diff --git a/libc/stdio/setvbuf.c b/libc/stdio/setvbuf.c
index 9b92bf0..2fb76af 100644
--- a/libc/stdio/setvbuf.c
+++ b/libc/stdio/setvbuf.c
@@ -61,6 +61,7 @@
 	 * malloc()ed.  We also clear any eof condition, as if this were
 	 * a seek.
 	 */
+	FLOCKFILE(fp);
 	ret = 0;
 	(void)__sflush(fp);
 	if (HASUB(fp))
@@ -107,6 +108,7 @@
 			fp->_w = 0;
 			fp->_bf._base = fp->_p = fp->_nbuf;
 			fp->_bf._size = 1;
+			FUNLOCKFILE(fp);
 			return (ret);
 		}
 		flags |= __SMBF;
@@ -145,6 +147,7 @@
 		/* begin/continue reading, or stay in intermediate state */
 		fp->_w = 0;
 	}
+	FUNLOCKFILE(fp);
 	__atexit_register_cleanup(_cleanup);
 
 	return (ret);