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WRITEV(3P)                 POSIX Programmer's Manual                WRITEV(3P)
PROLOG
       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.
NAME
       writev -- write a vector
SYNOPSIS
       #include <sys/uio.h>
       ssize_t writev(int fildes, const struct iovec *iov, int iovcnt);
DESCRIPTION
       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 oper-
       ation shall fail and no data shall be transferred.
RETURN VALUE
       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.
ERRORS
       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}.
       The following sections are informative.
EXAMPLES
   Writing Data from an Array
       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);
           ...
APPLICATION USAGE
       None.
RATIONALE
       Refer to write().
FUTURE DIRECTIONS
       None.
SEE ALSO
       readv(), write()
       The Base Definitions volume of POSIX.1-2008, <limits.h>, <sys_uio.h>
COPYRIGHT
       Portions  of  this text are reprinted and reproduced in electronic form
       from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
       --  Portable  Operating  System  Interface (POSIX), The Open Group Base
       Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri-
       cal  and  Electronics  Engineers,  Inc  and  The  Open Group.  (This is
       POSIX.1-2008 with the 2013 Technical Corrigendum  1  applied.)  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.unix.org/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.ker-
       nel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group                  2013                           WRITEV(3P)