The writev function is equivalent to write but gathers the output
    data from the iovcnt buffers specified by the members of the iov
    array: iov[0], iov[1], ..., iov[iovcnt-1]. The iovcnt argument
    is valid if greater than 0 and less than or equal to {IOV_MAX},
    defined in <limits.h>.

    Each iovec entry specifies the base address and length of an area
    in memory from which data should be written. The writev function
    writes a complete area before proceeding to the next.

    If filedes refers to a regular file and all of the iov_len
    members in the array pointed to by iov are 0, writev returns 0
    and has no other effect.

    For other file types, the behavior is unspecified.

    If the sum of the iov_len values is greater than SSIZE_MAX, the
    operation fails and no data is transferred.

    Upon successful completion, writev returns the number of bytes
    actually written. Otherwise, it returns a value of -1, the file
    pointer remains unchanged, and errno is set to indicate an error.