xsnprintf
xsnprintf(val.s, sizeof(val.s), "%" PRIu64, val.q);
xsnprintf(val.s, sizeof(val.s), "%d", val.i);
void xsnprintf(char *, size_t, const char *, ...)
xsnprintf(buf, sizeof(buf), "Unknown error: %d", error);
xsnprintf(buf, len, fmt, va_alist)
size_t xsnprintf(char *buf, size_t len, const char *fmt, ...);