int xwrite(int fd, const char *buf, size_t len);
int xwritev(int fd, struct iovec *iov, int iovcnt);
int for_each_file_in_dir(const char *dirname,
int xwrite(int fd, const char *buf, size_t len);
int xwritev(int fd, struct iovec *iov, int iovcnt);
int for_each_file_in_dir(const char *dirname,