'\" et
.TH WRITEV "3P" 2017 "IEEE/The Open Group" "POSIX Programmer's Manual"
.\"
.SH 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.
.\"
.SH NAME
writev
\(em write a vector
.SH SYNOPSIS
.LP
.nf
#include <sys/uio.h>
.P
ssize_t writev(int \fIfildes\fP, const struct iovec *\fIiov\fP, int \fIiovcnt\fP);
.fi
.SH DESCRIPTION
The
\fIwritev\fR()
function shall be equivalent to
\fIwrite\fR(),
except as described below. The
\fIwritev\fR()
function shall gather output data from the
.IR iovcnt
buffers specified by the members of the
.IR iov
array:
.IR iov [0],
.IR iov [1],
\&.\|.\|., \fIiov\fR[\fIiovcnt\fR\-1].
The
.IR iovcnt
argument is valid if greater than 0 and less than or equal to
{IOV_MAX},
as defined in
.IR <limits.h> .
.P
Each
.IR iovec
entry specifies the base address and length of an area in memory from
which data should be written. The
\fIwritev\fR()
function shall always write a complete area before proceeding to the
next.
.P
If
.IR fildes
refers to a regular file and all of the
.IR iov_len
members in the array pointed to by
.IR iov
are 0,
\fIwritev\fR()
shall return 0 and have no other effect. For other file types, the
behavior is unspecified.
.P
If the sum of the
.IR iov_len
values is greater than
{SSIZE_MAX},
the operation shall fail and no data shall be transferred.
.SH "RETURN VALUE"
Upon successful completion,
\fIwritev\fR()
shall return the number of bytes actually written. Otherwise, it shall
return a value of \-1, the file-pointer shall remain unchanged, and
.IR errno
shall be set to indicate an error.
.SH ERRORS
Refer to
.IR "\fIwrite\fR\^(\|)".
.P
In addition, the
\fIwritev\fR()
function shall fail if:
.TP
.BR EINVAL
The sum of the
.IR iov_len
values in the
.IR iov
array would overflow an
.BR ssize_t .
.P
The
\fIwritev\fR()
function may fail and set
.IR errno
to:
.TP
.BR EINVAL
The
.IR iovcnt
argument was less than or equal to 0, or greater than
{IOV_MAX}.
.LP
.IR "The following sections are informative."
.SH EXAMPLES
.SS "Writing Data from an Array"
.P
The following example writes data from the buffers specified by members
of the
.IR iov
array to the file associated with the file descriptor
.IR fd .
.sp
.RS 4
.nf

#include <sys/types.h>
#include <sys/uio.h>
#include <unistd.h>
\&...
ssize_t bytes_written;
int fd;
char *buf0 = "short string\en";
char *buf1 = "This is a longer string\en";
char *buf2 = "This is the longest string in this example\en";
int iovcnt;
struct iovec iov[3];
.P
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);
.P
bytes_written = writev(fd, iov, iovcnt);
\&...
.fi
.P
.RE
.SH "APPLICATION USAGE"
None.
.SH RATIONALE
Refer to
.IR "\fIwrite\fR\^(\|)".
.SH "FUTURE DIRECTIONS"
None.
.SH "SEE ALSO"
.IR "\fIreadv\fR\^(\|)",
.IR "\fIwrite\fR\^(\|)"
.P
The Base Definitions volume of POSIX.1\(hy2017,
.IR "\fB<limits.h>\fP",
.IR "\fB<sys_uio.h>\fP"
.\"
.SH 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 .
.PP
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 .