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14.14.1.16 Incomplete Types

Incomplete Types are structures, unions or arrays whose members are not yet specified. In C, they are specified by forward declarations, which are defined later:

    struct cell; /* forward declaration */

    struct {
        char *name;
        struct cell *next;
    } cell;

The straightforward translation into ctypes code would be this, but it does not work:

    >>> class cell(Structure):
    ...     _fields_ = [("name", c_char_p),
    ...                 ("next", POINTER(cell))]
    ...
    Traceback (most recent call last):
      File "<stdin>", line 1, in ?
      File "<stdin>", line 2, in cell
    NameError: name 'cell' is not defined
    >>>

because the new `class cell` is not available in the class statement itself. In `ctypes`, we can define the `cell` class and set the `_fields_` attribute later, after the class statement:

    >>> from ctypes import *
    >>> class cell(Structure):
    ...     pass
    ...
    >>> cell._fields_ = [("name", c_char_p),
    ...                  ("next", POINTER(cell))]
    >>>

Lets try it. We create two instances of `cell`, and let them point to each other, and finally follow the pointer chain a few times:

    >>> 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
    >>>