):

>In an earlier post, I was interested in passing a pointer to a structure
>to fcntl.ioctl.
>>
>This works:
>>
>c = create_string_b uffer (...)
>args = struct.pack("iP ", len(c), cast (pointer (c), c_void_p).value )
>err = fcntl.ioctl(eos _fd, request, args)
>>
>Now to do the same with ctypes, I have one problem.
>>
>class eos_dl_args_t (Structure):
> _fields_ = [("length", c_ulong),
> ("data", c_void_p)]
>...
>args = eos_dl_args_t()
>args_p = cast(pointer(ar gs), c_void_ptr).val ue
>fcntl.ioctl(fd , request, args_p) <<< May fail here
>>
>That last may fail, because .value creates a long int, and ioctl needs a
>string.
>>
>How can I convert my ctypes structure to a string? It _cannot_ be a
>c-style 0-terminate string, because the structure may contain '0' values.

>
Hello. I'm not completely sure what your problem is ("may fail" is not
a good description. Does it fail or does it not?).

May fail is exactly what I meant. As written, ioctl will accept a 3rd arg
that is either a string or int. In this case, it is int. If the address
fits in an int it works, but if the address (which is a long) doesn't fit
it fails (gives an overflow error). Seems to be random (as well as
platform dependent).

[Neal Becker]

Re: [ctypes] convert pointer to string?

marek.rocki@wp. pl wrote:

Neal Becker napisał(a):

>In an earlier post, I was interested in passing a pointer to a structure
>to fcntl.ioctl.
>>
>This works:
>>
>c = create_string_b uffer (...)
>args = struct.pack("iP ", len(c), cast (pointer (c), c_void_p).value )
>err = fcntl.ioctl(eos _fd, request, args)
>>
>Now to do the same with ctypes, I have one problem.
>>
>class eos_dl_args_t (Structure):
> _fields_ = [("length", c_ulong),
> ("data", c_void_p)]
>...
>args = eos_dl_args_t()
>args_p = cast(pointer(ar gs), c_void_ptr).val ue
>fcntl.ioctl(fd , request, args_p) <<< May fail here
>>
>That last may fail, because .value creates a long int, and ioctl needs a
>string.
>>
>How can I convert my ctypes structure to a string? It _cannot_ be a
>c-style 0-terminate string, because the structure may contain '0' values.

>
Hello. I'm not completely sure what your problem is ("may fail" is not
a good description. Does it fail or does it not?). If you want a
pointer to a structure as the third argument to ioctl, this would be
imho the easiest way to do it with ctypes:
>

>class eos_dl_args_t(S tructure):
> _fields_ = [("length", c_ulong), ("data", c_void_p)]
>args = eos_dl_args_t()
>fcntl.ioctl(fd , request, byref(args))

>

Seems to be a problem with that:
---59 err = fcntl.ioctl(eos _fd, request, byref(args))
60

TypeError: an integer is required