diff --git a/libc/include/sys/system_properties.h b/libc/include/sys/system_properties.h
index fb90251..b55566e 100644
--- a/libc/include/sys/system_properties.h
+++ b/libc/include/sys/system_properties.h
@@ -30,6 +30,7 @@
 #define _INCLUDE_SYS_SYSTEM_PROPERTIES_H
 
 #include <sys/cdefs.h>
+#include <stdbool.h>
 #include <stddef.h>
 #include <stdint.h>
 
@@ -68,6 +69,25 @@
 int __system_property_foreach(void (*propfn)(const prop_info* pi, void* cookie), void* cookie)
   __INTRODUCED_IN(19);
 
+/*
+ * Waits for the specific system property identified by `pi` to be updated
+ * past `old_serial`. Waits no longer than `relative_timeout`, or forever
+ * if `relaive_timeout` is null.
+ *
+ * If `pi` is null, waits for the global serial number instead.
+ *
+ * If you don't know the current serial, use 0.
+ *
+ * Returns true and updates `*new_serial_ptr` on success, or false if the call
+ * timed out.
+ */
+struct timespec;
+bool __system_property_wait(const prop_info* pi,
+                            uint32_t old_serial,
+                            uint32_t* new_serial_ptr,
+                            const struct timespec* relative_timeout)
+    __INTRODUCED_IN_FUTURE;
+
 /* Deprecated. In Android O and above, there's no limit on property name length. */
 #define PROP_NAME_MAX   32
 /* Deprecated. Use __system_property_read_callback instead. */