.\" Copyright (c) 2001-2003 The Open Group, All Rights Reserved 
.TH "WRITEV" 3P 2003 "IEEE/The Open Group" "POSIX Programmer's Manual"
.\" writev 
.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 \- write a vector
.SH SYNOPSIS
.LP
\fB#include <sys/uio.h>
.br
.sp
ssize_t writev(int\fP \fIfildes\fP\fB, const struct iovec *\fP\fIiov\fP\fB,
int\fP \fIiovcnt\fP\fB); \fP
\fB
.br
\fP
.SH DESCRIPTION
.LP
The \fIwritev\fP() function shall be equivalent to \fIwrite\fP(),
except as described
below. The \fIwritev\fP() function shall gather output data from the
\fIiovcnt\fP buffers specified by the members of the
\fIiov\fP array: \fIiov\fP[0], \fIiov\fP[1], ..., \fIiov\fP[\fIiovcnt\fP-1].
The \fIiovcnt\fP argument is valid if greater
than 0 and less than or equal to {IOV_MAX}, as defined in \fI<limits.h>\fP.
.LP
Each \fIiovec\fP entry specifies the base address and length of an
area in memory from which data should be written. The
\fIwritev\fP() function shall always write a complete area before
proceeding to the next.
.LP
If \fIfildes\fP refers to a regular file and all of the \fIiov_len\fP
members in the array pointed to by \fIiov\fP are 0,
\fIwritev\fP() shall return 0 and have no other effect. For other
file types, the behavior is unspecified.
.LP
If the sum of the \fIiov_len\fP values is greater than {SSIZE_MAX},
the operation shall fail and no data shall be
transferred.
.SH RETURN VALUE
.LP
Upon successful completion, \fIwritev\fP() shall return the number
of bytes actually written. Otherwise, it shall return a
value of -1, the file-pointer shall remain unchanged, and \fIerrno\fP
shall be set to indicate an error.
.SH ERRORS
.LP
Refer to \fIwrite\fP().
.LP
In addition, the \fIwritev\fP() function shall fail if:
.TP 7
.B EINVAL
The sum of the \fIiov_len\fP values in the \fIiov\fP array would overflow
an \fBssize_t\fP.
.sp
.LP
The \fIwritev\fP() function may fail and set \fIerrno\fP to:
.TP 7
.B EINVAL
The \fIiovcnt\fP argument was less than or equal to 0, or greater
than {IOV_MAX}.
.sp
.LP
\fIThe following sections are informative.\fP
.SH EXAMPLES
.SS Writing Data from an Array
.LP
The following example writes data from the buffers specified by members
of the \fIiov\fP array to the file associated with the
file descriptor \fIfd\fP.
.sp
.RS
.nf

\fB#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];
.sp

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);
.sp

bytes_written = writev(fd, iov, iovcnt);
\&...
\fP
.fi
.RE
.SH APPLICATION USAGE
.LP
None.
.SH RATIONALE
.LP
Refer to \fIwrite\fP().
.SH FUTURE DIRECTIONS
.LP
None.
.SH SEE ALSO
.LP
\fIreadv\fP(), \fIwrite\fP(), the Base Definitions volume of
IEEE\ Std\ 1003.1-2001, \fI<limits.h>\fP, \fI<sys/uio.h>\fP
.SH 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, Copyright (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.opengroup.org/unix/online.html .