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 operation 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.1‐2017, <limits.h>, <sys_uio.h>

COPYRIGHT

       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 .