diff --git a/libc/include/malloc.h b/libc/include/malloc.h
index 58a9b1d..42237d0 100644
--- a/libc/include/malloc.h
+++ b/libc/include/malloc.h
@@ -161,8 +161,18 @@
  */
 int malloc_info(int __must_be_zero, FILE* __fp) __INTRODUCED_IN(23);
 
-/** mallopt() option to set the decay time. Valid values are 0 and 1. */
+/**
+ * mallopt() option to set the decay time. Valid values are 0 and 1.
+ *
+ * Available since API level 27.
+ */
 #define M_DECAY_TIME -100
+/**
+ * mallopt() option to immediately purge any memory not in use. This
+ * will release the memory back to the kernel. The value is ignored.
+ *
+ * Available since API level 28.
+ */
 #define M_PURGE -101
 
 /**