I get a segfault when trying to cast a string to a structure. 
>>> import ctypes
>>> class Struct(ctypes.Structure):
...     _fields_ = [('a', ctypes.c_uint32)]
... 
>>> s = '0'*100
>>> ctypes.cast(s,Struct)
Segmentation fault

The docs (https://docs.python.org/2/library/ctypes.html#ctypes.cast) say that `obj` "must be an object that can be interpreted as a pointer", so I assume this should return the same exception you get when trying to cast a list:

>>> ctypes.cast(range(10),Struct)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/lib/python3.4/ctypes/__init__.py", line 488, in cast
    return _cast(obj, obj, typ)
ctypes.ArgumentError: argument 1: <class 'TypeError'>: wrong type

You need to cast to a pointer type, i.e. POINTER(Struct). Trying to cast to just Struct should raise a TypeError. Instead this revealed a bug in cast_check_pointertype (3.4.1):

http://hg.python.org/cpython/file/c0e311e010fc/Modules/_ctypes/_ctypes.c#l5225

dict->proto is NULL in the Struct type's stgdict, so PyUnicode_Check(dict->proto) segfaults. A simple fix is to add a check for this on line 5235:

    if (dict && dict->proto) {

Then cast will raise the expected TypeError from line 5242 on return from line 5255.

I'll provide a patch but I don't know which test file to use, can somebody please advise.

IMHO, Lib/ctypes/test/test_cast.py is the relevant test.

Mark, do you still wish to provide a fix for that?
(Otherwise, i would be happy to open a PR.)

New changeset d518d8bc8d5dac1a1270612f424d33e0e5afc2b5 by Steve Dower (Oren Milman) in branch 'master':
bpo-21983: Fix a crash in ctypes.cast() when passed a ctypes structured data type (GH-3859)
https://github.com/python/cpython/commit/d518d8bc8d5dac1a1270612f424d33e0e5afc2b5

The backport to 2.7 needs some help. I can't do it on my laptop for the next week, but I'll try to get to it eventually. Feel free to get there first.

New changeset 8ac158a6dfb86880e22003afe0ff39ec31b0a094 by Miss Islington (bot) in branch '3.6':
bpo-21983: Fix a crash in ctypes.cast() when passed a ctypes structured data type (GH-3859)
https://github.com/python/cpython/commit/8ac158a6dfb86880e22003afe0ff39ec31b0a094

New changeset e60f6e1864d80d80f8928afadec617d161a64150 by Miss Islington (bot) in branch '3.7':
bpo-21983: Fix a crash in ctypes.cast() when passed a ctypes structured data type (GH-3859)
https://github.com/python/cpython/commit/e60f6e1864d80d80f8928afadec617d161a64150

Still needs a backport to 2.7

> Still needs a backport to 2.7

No longer needed, I close the issue.