diff --git a/Modules/posixmodule.c b/Modules/posixmodule.c
--- a/Modules/posixmodule.c
+++ b/Modules/posixmodule.c
@@ -6622,12 +6622,24 @@ posix_confstr(PyObject *self, PyObject *
     PyObject *result = NULL;
     int name;
     char buffer[256];
+    char *allocated = NULL;
 
     if (PyArg_ParseTuple(args, "O&:confstr", conv_confstr_confname, &name)) {
-        int len;
+        char *recvbuf = buffer;
+        size_t len, buflen = sizeof(buffer);
 
         errno = 0;
-        len = confstr(name, buffer, sizeof(buffer));
+        while ((len = confstr(name, recvbuf, buflen)) > buflen) {
+            recvbuf = PyMem_Realloc(allocated, len);
+            if (recvbuf == NULL) {
+                PyErr_NoMemory();
+                goto finally;
+            }
+            allocated = recvbuf;
+            buflen = len;
+            errno = 0;
+        }
+
         if (len == 0) {
             if (errno) {
                 posix_error();
@@ -6638,15 +6650,11 @@ posix_confstr(PyObject *self, PyObject *
             }
         }
         else {
-            if ((unsigned int)len >= sizeof(buffer)) {
-                result = PyUnicode_FromStringAndSize(NULL, len-1);
-                if (result != NULL)
-                    confstr(name, _PyUnicode_AsString(result), len);
-            }
-            else
-                result = PyUnicode_FromStringAndSize(buffer, len-1);
+            result = PyUnicode_FromStringAndSize(recvbuf, len - 1);
         }
     }
+finally:
+    PyMem_Free(allocated);
     return result;
 }
 #endif