readx
readx(fd[1], buf, int_arg);
readx(fd[0], buf, int_arg);
readx(fd, buf, int_arg);
readx(fd[0], buf, int_arg);
static size_t readx(int, void *, size_t, off_t);
if (readx(fd, btx, sizeof(*btx), 0) != sizeof(*btx) ||
if (readx(fdi, buf, n, offset) != n)