Index: uspace/lib/c/generic/io/io.c
===================================================================
--- uspace/lib/c/generic/io/io.c	(revision 02458965e345492486c6a3bf78d2e11a99e4315a)
+++ 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;
 }
 
Index: uspace/lib/c/include/stdio.h
===================================================================
--- uspace/lib/c/include/stdio.h	(revision 02458965e345492486c6a3bf78d2e11a99e4315a)
+++ uspace/lib/c/include/stdio.h	(revision 58daded42983dc7a64d6d8abca0485d5d85fe7a6)
@@ -170,5 +170,5 @@
 extern void perror(const char *);
 
-extern void setvbuf(FILE *, void *, int, size_t);
+extern int setvbuf(FILE *, void *, int, size_t);
 extern void setbuf(FILE *, void *);