Reads from a file.

    This function is supported on OpenVMS Integrity servers and
    Alpha only.

    Format

      #include  <sys/uio.h>

      ssize_t readv  (int file_desc, const struct iovec *iov, int iovcnt);

      ssize_t _readv64  (int file_desc, struct __iovec64 *iov, int iovcnt);