diff --git a/include/mimalloc.h b/include/mimalloc.h
index f63038e5..194edb47 100644
--- a/include/mimalloc.h
+++ b/include/mimalloc.h
@@ -114,10 +114,8 @@ mi_decl_export void mi_register_deferred_free(mi_deferred_free_fun* deferred_fre
 typedef void (mi_output_fun)(const char* msg);
 mi_decl_export void mi_register_output(mi_output_fun* out) mi_attr_noexcept;
 
-#if MI_USER_CLEANUP
 typedef void (mi_cleanup_fun)(void* user_data, void* p, size_t size);
-mi_decl_export void mi_register_user_cleanup(mi_cleanup_fun* out, void* user_data) mi_attr_noexcept;
-#endif
+mi_decl_export void mi_register_user_cleanup(mi_cleanup_fun* cleanup, void* user_data) mi_attr_noexcept;
 
 mi_decl_export void mi_collect(bool force)    mi_attr_noexcept;
 mi_decl_export int  mi_version(void)          mi_attr_noexcept;
diff --git a/src/os.c b/src/os.c
index 2ad0648f..4d08c7bd 100644
--- a/src/os.c
+++ b/src/os.c
@@ -177,6 +177,39 @@ void _mi_os_init() {
 }
 #endif
 
+#if MI_USER_CLEANUP
+static mi_cleanup_fun* user_cleanup = NULL;
+static void* cleanup_udata = NULL;
+static void _mi_call_user_cleanup(void* p, size_t size)
+{
+  if(user_cleanup)
+    user_cleanup(cleanup_udata, p, size);
+#if (MI_DEBUG>1)
+  else
+    memset(start, 0, csize);
+#endif
+}
+#else
+static void _mi_call_user_cleanup(void* p, size_t size)
+{
+  (void)p;
+  (void)size;
+#if (MI_DEBUG>1)
+  if (MI_SECURE==0)
+    memset(start, 0, csize); // pretend it is eagerly reset
+#endif
+}
+#endif
+
+void mi_register_user_cleanup(mi_cleanup_fun* cleanup, void* user_data)
+{
+  (void)cleanup;
+  (void)user_data;
+#if MI_USER_CLEANUP
+  user_cleanup = cleanup;
+  cleanup_udata = user_data;
+#endif
+}
 
 /* -----------------------------------------------------------
   Raw allocation on Windows (VirtualAlloc) and Unix's (mmap).
@@ -186,6 +219,7 @@ static bool mi_os_mem_free(void* addr, size_t size, bool was_committed, mi_stats
 {
   if (addr == NULL || size == 0 || _mi_os_is_huge_reserved(addr)) return true;
   bool err = false;
+  _mi_call_user_cleanup(addr, size);
 #if defined(_WIN32)
   err = (VirtualFree(addr, 0, MEM_RELEASE) == 0);
 #elif defined(__wasi__)
@@ -687,11 +721,7 @@ static bool mi_os_resetx(void* addr, size_t size, bool reset, mi_stats_t* stats)
         else _mi_stat_decrease(&stats->reset, csize);
   if (!reset) return true; // nothing to do on unreset!
 
-  #if (MI_DEBUG>1)
-  if (MI_SECURE==0) {
-    memset(start, 0, csize); // pretend it is eagerly reset
-  }
-  #endif
+  _mi_call_user_cleanup(start, size);
 
 #if defined(_WIN32)
   // Testing shows that for us (on `malloc-large`) MEM_RESET is 2x faster than DiscardVirtualMemory