diff --git a/libc/include/signal.h b/libc/include/signal.h
index 7ae50de..ffce1fa 100644
--- a/libc/include/signal.h
+++ b/libc/include/signal.h
@@ -150,6 +150,7 @@
 int raise(int);
 int kill(pid_t, int);
 int killpg(int, int);
+int tgkill(int tgid, int tid, int sig) __INTRODUCED_IN_32(16);
 
 int sigaltstack(const stack_t*, stack_t*);