diff --git a/libc/include/malloc.h b/libc/include/malloc.h
index f5fbedf..58a9b1d 100644
--- a/libc/include/malloc.h
+++ b/libc/include/malloc.h
@@ -163,6 +163,7 @@
 
 /** mallopt() option to set the decay time. Valid values are 0 and 1. */
 #define M_DECAY_TIME -100
+#define M_PURGE -101
 
 /**
  * [mallopt(3)](http://man7.org/linux/man-pages/man3/mallopt.3.html) modifies