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 (con-
       sult 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 out-
       put 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,  Copy-
       right (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.open-
       group.org/unix/online.html .



IEEE/The Open Group                                           2003                                                    WRITEV(3P)