add support for caller-provided buffers to setvbuf

src/stdio/setvbuf.c

 #include "stdio_impl.h"
 
-/* This function makes no attempt to protect the user from his/her own
- * stupidity. If called any time but when then ISO C standard specifically
- * allows it, all hell can and will break loose, especially with threads!
- *
- * This implementation ignores all arguments except the buffering type,
- * and uses the existing buffer allocated alongside the FILE object.
- * In the case of stderr where the preexisting buffer is length 1, it
- * is not possible to set line buffering or full buffering. */
+/* The behavior of this function is undefined except when it is the first
+ * operation on the stream, so the presence or absence of lockign is not
+ * observable in a program whose behavior is defined. Thus no locking is
+ * performed here. No allocation of buffers is performed, but a buffer
+ * provided by the caller is used as long as it is suitably sized. */
 
 int setvbuf(FILE *restrict f, char *restrict buf, int type, size_t size)
 {
 	f->lbf = EOF;
 
-	if (type == _IONBF)
+	if (type == _IONBF) {
 		f->buf_size = 0;
-	else if (type == _IOLBF)
-		f->lbf = '\n';
+	} else {
+		if (buf && size >= UNGET) {
+			f->buf = (void *)buf;
+			f->buf_size = size - UNGET;
+		}
+		if (type == _IOLBF && f->buf_size)
+			f->lbf = '\n';
+	}
 
 	f->flags |= F_SVB;