diff --git a/libc/sysv/consts.sh b/libc/sysv/consts.sh
index cc7972b84..7c811417e 100755
--- a/libc/sysv/consts.sh
+++ b/libc/sysv/consts.sh
@@ -272,6 +272,7 @@ syscon	madv	MADV_PAGEOUT				21			21			127			127			127			127			127			127			# TODO
 syscon	madv	MADV_POPULATE_READ				22			22			127			127			127			127			127			127			# TODO: add support ?
 syscon	madv	MADV_POPULATE_WRITE				23			23			127			127			127			127			127			127			# TODO: add support ?
 syscon	madv	MADV_DONTNEED_LOCKED				24			24			127			127			127			127			127			127			# TODO: add support ?
+syscon	madv	MADV_COLLAPSE				25			25			127			127			127			127			127			127			# TODO: add support ?
 syscon	madv	MADV_DOFORK				11			11			127			127			127			127			127			127			# TODO(jart): what is it?
 syscon	madv	MADV_DONTDUMP				16			16			127			127			127			127			127			127			# see MAP_CONCEAL in OpenBSD; TODO(jart): what is it?
 syscon	madv	MADV_DONTFORK				10			10			127			127			127			127			127			127			# TODO(jart): what is it?
diff --git a/libc/sysv/consts/MADV_COLLAPSE.S b/libc/sysv/consts/MADV_COLLAPSE.S
new file mode 100644
index 000000000..c822224ac
--- /dev/null
+++ b/libc/sysv/consts/MADV_COLLAPSE.S
@@ -0,0 +1,2 @@
+#include "libc/sysv/consts/syscon.internal.h"
+.syscon madv,MADV_COLLAPSE,25,25,127,127,127,127,127,127
diff --git a/libc/sysv/consts/madv.h b/libc/sysv/consts/madv.h
index 59c00f4d1..df2f5b9f8 100644
--- a/libc/sysv/consts/madv.h
+++ b/libc/sysv/consts/madv.h
@@ -27,6 +27,7 @@ extern const unsigned MADV_PAGEOUT;
 extern const unsigned MADV_POPULATE_READ;
 extern const unsigned MADV_POPULATE_WRITE;
 extern const unsigned MADV_DONTNEED_LOCKED;
+extern const unsigned MADV_COLLAPSE;
 
 #define MADV_NORMAL     0
 #define MADV_RANDOM     1