Description

   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.