diff --git a/libc/include/unistd.h b/libc/include/unistd.h
index b4982cb..ee6d7b1 100644
--- a/libc/include/unistd.h
+++ b/libc/include/unistd.h
@@ -54,7 +54,7 @@
 extern pid_t  fork(void);
 extern pid_t  vfork(void);
 extern pid_t  getpid(void);
-extern pid_t  gettid(void);
+extern pid_t  gettid(void) __pure2;
 extern pid_t  getpgid(pid_t);
 extern int    setpgid(pid_t, pid_t);
 extern pid_t  getppid(void);