Index: Modules/posixmodule.c
===================================================================
--- Modules/posixmodule.c	(revision 78371)
+++ Modules/posixmodule.c	(working copy)
@@ -3841,17 +3841,41 @@
 #define MAX_GROUPS 64
 #endif
         gid_t grouplist[MAX_GROUPS];
+	gid_t* alt_grouplist = NULL;
         int n;
 
         n = getgroups(MAX_GROUPS, grouplist);
-        if (n < 0)
-            posix_error();
-        else {
-            result = PyList_New(n);
-            if (result != NULL) {
+        if (n < 0) {
+	    if (errno == EINVAL) {
+		    n = getgroups(0, grouplist);
+		    if (n == -1) {
+			    return posix_error();
+		    }
+		    if (n == 0) {
+			    alt_grouplist = grouplist;
+	            } else {
+			    alt_grouplist = PyMem_Malloc(n);
+			    if (alt_grouplist == NULL) {
+				    errno = EINVAL;
+				    return posix_error();
+			    }
+			    n = getgroups(n, grouplist);
+			    if (n == -1) {
+				    PyMem_Free(alt_grouplist);
+				    return posix_error;
+			    }
+		    }
+            } else {
+		    return posix_error();
+	    }
+	} else {
+		alt_grouplist = grouplist;
+	}
+	result = PyList_New(n);
+        if (result != NULL) {
                 int i;
                 for (i = 0; i < n; ++i) {
-                    PyObject *o = PyInt_FromLong((long)grouplist[i]);
+                    PyObject *o = PyInt_FromLong((long)alt_grouplist[i]);
                     if (o == NULL) {
                         Py_DECREF(result);
                         result = NULL;
@@ -3859,10 +3883,13 @@
                     }
                     PyList_SET_ITEM(result, i, o);
                 }
-            }
-        }
+	}
 
-    return result;
+	if (alt_grouplist != grouplist) {
+		PyMem_Free(alt_grouplist);
+	}
+
+	return result;
 }
 #endif