diff --git a/lib/std/c/dragonfly.zig b/lib/std/c/dragonfly.zig
index c3086ff32930f4e3f074d41ae28c2d3c853348a8..457e817b682cfa23eca663b2012f8f9b42fb73f0 100644
--- a/lib/std/c/dragonfly.zig
+++ b/lib/std/c/dragonfly.zig
@@ -21,6 +21,7 @@ pub extern "c" fn dl_iterate_phdr(callback: dl_iterate_phdr_callback, data: ?*an
 pub extern "c" fn lwp_gettid() c_int;
 
 pub extern "c" fn posix_memalign(memptr: *?*anyopaque, alignment: usize, size: usize) c_int;
+pub extern "c" fn malloc_usable_size(?*const anyopaque) usize;
 
 pub const pthread_mutex_t = extern struct {
     inner: ?*anyopaque = null,
diff --git a/lib/std/c/haiku.zig b/lib/std/c/haiku.zig
index 53930cdca72de1d7b86f460a1ab69bae63592a28..2e0e02a20f1d2170993b62e9fc7113708598d626 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;