diff --git a/libc/include/unistd.h b/libc/include/unistd.h
index 12e6257..82c53e8 100644
--- a/libc/include/unistd.h
+++ b/libc/include/unistd.h
@@ -170,7 +170,7 @@
 extern int optind, opterr, optopt;
 
 extern int isatty(int);
-extern char* ttyname(int);
+extern char* ttyname(int) __warnattr("ttyname is not thread-safe; use ttyname_r instead");
 extern int ttyname_r(int, char*, size_t);
 
 extern int  acct(const char*  filepath);