diff --git a/Doc/library/dbm.rst b/Doc/library/dbm.rst
index 6f548fbb1b39d8..39e287b15214e4 100644
--- a/Doc/library/dbm.rst
+++ b/Doc/library/dbm.rst
@@ -254,6 +254,9 @@ functionality like crash tolerance.
* ``'s'``: Synchronized mode.
Changes to the database will be written immediately to the file.
* ``'u'``: Do not lock database.
+ * ``'m'``: Do not use :manpage:`mmap(2)`.
+ This may harm performance, but improve crash tolerance.
+ .. versionadded:: next
Not all flags are valid for all versions of GDBM.
See the :data:`open_flags` member for a list of supported flag characters.
diff --git a/Doc/whatsnew/3.15.rst b/Doc/whatsnew/3.15.rst
index 244ce327763f57..e2006d1674c680 100644
--- a/Doc/whatsnew/3.15.rst
+++ b/Doc/whatsnew/3.15.rst
@@ -96,6 +96,10 @@ dbm
which allow to recover unused free space previously occupied by deleted entries.
(Contributed by Andrea Oliveri in :gh:`134004`.)
+* Add the ``'m'`` flag for :func:`dbm.gnu.open` which allows to disable
+ the use of :manpage:`mmap(2)`.
+ This may harm performance, but improve crash tolerance.
+ (Contributed by Serhiy Storchaka in :gh:`66234`.)
difflib
-------
diff --git a/Lib/test/test_dbm_gnu.py b/Lib/test/test_dbm_gnu.py
index 66268c42a300b5..e0b988b7b95bbd 100644
--- a/Lib/test/test_dbm_gnu.py
+++ b/Lib/test/test_dbm_gnu.py
@@ -74,12 +74,12 @@ def test_flags(self):
# Test the flag parameter open() by trying all supported flag modes.
all = set(gdbm.open_flags)
# Test standard flags (presumably "crwn").
- modes = all - set('fsu')
+ modes = all - set('fsum')
for mode in sorted(modes): # put "c" mode first
self.g = gdbm.open(filename, mode)
self.g.close()
- # Test additional flags (presumably "fsu").
+ # Test additional flags (presumably "fsum").
flags = all - set('crwn')
for mode in modes:
for flag in flags:
@@ -217,6 +217,29 @@ def test_localized_error(self):
create_empty_file(os.path.join(d, 'test'))
self.assertRaises(gdbm.error, gdbm.open, filename, 'r')
+ @unittest.skipUnless('m' in gdbm.open_flags, "requires 'm' in open_flags")
+ def test_nommap_no_crash(self):
+ self.g = g = gdbm.open(filename, 'nm')
+ os.truncate(filename, 0)
+
+ g.get(b'a', b'c')
+ g.keys()
+ g.firstkey()
+ g.nextkey(b'a')
+ with self.assertRaises(KeyError):
+ g[b'a']
+ with self.assertRaises(gdbm.error):
+ len(g)
+
+ with self.assertRaises(gdbm.error):
+ g[b'a'] = b'c'
+ with self.assertRaises(gdbm.error):
+ del g[b'a']
+ with self.assertRaises(gdbm.error):
+ g.setdefault(b'a', b'c')
+ with self.assertRaises(gdbm.error):
+ g.reorganize()
+
if __name__ == '__main__':
unittest.main()
diff --git a/Misc/NEWS.d/next/Library/2025-06-01-15-13-07.gh-issue-66234.Jw7OdC.rst b/Misc/NEWS.d/next/Library/2025-06-01-15-13-07.gh-issue-66234.Jw7OdC.rst
new file mode 100644
index 00000000000000..1defb9a72e04e7
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2025-06-01-15-13-07.gh-issue-66234.Jw7OdC.rst
@@ -0,0 +1,3 @@
+Add the ``'m'`` flag for :func:`dbm.gnu.open` which allows to disable the
+use of :manpage:`mmap(2)`. This may harm performance, but improve crash
+tolerance.
diff --git a/Modules/_gdbmmodule.c b/Modules/_gdbmmodule.c
index 9c402e20e513b9..6a4939512b22fc 100644
--- a/Modules/_gdbmmodule.c
+++ b/Modules/_gdbmmodule.c
@@ -813,6 +813,11 @@ dbmopen_impl(PyObject *module, PyObject *filename, const char *flags,
case 'u':
iflags |= GDBM_NOLOCK;
break;
+#endif
+#ifdef GDBM_NOMMAP
+ case 'm':
+ iflags |= GDBM_NOMMAP;
+ break;
#endif
default:
PyErr_Format(state->gdbm_error,
@@ -846,6 +851,9 @@ static const char gdbmmodule_open_flags[] = "rwcn"
#endif
#ifdef GDBM_NOLOCK
"u"
+#endif
+#ifdef GDBM_NOMMAP
+ "m"
#endif
;
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