This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface

       None.

       Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard
       for Information  Technology  --  Portable  Operating  System  Interface  (POSIX),  The  Open  Group  Base
       Specifications  Issue  7, 2018 Edition, Copyright (C) 2018 by the Institute of Electrical and Electronics
       Engineers, Inc and The Open Group.  In the event of any discrepancy between this version and the original
       IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee  document.
       The original Standard can be obtained online at http://www.opengroup.org/unix/online.html .

       Any  typographical  or formatting errors that appear in this page are most likely to have been introduced
       during  the  conversion  of  the  source  files  to  man  page  format.  To  report  such   errors,   see
       https://www.kernel.org/doc/man-pages/reporting_bugs.html .

IEEE/The Open Group                                   2017                                        WRITEV(3POSIX)

       The  writev()  function  shall be equivalent to write(), except as described below. The writev() function
       shall gather 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}, as 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 shall always write a complete area before proceeding to the next.

       If  fildes  refers to a regular file and all of the iov_len members in the array pointed to by iov are 0,
       writev() shall return 0 and have 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 shall fail and no data  shall
       be transferred.

       Refer to write().

       In addition, the writev() function shall fail if:

       EINVAL The sum of the iov_len values in the iov array would overflow an ssize_t.

       The writev() function may fail and set errno to:

       EINVAL The iovcnt argument was less than or equal to 0, or greater than {IOV_MAX}.

       Thefollowingsectionsareinformative.

WritingDatafromanArray
       The  following  example  writes  data  from the buffers specified by members of the iov array to the file
       associated with the file descriptor fd.

           #include <sys/types.h>
           #include <sys/uio.h>
           #include <unistd.h>
           ...
           ssize_t bytes_written;
           int fd;
           char *buf0 = "short string\n";
           char *buf1 = "This is a longer string\n";
           char *buf2 = "This is the longest string in this example\n";
           int iovcnt;
           struct iovec iov[3];

           iov[0].iov_base = buf0;
           iov[0].iov_len = strlen(buf0);
           iov[1].iov_base = buf1;
           iov[1].iov_len = strlen(buf1);
           iov[2].iov_base = buf2;
           iov[2].iov_len = strlen(buf2);
           ...
           iovcnt = sizeof(iov) / sizeof(struct iovec);

           bytes_written = writev(fd, iov, iovcnt);
           ...

       None.

       writev — write a vector

       This  manual  page  is part of the POSIX Programmer's Manual.  The Linux implementation of this interface
       may differ (consult the corresponding Linux manual page for details of Linux behavior), or the  interface
       may not be implemented on Linux.

       Refer to write().

       Upon  successful  completion,  writev()  shall return the number of bytes actually written. Otherwise, it
       shall return a value of -1, the file-pointer shall remain unchanged, and errno shall be set  to  indicate
       an error.

readv(), write()

       The Base Definitions volume of POSIX.1‐2017, <limits.h>, <sys_uio.h>

       #include <sys/uio.h>

       ssize_t writev(int fildes, const struct iovec *iov, int iovcnt);