Index: configure.in
===================================================================
--- configure.in	(revision 76628)
+++ configure.in	(working copy)
@@ -2547,7 +2547,7 @@
  clock confstr ctermid execv fchmod fchown fork fpathconf ftime ftruncate \
  gai_strerror getgroups getlogin getloadavg getpeername getpgid getpid \
  getpriority getresuid getresgid getpwent getspnam getspent getsid getwd \
- kill killpg lchmod lchown lstat mkfifo mknod mktime \
+ initgroups kill killpg lchmod lchown lstat mkfifo mknod mktime \
  mremap nice pathconf pause plock poll pthread_init \
  putenv readlink realpath \
  select sem_open sem_timedwait sem_getvalue sem_unlink setegid seteuid \
Index: Doc/library/os.rst
===================================================================
--- Doc/library/os.rst	(revision 76628)
+++ Doc/library/os.rst	(working copy)
@@ -136,6 +136,13 @@
    Availability: Unix.
 
 
+.. function:: initgroups(username, gid)
+
+   Call the system initgroups() to initialize the group access list with all of
+   the groups of which the specified username is a member, plus the specified
+   group id. Availability: Unix.
+
+
 .. function:: getlogin()
 
    Return the name of the user logged in on the controlling terminal of the
Index: Lib/test/test_posix.py
===================================================================
--- Lib/test/test_posix.py	(revision 76628)
+++ Lib/test/test_posix.py	(working copy)
@@ -5,6 +5,7 @@
 # Skip these tests if there is no posix module.
 posix = test_support.import_module('posix')
 
+import errno
 import time
 import os
 import pwd
@@ -83,6 +84,26 @@
                 new_group_ids = (current_group_ids[0]+1, -1, -1)
                 self.assertRaises(OSError, posix.setresgid, *new_group_ids)
 
+    @unittest.skipUnless(hasattr(posix, 'initgroups'))
+    def test_initgroups(self):
+        # It takes a string and an integer; check that it raises a TypeError
+        # for other argument lists.
+        self.assertRaises(TypeError, posix.initgroups)
+        self.assertRaises(TypeError, posix.initgroups, None)
+        self.assertRaises(TypeError, posix.initgroups, 3, "foo")
+        self.assertRaises(TypeError, posix.initgroups, "foo", 3, object())
+
+        # If a non-privileged user invokes it, it should fail with OSError
+        # EPERM.
+        if os.getuid() != 0:
+            name = pwd.getpwuid(posix.getuid()).pw_nam
+            try:
+                posix.initgroups(name, 13)
+            except OSError, e:
+                self.assertEquals(e.errno, errno.EPERM)
+            else:
+                self.fail("Expected OSError to be raised by initgroups")
+
     def test_statvfs(self):
         if hasattr(posix, 'statvfs'):
             self.assertTrue(posix.statvfs(os.curdir))
Index: Modules/posixmodule.c
===================================================================
--- Modules/posixmodule.c	(revision 76628)
+++ Modules/posixmodule.c	(working copy)
@@ -3861,6 +3861,33 @@
 }
 #endif
 
+#ifdef HAVE_INITGROUPS
+PyDoc_STRVAR(posix_initgroups__doc__,
+"initgroups(username, gid) -> None\n\n\
+Call the system initgroups() to initialize the group access list with all of\n\
+the groups of which the specified username is a member, plus the specified\n\
+group id.");
+
+static PyObject *
+posix_initgroups(PyObject *self, PyObject *args)
+{
+        char *username;
+        unsigned int igid;
+        gid_t gid;
+
+        if (!PyArg_ParseTuple(args, "sI:initgroups", &username, &igid))
+                return NULL;
+
+        gid = igid;
+
+        if (initgroups(username, gid) == -1)
+                return PyErr_SetFromErrno(PyExc_OSError);
+
+        Py_INCREF(Py_None);
+        return Py_None;
+}
+#endif
+
 #ifdef HAVE_GETPGID
 PyDoc_STRVAR(posix_getpgid__doc__,
 "getpgid(pid) -> pgid\n\n\
@@ -8629,6 +8656,9 @@
 #ifdef HAVE_SETGROUPS
 	{"setgroups",	posix_setgroups, METH_O, posix_setgroups__doc__},
 #endif /* HAVE_SETGROUPS */
+#ifdef HAVE_INITGROUPS
+	{"initgroups",	posix_initgroups, METH_VARARGS, posix_initgroups__doc__},
+#endif /* HAVE_INITGROUPS */
 #ifdef HAVE_GETPGID
 	{"getpgid",	posix_getpgid, METH_VARARGS, posix_getpgid__doc__},
 #endif /* HAVE_GETPGID */
Index: pyconfig.h.in
===================================================================
--- pyconfig.h.in	(revision 76628)
+++ pyconfig.h.in	(working copy)
@@ -301,6 +301,9 @@
 /* Define to 1 if you have the `getpeername' function. */
 #undef HAVE_GETPEERNAME
 
+/* Define to 1 if you have the `initgroups' function. */
+#undef HAVE_INITGROUPS
+
 /* Define to 1 if you have the `getpgid' function. */
 #undef HAVE_GETPGID