evbuffer_add_vprintf
res = evbuffer_add_vprintf(buf, fmt, ap);
int evbuffer_add_vprintf(struct evbuffer *, const char *fmt, va_list ap);
evbuffer_add_vprintf(evb, fmt, ap);
evbuffer_add_vprintf(cf->buffer, fmt, ap);
evbuffer_add_vprintf(cf->buffer, fmt, ap);
evbuffer_add_vprintf(cf->buffer, fmt, ap);
evbuffer_add_vprintf(cf->buffer, fmt, ap);
ret = evbuffer_add_vprintf(cre->output, fmt, ap);