int ctl_vprintf(const char *fmt, va_list ap);
ctl_vprintf writes to the standard output stream under control of the string pointed to by fmt that specifies how subsequent arguments are converted for output. Before calling ctl_vprintf, ap must be initialized by the va_start macro (and possibly subsequent va_arg calls). ctl_vprintf does not invoke the va_end macro.
ctl_vprintf returns the number of characters transmitted, or a negative value if an output error occurred.