Exploring ctypes (5)

I was a little snarky about how the ctypes docs start with all Windows examples, but once you get to here they are really excellent. So I won't say too much more about them except to reproduce a fun example:

>>> from ctypes import * >>> class cell(Structure): ... pass ... >>> cell._fields_ = [('name', c_char_p), ... ('next', POINTER(cell))] >>> >>> c1 = cell() >>> c1.name = 'foo' >>> c2 = cell() >>> c2.name = 'bar' >>> c1.next = pointer(c2) >>> c2.next = pointer(c1) >>> p = c1 >>> for i in range(8): ... print p.name, ... p = p.next[0] ... foo bar foo bar foo bar foo bar

This linked list is a classic example of something that needs a "forward declaration" of the cell type in C.

One other small point, from the docs:

Either the LoadLibrary() method of the dll loaders should be used, or you should load the library by creating an instance of CDLL by calling the constructor:

We used the second method before. To use the first with the example from the other day (here):

>>> import ctypes >>> libadd = ctypes.cdll.LoadLibrary('add1.so') >>> libadd.f1(2) 3 >>> libadd.f1(2,3) 3 >>> libadd.f1() 1858614497 >>> libadd.f1.argtypes = [ctypes.c_int] >>> libadd.f1(2,3) 3 >>> libadd.f1() Traceback (most recent call last): File "", line 1, in TypeError: this function takes at least 1 argument (0 given) >>>

You can get some error checking with the argtypes call, but not everything.

A developer of Cython gave a nice summary of why you might want to start with that in the SO post cited the other day. But for my small purposes, I think ctypes will probably be everything I'll need.