diff --git a/lib/std/c/haiku.zig b/lib/std/c/haiku.zig
index 53930cdca7..2e0e02a20f 100644
--- a/lib/std/c/haiku.zig
+++ b/lib/std/c/haiku.zig
@@ -1071,3 +1071,5 @@ pub const sigevent = extern struct {
 
 /// TODO refines if necessary
 pub const PTHREAD_STACK_MIN = 2 * 4096;
+
+pub extern "c" fn malloc_usable_size(?*anyopaque) usize;