Issue #1798: Add ctypes calling convention that allows safe access to

errno (and LastError, on Windows).

ctypes maintains a module-global, but thread-local, variable that
contains an error number; called 'ctypes_errno' for this discussion.
This variable is a private copy of the systems 'errno' value; the copy
is swapped with the 'errno' variable on several occasions.

Foreign functions created with CDLL(..., use_errno=True), when called,
swap the values just before the actual function call, and swapped
again immediately afterwards.  The 'use_errno' parameter defaults to
False, in this case 'ctypes_errno' is not touched.

The values are also swapped immeditately before and after ctypes
callback functions are called, if the callbacks are constructed using
the new optional use_errno parameter set to True: CFUNCTYPE(..., use_errno=TRUE)
or WINFUNCTYPE(..., use_errno=True).

Two new ctypes functions are provided to access the 'ctypes_errno'
value from Python:

- ctypes.set_errno(value) sets ctypes_errno to 'value', the previous
  ctypes_errno value is returned.

- ctypes.get_errno() returns the current ctypes_errno value.

---

On Windows, the same scheme is implemented for the error value which
is managed by the GetLastError() and SetLastError() windows api calls.

The ctypes functions are 'ctypes.set_last_error(value)' and
'ctypes.get_last_error()', the CDLL and WinDLL optional parameter is
named 'use_last_error', defaults to False.

---

On Windows, TlsSetValue and TlsGetValue calls are used to provide
thread local storage for the variables; ctypes compiled with __GNUC__
uses __thread variables.

2 files changed, 125 insertions, 23 deletions

diff --git a/Lib/ctypes/__init__.py b/Lib/ctypes/__init__.py
index 41d39dc..d3e11dc 100644
--- a/Lib/ctypes/__init__.py
+++ b/Lib/ctypes/__init__.py
@@ -33,7 +33,9 @@ if _os.name == "posix" and _sys.platform == "darwin":
         DEFAULT_MODE = RTLD_GLOBAL
 
 from _ctypes import FUNCFLAG_CDECL as _FUNCFLAG_CDECL, \
-     FUNCFLAG_PYTHONAPI as _FUNCFLAG_PYTHONAPI
+     FUNCFLAG_PYTHONAPI as _FUNCFLAG_PYTHONAPI, \
+     FUNCFLAG_USE_ERRNO as _FUNCFLAG_USE_ERRNO, \
+     FUNCFLAG_USE_LASTERROR as _FUNCFLAG_USE_LASTERROR
 
 """
 WINOLEAPI -> HRESULT
@@ -73,8 +75,9 @@ def c_buffer(init, size=None):
     return create_string_buffer(init, size)
 
 _c_functype_cache = {}
-def CFUNCTYPE(restype, *argtypes):
-    """CFUNCTYPE(restype, *argtypes) -> function prototype.
+def CFUNCTYPE(restype, *argtypes, **kw):
+    """CFUNCTYPE(restype, *argtypes,
+                 use_errno=False, use_last_error=False) -> function prototype.
 
     restype: the result type
     argtypes: a sequence specifying the argument types
@@ -88,14 +91,21 @@ def CFUNCTYPE(restype, *argtypes):
     prototype((ordinal number, dll object)[, paramflags]) -> foreign function exported by ordinal
     prototype((function name, dll object)[, paramflags]) -> foreign function exported by name
     """
+    flags = _FUNCFLAG_CDECL
+    if kw.pop("use_errno", False):
+        flags |= _FUNCFLAG_USE_ERRNO
+    if kw.pop("use_last_error", False):
+        flags |= _FUNCFLAG_USE_LASTERROR
+    if kw:
+        raise ValueError("unexpected keyword argument(s) %s" % kw.keys())
     try:
-        return _c_functype_cache[(restype, argtypes)]
+        return _c_functype_cache[(restype, argtypes, flags)]
     except KeyError:
         class CFunctionType(_CFuncPtr):
             _argtypes_ = argtypes
             _restype_ = restype
-            _flags_ = _FUNCFLAG_CDECL
-        _c_functype_cache[(restype, argtypes)] = CFunctionType
+            _flags_ = flags
+        _c_functype_cache[(restype, argtypes, flags)] = CFunctionType
         return CFunctionType
 
 if _os.name in ("nt", "ce"):
@@ -106,16 +116,23 @@ if _os.name in ("nt", "ce"):
         _FUNCFLAG_STDCALL = _FUNCFLAG_CDECL
 
     _win_functype_cache = {}
-    def WINFUNCTYPE(restype, *argtypes):
+    def WINFUNCTYPE(restype, *argtypes, **kw):
         # docstring set later (very similar to CFUNCTYPE.__doc__)
+        flags = _FUNCFLAG_STDCALL
+        if kw.pop("use_errno", False):
+            flags |= _FUNCFLAG_USE_ERRNO
+        if kw.pop("use_last_error", False):
+            flags |= _FUNCFLAG_USE_LASTERROR
+        if kw:
+            raise ValueError("unexpected keyword argument(s) %s" % kw.keys())
         try:
-            return _win_functype_cache[(restype, argtypes)]
+            return _win_functype_cache[(restype, argtypes, flags)]
         except KeyError:
             class WinFunctionType(_CFuncPtr):
                 _argtypes_ = argtypes
                 _restype_ = restype
-                _flags_ = _FUNCFLAG_STDCALL
-            _win_functype_cache[(restype, argtypes)] = WinFunctionType
+                _flags_ = flags
+            _win_functype_cache[(restype, argtypes, flags)] = WinFunctionType
             return WinFunctionType
     if WINFUNCTYPE.__doc__:
         WINFUNCTYPE.__doc__ = CFUNCTYPE.__doc__.replace("CFUNCTYPE", "WINFUNCTYPE")
@@ -124,6 +141,7 @@ elif _os.name == "posix":
     from _ctypes import dlopen as _dlopen
 
 from _ctypes import sizeof, byref, addressof, alignment, resize
+from _ctypes import get_errno, set_errno
 from _ctypes import _SimpleCData
 
 def _check_size(typ, typecode=None):
@@ -313,12 +331,24 @@ class CDLL(object):
     Calling the functions releases the Python GIL during the call and
     reacquires it afterwards.
     """
-    class _FuncPtr(_CFuncPtr):
-        _flags_ = _FUNCFLAG_CDECL
-        _restype_ = c_int # default, can be overridden in instances
+    _func_flags_ = _FUNCFLAG_CDECL
+    _func_restype_ = c_int
 
-    def __init__(self, name, mode=DEFAULT_MODE, handle=None):
+    def __init__(self, name, mode=DEFAULT_MODE, handle=None,
+                 use_errno=False,
+                 use_last_error=False):
         self._name = name
+        flags = self._func_flags_
+        if use_errno:
+            flags |= _FUNCFLAG_USE_ERRNO
+        if use_last_error:
+            flags |= _FUNCFLAG_USE_LASTERROR
+
+        class _FuncPtr(_CFuncPtr):
+            _flags_ = flags
+            _restype_ = self._func_restype_
+        self._FuncPtr = _FuncPtr
+
         if handle is None:
             self._handle = _dlopen(self._name, mode)
         else:
@@ -348,9 +378,7 @@ class PyDLL(CDLL):
     access Python API functions.  The GIL is not released, and
     Python exceptions are handled correctly.
     """
-    class _FuncPtr(_CFuncPtr):
-        _flags_ = _FUNCFLAG_CDECL | _FUNCFLAG_PYTHONAPI
-        _restype_ = c_int # default, can be overridden in instances
+    _func_flags_ = _FUNCFLAG_CDECL | _FUNCFLAG_PYTHONAPI
 
 if _os.name in ("nt", "ce"):
 
@@ -358,9 +386,7 @@ if _os.name in ("nt", "ce"):
         """This class represents a dll exporting functions using the
         Windows stdcall calling convention.
         """
-        class _FuncPtr(_CFuncPtr):
-            _flags_ = _FUNCFLAG_STDCALL
-            _restype_ = c_int # default, can be overridden in instances
+        _func_flags_ = _FUNCFLAG_STDCALL
 
     # XXX Hm, what about HRESULT as normal parameter?
     # Mustn't it derive from c_long then?
@@ -384,9 +410,8 @@ if _os.name in ("nt", "ce"):
         HRESULT error values are automatically raised as WindowsError
         exceptions.
         """
-        class _FuncPtr(_CFuncPtr):
-            _flags_ = _FUNCFLAG_STDCALL
-            _restype_ = HRESULT
+        _func_flags_ = _FUNCFLAG_STDCALL
+        _func_restype_ = HRESULT
 
 class LibraryLoader(object):
     def __init__(self, dlltype):
@@ -424,6 +449,7 @@ if _os.name in ("nt", "ce"):
         GetLastError = windll.kernel32.GetLastError
     else:
         GetLastError = windll.coredll.GetLastError
+    from _ctypes import get_last_error, set_last_error
 
     def WinError(code=None, descr=None):
         if code is None:
diff --git a/Lib/ctypes/test/test_errno.py b/Lib/ctypes/test/test_errno.py
new file mode 100644
index 0000000..b80656b
--- /dev/null
+++ b/Lib/ctypes/test/test_errno.py
@@ -0,0 +1,76 @@
+import unittest, os, errno
+from ctypes import *
+from ctypes.util import find_library
+import threading
+
+class Test(unittest.TestCase):
+    def test_open(self):
+        libc_name = find_library("c")
+        if libc_name is not None:
+            libc = CDLL(libc_name, use_errno=True)
+            if os.name == "nt":
+                libc_open = libc._open
+            else:
+                libc_open = libc.open
+
+            libc_open.argtypes = c_char_p, c_int
+
+            self.failUnlessEqual(libc_open("", 0), -1)
+            self.failUnlessEqual(get_errno(), errno.ENOENT)
+
+            self.failUnlessEqual(set_errno(32), errno.ENOENT)
+            self.failUnlessEqual(get_errno(), 32)
+
+
+            def _worker():
+                set_errno(0)
+
+                libc = CDLL(libc_name, use_errno=False)
+                if os.name == "nt":
+                    libc_open = libc._open
+                else:
+                    libc_open = libc.open
+                libc_open.argtypes = c_char_p, c_int
+                self.failUnlessEqual(libc_open("", 0), -1)
+                self.failUnlessEqual(get_errno(), 0)
+
+            t = threading.Thread(target=_worker)
+            t.start()
+            t.join()
+
+            self.failUnlessEqual(get_errno(), 32)
+            set_errno(0)
+
+    if os.name == "nt":
+
+        def test_GetLastError(self):
+            dll = WinDLL("kernel32", use_last_error=True)
+            GetModuleHandle = dll.GetModuleHandleA
+            GetModuleHandle.argtypes = [c_wchar_p]
+
+            self.failUnlessEqual(0, GetModuleHandle("foo"))
+            self.failUnlessEqual(get_last_error(), 126)
+
+            self.failUnlessEqual(set_last_error(32), 126)
+            self.failUnlessEqual(get_last_error(), 32)
+
+            def _worker():
+                set_last_error(0)
+
+                dll = WinDLL("kernel32", use_last_error=False)
+                GetModuleHandle = dll.GetModuleHandleW
+                GetModuleHandle.argtypes = [c_wchar_p]
+                GetModuleHandle("bar")
+
+                self.failUnlessEqual(get_last_error(), 0)
+
+            t = threading.Thread(target=_worker)
+            t.start()
+            t.join()
+
+            self.failUnlessEqual(get_last_error(), 32)
+
+            set_last_error(0)
+
+if __name__ == "__main__":
+    unittest.main()