Index: uspace/lib/c/generic/io/io.c
===================================================================
--- uspace/lib/c/generic/io/io.c	(revision 984a9ba2fa05c490a7066737655ed19130c714e8)
+++ uspace/lib/c/generic/io/io.c	(revision 58daded42983dc7a64d6d8abca0485d5d85fe7a6)
@@ -242,6 +242,9 @@
 
 /** Set stream buffer. */
-void setvbuf(FILE *stream, void *buf, int mode, size_t size)
-{
+int setvbuf(FILE *stream, void *buf, int mode, size_t size)
+{
+	if (mode != _IONBF && mode != _IOLBF && mode != _IOFBF)
+		return -1;
+
 	stream->btype = mode;
 	stream->buf = buf;
@@ -250,4 +253,6 @@
 	stream->buf_tail = stream->buf;
 	stream->buf_state = _bs_empty;
+
+	return 0;
 }