WRITEV(3)                  POSIX Programmer's Manual                 WRITEV(3)



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

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 .



IEEE/The Open Group                  2017                            WRITEV(3)
