PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT

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

PROLOG         top

       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         top

       writev — write a vector

SYNOPSIS         top

       #include <sys/uio.h>

       ssize_t writev(int fildes, const struct iovec *iov, int iovcnt);

DESCRIPTION         top

       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         top

       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         top

       Refer to write(3p).

       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         top

   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         top

       None.

RATIONALE         top

       Refer to write(3p).

FUTURE DIRECTIONS         top

       None.

SEE ALSO         top

       readv(3p), write(3p)

       The Base Definitions volume of POSIX.1‐2008, limits.h(0p),
       sys_uio.h(0p)

COPYRIGHT         top

       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 Electrical 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.kernel.org/doc/man-pages/reporting_bugs.html .

IEEE/The Open Group                 2013                          WRITEV(3P)