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  IEEE Std 1003.1-2001,  <limits.h>,
       <sys/uio.h>

COPYRIGHT

       Portions  of  this  text  are  reprinted  and  reproduced in electronic form from IEEE Std
       1003.1, 2003 Edition, Standard for Information Technology  --  Portable  Operating  System
       Interface  (POSIX), The Open Group Base Specifications Issue 6, Copyright (C) 2001-2003 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 .