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I did some debugging and I found out that Python3 correctly gets a return value of XOpenDisplay() but passes the argument of XCloseDisplay() truncated to 4 Bytes because of a bug in Modules/_ctypes/callproc.c at line 1795 where a unsigned long rvalue is assigned to an int lvalue:

   pa->value.i = (long)PyLong_AsUnsignedLong(obj);

pa->value : struct param { ... union value; ... }  : union value { ... int i; long l; ...}

Perhaps this should be reported upstream to python?

Actually, there is one more problem in Python ctypes. The return type of all functions is 'c_int' and that leads to truncation of the return value too.

Lib/ctypes/__init__.py
  class CDLL(object):
     _func_restype_ = c_int

When I run a modified reproducer from bug #1020505 comment #2 I get this:

print(XLIB.XOpenDisplay._restype_._type_) : i

Breakpoint 1, XOpenDisplay (display=0x0) at OpenDis.c:66
66      {
(gdb) n
...
115             if ((dpy = Xcalloc(1, sizeof(Display))) == NULL) {
(gdb) 
119             if ((dpy->display_name = strdup(display_name)) == NULL) {
(gdb) p dpy
$1 = (Display *) 0x55555590e0d0
(gdb) c
Continuing.
print(type(DISPLAY)) : <class 'int'>
print(hex(DISPLAY))  : 0x5590e0d0

Program received signal SIGSEGV, Segmentation fault.
XCloseDisplay (dpy=0x5590e0d0) at ClDisplay.c:51
51              if (!(dpy->flags & XlibDisplayClosing))




If I change _func_restype_ to c_long and run the reproducere again I get this:

print(XLIB.XOpenDisplay._restype_._type_) : l

Breakpoint 1, XOpenDisplay (display=0x0) at OpenDis.c:66
66      {
(gdb) n
... 
115             if ((dpy = Xcalloc(1, sizeof(Display))) == NULL) {
(gdb) 
119             if ((dpy->display_name = strdup(display_name)) == NULL) {
(gdb) p dpy
$1 = (Display *) 0x55555590dbf0
(gdb) c
Continuing.
print(type(DISPLAY)) : <class 'int'>
print(hex(DISPLAY))  : 0x55555590dbf0

Program received signal SIGSEGV, Segmentation fault.
XCloseDisplay (dpy=0x5590dbf0) at ClDisplay.c:51
51              if (!(dpy->flags & XlibDisplayClosing))

I noticed the documentation says that Python ints are always assumed to C ints.
There is indeed a Python bug, but in validation: using Python int that fit into a C long but not a C long is quietly truncated. Passing a Python2 long, or Python3 int, that is larger than a C long will fail with an OverflowError.

To pass a C long, there are two alternatives:
- explicitly wrap the number in c_long [0]:
  dll.function(ctyles.c_long(param))

- use prototypes [1]:

  func = dll.function
  func.argtypes = [c_long]
  func(param)


[0]: https://docs.python.org/2/library/ctypes.html#fundamental-data-types
[1]: https://docs.python.org/2/library/ctypes.html#specifying-the-required-argument-types-function-prototypes

D`oh, sorry for the noise!

Adding these lines to the reproducer fixes the crash: 

XOpenDisplay = XLIB.XOpenDisplay                                                 
XOpenDisplay.restype = c_void_p                                                  
                                                                                 
XCloseDisplay = XLIB.XCloseDisplay                                               
XCloseDisplay.argtypes = [c_void_p]

Validation error reported upstream as http://bugs.python.org/issue24747

I'll write a patch for it after vacation, if it's still open.

I have opened a github pull request for the bug in fros:
https://github.com/mozeq/fros/pull/12