how to get the memory address of a numpy array for C

Question

I constructed an numpy array::

a=np.ndarray([2,3])

then i want to see where its data are::

a.data 
>>>Out[213]: <read-write buffer for 0x0482C1D0, size 48, offset 0 at 0x049E87A0> 
a.data 
>>>Out[214]: <read-write buffer for 0x0482C1D0, size 48, offset 0 at 0x049E82A0> 
a.data 
>>>Out[215]: <read-write buffer for 0x0482C1D0, size 48, offset 0 at 0x049E81C0>

...

why every time the offset address is different? if i want to transfer the data to a c function using c_types by::

ctypes_array = (ctypes.c_char * a.size * 8).from_address(ptr)

how should i get the value of ptr?

related stackoverflow.com/a/3671889

jfs
– jfs

2012-06-29 15:57:32 +00:00
Commented Jun 29, 2012 at 15:57 — jfs
– jfs, Commented Jun 29, 2012 at 15:57

Joe Kington · Accepted Answer · 2012-06-29 17:18:08Z

39

Also, have a look at ndarray.__array_interface__, which is a dict that contains all of the information you're after.

In your case,

pointer, read_only_flag = a.__array_interface__['data']

answered Jun 29, 2012 at 17:18

Joe Kington

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4 Comments

Joe Kington Over a year ago

Glad it helped! It may not be the best or most effective way to do it, though. As J.F.Sebastian mentioned, have a look at numpy.ctypeslib (Though if I recall correctly, it uses the __array_interface__, as well.).

Samuel Over a year ago

@Joe The result of a.__array_interface__['data'] is not equal to the address in the echo of a.data, like following

>>>a=array([(1,2)])     >>> a.data     <read-write buffer for 0x8b174c0, size 8, offset 0 at 0x8af6120> >>> print hex(a.__array_interface__['data'][0]) 0x893ff38

Joe Kington Over a year ago

@Samuel - That's because a.data is a buffer object, rather than the the actual memory buffer itself. Notice that a new, different buffer object is created each time you call a.data (as J.F. Sebastian notes in his answer). Have a look at: docs.python.org/2/library/functions.html#buffer

Maksym Ganenko Over a year ago

That's bad practice because, obviously, __array_interface__ is a hidden and undocumented field which may be changed in future versions without any warning. But numpy arrays have ctypes.get_data() method which returns the address of the first element of the array.

jfs · Accepted Answer · 2012-06-29 16:17:50Z

3

a.data might be a property whose getter function creates a new buffer object (meta data) on each call.

To get the address see how numpy.ctypeslib.as_ctypes() is implemented.

answered Jun 29, 2012 at 16:17

jfs

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Maksym Ganenko · Accepted Answer · 2022-09-07 18:35:27Z

3

NumPy currently has documented interface to get raw pointer address to an array like this:

a = np.asarray([2, 3])
address = a.ctypes.data

answered Sep 7, 2022 at 18:35

Maksym Ganenko

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