cytpes **int

[VernM]

I am using ctypes to wrap a set of functions in a DLL. It has been
going very well, and I am very impressed with ctypes. I want to call a
c function with a signature of: void func(int **cube), where the array
if ints in cube is modified by func. I want to setup cube with int
values, and access them after the call to func. I unerstand how to
setup the ctypes array, but how do I pass **cube to the function, and
how do I access the results?

[Diez B. Roggisch]

VernM schrieb:

I am using ctypes to wrap a set of functions in a DLL. It has been
going very well, and I am very impressed with ctypes. I want to call a
c function with a signature of: void func(int **cube), where the array
if ints in cube is modified by func. I want to setup cube with int
values, and access them after the call to func. I unerstand how to
setup the ctypes array, but how do I pass **cube to the function, and
how do I access the results?

it should be simple.

use something like (untestet):

b = POINTER(c_int)()
func(byref(b))
Or

a = c_int()
b = pointer(a)
func(byref(b)

Or

b = POINTER(c_int)()
func(pointer(b))
Diez

[Gabriel Genellina]

En Mon, 28 Apr 2008 18:55:15 -0300, Diez B. Roggisch <de***@nospam.web.de>
escribió:

VernM schrieb:

>I am using ctypes to wrap a set of functions in a DLL. It has been
going very well, and I am very impressed with ctypes. I want to call a
c function with a signature of: void func(int **cube), where the array
if ints in cube is modified by func. I want to setup cube with int
values, and access them after the call to func. I unerstand how to
setup the ctypes array, but how do I pass **cube to the function, and
how do I access the results?

it should be simple.

use something like (untestet):

b = POINTER(c_int)()
func(byref(b))

[snip two other similar alternatives]

That's true for "a pointer to a pointer to int", and it's valid if the
functions references **b or b[0][0] - but in this case int** probably
means "[pointer to] an array of arrays of int" and presumibly the function
will try to access b[3][2] (or whatever indices are in range).
The duality pointer/array in C is dangerous when defining interfases - you
have to know how the value is intended to be accessed.
(I assume the function modifies the integer values, but not the pointers
themselves)

# build an array of 10x10 ints
Arr10int = c_int * 10
Pint = POINTER(c_int)
PPint = POINTER(Pint)
Arr10pint = Pint * 10
a = Arr10pint()
for i in range(10):
a[i] = Arr10int()
.... initialize the array ...
.... load the function ...
# call the function
somefunction.argtypes = (PPint,)
somefunction.restype = None
somefunction(a)

--
Gabriel Genellina

[Diez B. Roggisch]

Gabriel Genellina schrieb:

En Mon, 28 Apr 2008 18:55:15 -0300, Diez B. Roggisch
<de***@nospam.web.deescribió:

>VernM schrieb:

>>I am using ctypes to wrap a set of functions in a DLL. It has been
going very well, and I am very impressed with ctypes. I want to call a
c function with a signature of: void func(int **cube), where the array
if ints in cube is modified by func. I want to setup cube with int
values, and access them after the call to func. I unerstand how to
setup the ctypes array, but how do I pass **cube to the function, and
how do I access the results?

it should be simple.

use something like (untestet):

b = POINTER(c_int)()
func(byref(b))

[snip two other similar alternatives]

That's true for "a pointer to a pointer to int", and it's valid if the
functions references **b or b[0][0] - but in this case int** probably
means "[pointer to] an array of arrays of int" and presumibly the
function will try to access b[3][2] (or whatever indices are in range).
The duality pointer/array in C is dangerous when defining interfases -
you have to know how the value is intended to be accessed.
(I assume the function modifies the integer values, but not the pointers
themselves)

# build an array of 10x10 ints
Arr10int = c_int * 10
Pint = POINTER(c_int)
PPint = POINTER(Pint)
Arr10pint = Pint * 10
a = Arr10pint()
for i in range(10):
a[i] = Arr10int()
... initialize the array ...
... load the function ...
# call the function
somefunction.argtypes = (PPint,)
somefunction.restype = None
somefunction(a)

Yup, you are right - I somehow missed the access description and thought
of the pointer-to-a-pointer as out-parameter-spec.

Diez

[VernM]

On Apr 28, 11:57*pm, "Diez B. Roggisch" <[e-mail]:

Gabriel Genellina schrieb:

[snip repetition]

That's true for "a pointer to a pointer to int", and it's valid if the
functions references **b or b[0][0] - but in this case int** probably
means "[pointer to] an array of arrays of int" and presumibly the
function will try to access b[3][2] (or whatever indices are in range).
The duality pointer/array in C is dangerous when defining interfases -
you have to know how the value is intended to be accessed.
(I assume the function modifies the integer values, but not the pointers
themselves)

# build an array of 10x10 ints
Arr10int = c_int * 10
Pint = POINTER(c_int)
PPint = POINTER(Pint)
Arr10pint = Pint * 10
a = Arr10pint()
for i in range(10):
* * a[i] = Arr10int()
... initialize the array ...
... load the function ...
# call the function
somefunction.argtypes = (PPint,)
somefunction.restype = None
somefunction(a)

Yup, you are right - I somehow missed the access description and thought
of the pointer-to-a-pointer as out-parameter-spec.

Diez- Hide quoted text -

- Show quoted text -

To provide a little more detail: currently, a DLL function uses malloc
to create a pointer to a block of memory where the ints are stored
This pointer is returned to python. Then a pointer to this pointer is
passed to another C function which manipulates the ints. When that C
function returns, python needs to access the int values. I am now able
to get this to work with this grossly ugly code.

# Setup a place to store the *int pointer
pt = (ctypes.c_int * 1)
cube = pt()
cube[0] = dll.AllocCube() # Get the pointer to the ints

# Call the function that manipulates the ints
dll.FirstPrime(ctypes.byref(cube))

# Create a python list of the ints
result = [ctypes.c_int.from_address(cube[0]+i*4).value for i in
range(5)]

I appreciate the suggestions so far. I know there must be a cleaner
way to express this. I would prefer the array of ints to be built by
cytpes, rather than by a C function in the DLL.