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.

1 files changed, 76 insertions, 0 deletions

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()