What does (ctypes.c_int * len(x))(*x) do?

Question

I am working with pyOpenGL, and OpenGL requires me to transfer data by passing a pointer and the number of bytes to transfer.

I understand that python doesn't store variables in memory the same way c does. I have found the following code that makes my program work:

x = [1, 2, ... ]              # some list
(ctypes.c_int * len(x))(*x)

However I have no idea why it works (and I don't just want to trust that I haven't just gotten lucky with how everything fell into memory). What is this code actually doing?

You don't mention what you're using that works, but you can use `sizeof` to get the number of bytes to transfer. For example: `cx = (ctypes.c_int * len(x))(*x);` `num_bytes = ctypes.sizeof(cx)`. — Eryk Sun, Feb 06 '16 at 14:28
I assume you're passing the array as a C function argument. This works because ctypes arrays, just like C arrays, are passed as a pointer to the first element. — Eryk Sun, Feb 06 '16 at 14:31

score 5 · Accepted Answer · edited May 23 '17 at 10:32

According to the Python documentation:

The recommended way to create concrete array types is by multiplying any ctypes data type with a positive integer. Alternatively, you can subclass this type and define length and type class variables. Array elements can be read and written using standard subscript and slice accesses; for slice reads, the resulting object is not itself an Array.

Example:
>>> from ctypes import *
>>> TenIntegers = c_int * 10
>>> ii = TenIntegers(1, 2, 3, 4, 5, 6, 7, 8, 9, 10)
>>> print ii
<c_long_Array_10 object at 0x...>
>>> for i in ii: print i,
...
1 2 3 4 5 6 7 8 9 10
>>>

So, the first part ctypes.c_int * len(x) creates an array type with len(x) elements:

In [17]: ctypes.c_int * 10
Out[17]: __main__.c_int_Array_10

In [18]: ctypes.c_int * 100
Out[18]: __main__.c_int_Array_100

After the type creation, you should call it and pass array elements:

(ctypes.c_int * len(x))(*x)
#                      ^^^^

Created array type accepts variadic number of elements, so, you should expand list x using the *x form:

In [24]: x = [1, 2, 3]

In [25]: (ctypes.c_int * len(x))(*x)
Out[25]: <__main__.c_int_Array_3 at 0x7f0b34171ae8>

In [26]: list((ctypes.c_int * len(x))(*x))
Out[26]: [1, 2, 3]

In [27]: (ctypes.c_int * len(x))(*x)[1]
Out[27]: 2

You cannot pass x, since __init__ expects integers:

In [28]: (ctypes.c_int * len(x))(x)
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-28-ff45cb7481e4> in <module>()
----> 1 (ctypes.c_int * len(x))(x)

TypeError: an integer is required (got type list)

What does (ctypes.c_int * len(x))(*x) do?

1 Answers1