The readv function is equivalent to read, but places the input
    data into the iovcnt buffers specified by the members of the iov
    array: iov[0], iov[1], ..., iov[iovcnt-1]. The iovcnt argument is
    valid if it is greater than 0 and less than or equal to IOV_MAX.

    Each iovec entry specifies the base address and length of an area
    in memory where data should be placed. The readv function always
    fills an area completely before proceeding to the next.

    Upon successful completion, readv marks for update the st_atime
    field of the file.

    If the Synchronized Input and Output option is supported:

          If the O_DSYNC and O_RSYNC bits have been set, read
          I/O operations on the file descriptor will complete
          as defined by synchronized I/O data integrity
          completion.

          If the O_SYNC and O_RSYNC bits have been set, read
          I/O operations on the file descriptor will complete
          as defined by synchronized I/O file integrity
          completion.

    If the Shared Memory Objects option is supported:

          If file_desc refers to a shared memory object, the
          result of the read function is unspecified.

    For regular files, no data transfer occurs past the offset
    maximum established in the open file description associated with
    file_desc.