[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_buffer (...)
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(args), c_void_ptr).value
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]

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_buffer (...)
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(args), c_void_ptr).value
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(Structure):
_fields_ = [("length", c_ulong), ("data", c_void_p)]
args = eos_dl_args_t()
fcntl.ioctl(fd, request, byref(args))

Regards,
Marek

 

[Thomas Heller]

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

This works:

c = create_string_buffer (...)
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(args), c_void_ptr).value
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))

buffer(c_int(42))[:] '*\x00\x00\x00'

Thomas

 

[Neal Becker]

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_buffer (...)
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(args), c_void_ptr).value
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]

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_buffer (...)
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(args), c_void_ptr).value
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(Structure):
_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