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 (consult the corresponding
Linux manual page for details of Linux behavior), or the interface may
not be implemented on Linux.
NAMEwritev — 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 oper‐
ation 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 ALSOreadv(), write()
The Base Definitions volume of POSIX.1‐2008, <limits.h>, <sys_uio.h>
COPYRIGHT
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 Electri‐
cal 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.ker‐
nel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2013 WRITEV(3P)
