diff --git a/libc/include/sys/ucontext.h b/libc/include/sys/ucontext.h
index c65e2b6..94b7a68 100644
--- a/libc/include/sys/ucontext.h
+++ b/libc/include/sys/ucontext.h
@@ -74,9 +74,20 @@
   /* TODO: uc_regspace */
 } ucontext_t;
 
-#elif defined(__arm64__)
+#elif defined(__aarch64__)
 
-#error TODO
+/* TODO: gregset_t and fpregset_t. */
+
+#include <asm/sigcontext.h>
+typedef struct sigcontext mcontext_t;
+
+typedef struct ucontext {
+  unsigned long uc_flags;
+  struct ucontext *uc_link;
+  stack_t uc_stack;
+  sigset_t uc_sigmask;
+  mcontext_t uc_mcontext;
+} ucontext_t;
 
 #elif defined(__i386__)