Fix skipdata setup (capstone-engine#1320)

* Fix skipdata_setup for when _cb is None

ctypes prototype does not accept None value,
so if we want to get a NULL function pointer
then we should either call it with no arguments
or pass zero as an argument.

Fixes capstone-engine#1316

* Do store and return skipdata_setup data

* Add convenience wrappers for skipdata_setup

* Uncomment skipdata_setup tests

* Add alternate usage variants to test_skipdata.py

* document getter

bindings/python/capstone/__init__.py

@@ -828,6 +828,7 @@ def __init__(self, arch, mode):
 
         # default mnemonic for SKIPDATA
         self._skipdata_mnem = ".byte"
+        self._skipdata_cb = (None, None)
         self._skipdata = False
 
 
@@ -897,21 +898,45 @@ def skipdata(self, opt):
 
     @property
     def skipdata_setup(self):
-        return
+        return (self._skipdata_mnem,) + self._skipdata_cb
 
 
     @skipdata_setup.setter
     def skipdata_setup(self, opt):
         _skipdata_opt = _cs_opt_skipdata()
         _mnem, _cb, _ud = opt
         _skipdata_opt.mnemonic = _mnem.encode()
-        _skipdata_opt.callback = CS_SKIPDATA_CALLBACK(_cb)
+        _skipdata_opt.callback = CS_SKIPDATA_CALLBACK(_cb or 0)
         _skipdata_opt.user_data = ctypes.cast(_ud, ctypes.c_void_p)
         status = _cs.cs_option(self.csh, CS_OPT_SKIPDATA_SETUP, ctypes.cast(ctypes.byref(_skipdata_opt), ctypes.c_void_p))
         if status != CS_ERR_OK:
             raise CsError(status)
 
-        self._skipdata_opt = _skipdata_opt
+        self._skipdata_mnem = _mnem
+        self._skipdata_cb = (_cb, _ud)
+
+
+    @property
+    def skipdata_mnem(self):
+        return self._skipdata_mnem
+
+
+    @skipdata_mnem.setter
+    def skipdata_mnem(self, mnem):
+        self.skipdata_setup = (mnem,) + self._skipdata_cb
+
+
+    @property
+    def skipdata_cb(self):
+        return self._skipdata_cb
+
+
+    @skipdata_cb.setter
+    def skipdata_cb(self, val):
+        if not isinstance(val, tuple):
+            val = (val, None)
+        func, data = val
+        self.skipdata_setup = (self._skipdata_mnem, func, data)
 
 
     # customize instruction mnemonic

bindings/python/test_skipdata.py

@@ -39,13 +39,21 @@ def test_class():
 
             md.skipdata = True
 
-            # Default "data" instruction's name is ".byte". To rename it to "db", just uncomment
+            # Default "data" instruction's name is ".byte". To rename it to "db", just use
             # the code below.
-            # md.skipdata_setup = ("db", None, None)
+            md.skipdata_setup = ("db", None, None)
             # NOTE: This example ignores SKIPDATA's callback (first None) & user_data (second None)
-
-            # To customize the SKIPDATA callback, uncomment the line below.
-            # md.skipdata_setup = (".db", testcb, None)
+            # Can also use dedicated setter
+            md.skipdata_mnem = 'db'
+
+            # To customize the SKIPDATA callback, use the line below.
+            md.skipdata_setup = (".db", testcb, None)
+            # Or use dedicated setter with custom parameter
+            md.skipdata_cb = (testcb, 42)
+            # Or provide just a function
+            md.skipdata_cb = testcb
+            # Note that reading this property will always return a tuple
+            assert md.skipdata_cb == (testcb, None), md.skipdata_cb
 
             for insn in md.disasm(code, 0x1000):
                 #bytes = binascii.hexlify(insn.bytes)