man/man2/write.2

.\" Copyright (c) 1980, 1991, 1993
.\"	The Regents of the University of California.  All rights reserved.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\" 1. Redistributions of source code must retain the above copyright
.\"    notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\"    notice, this list of conditions and the following disclaimer in the
.\"    documentation and/or other materials provided with the distribution.
.\" 3. All advertising materials mentioning features or use of this software
.\"    must display the following acknowledgement:
.\"	This product includes software developed by the University of
.\"	California, Berkeley and its contributors.
.\" 4. Neither the name of the University nor the names of its contributors
.\"    may be used to endorse or promote products derived from this software
.\"    without specific prior written permission.
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
.\" ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
.\" SUCH DAMAGE.
.\"
.\"     @(#)write.2	8.5 (Berkeley) 4/2/94
.\" $FreeBSD: src/lib/libc/sys/write.2,v 1.12.2.7 2001/12/14 18:34:02 ru Exp $
.\"
.Dd June 3, 2021
.Dt WRITE 2
.Os
.Sh NAME
.Nm pwrite ,
.Nm write ,
.Nm pwritev ,
.Nm writev
.Nd write output
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In unistd.h
.Ft ssize_t
.Fo pwrite
.Fa "int fildes"
.Fa "const void *buf"
.Fa "size_t nbyte"
.Fa "off_t offset"
.Fc
.Ft ssize_t
.Fo write
.Fa "int fildes"
.Fa "const void *buf"
.Fa "size_t nbyte"
.Fc
.In sys/uio.h
.Ft ssize_t
.Fo writev
.Fa "int fildes"
.Fa "const struct iovec *iov"
.Fa "int iovcnt"
.Fc
.Ft ssize_t
.Fo pwritev
.Fa "int fildes"
.Fa "const struct iovec *iov"
.Fa "int iovcnt"
.Fa "off_t offset"
.Fc
.Sh DESCRIPTION
.Fn write
attempts to write
.Fa nbyte
of data to the object referenced by the descriptor
.Fa fildes
from the buffer pointed to by
.Fa buf .
.Fn writev
performs the same action, but gathers the output data
from the
.Fa iovcnt
buffers specified by the members of the
.Fa iov
array: iov[0], iov[1], ..., iov[iovcnt\|-\|1].
.Fn pwrite
and
.Fn pwritev
perform the same functions, but write to the specified position in
the file without modifying the file pointer.
.Pp
For
.Fn writev
and
.Fn pwritev ,
the
.Fa iovec
structure is defined as:
.Pp
.Bd -literal -offset indent -compact
struct iovec {
	char   *iov_base;  /* Base address. */
	size_t iov_len;    /* Length. */
};
.Ed
.Pp
Each
.Fa iovec
entry specifies the base address and length of an area
in memory from which data should be written.
.Fn writev
and
.Fn pwritev
will always write a complete area before proceeding
to the next.
.Pp
On objects capable of seeking, the
.Fn write
starts at a position
given by the pointer associated with
.Fa fildes ,
see
.Xr lseek 2 .
Upon return from
.Fn write ,
the pointer is incremented by the number of bytes which were written.
.Pp
Objects that are not capable of seeking always write from the current
position.  The value of the pointer associated with such an object
is undefined.
.Pp
If the real user is not the super-user, then
.Fn write
clears the set-user-id bit on a file.
This prevents penetration of system security
by a user who
.Dq captures
a writable set-user-id file
owned by the super-user.
.Pp
When using non-blocking I/O on objects, such as sockets,
that are subject to flow control,
.Fn write
and
.Fn writev
may write fewer bytes than requested;
the return value must be noted,
and the remainder of the operation should be retried when possible.
.Pp
.Fn write
and
.Fn pwrite
will fail if the parameter
.Fa nbyte
exceeds
.Dv INT_MAX ,
and they do not attempt a partial write.
.Sh RETURN VALUES
Upon successful completion the number of bytes
which were written is returned.
Otherwise, a -1 is returned and the global variable
.Va errno
is set to indicate the error.
.Sh ERRORS
The
.Fn write ,
.Fn writev ,
.Fn pwrite ,
and
.Fn pwritev
system calls will fail and the file pointer will remain unchanged if:
.Bl -tag -width Er
.\" ===========
.It Bq Er EDQUOT
The user's quota of disk blocks on the file system
containing the file is exhausted.
.\" ===========
.It Bq Er EFAULT
Part of
.Fa iov
or data to be written to the file
points outside the process's allocated address space.
.\" ===========
.It Bq Er EINVAL
The pointer associated with
.Fa fildes
is negative.
.El
.Pp
The
.Fn write
and
.Fn pwrite
system calls will fail and the file pointer will remain unchanged if:
.Bl -tag -width Er
.\" ===========
.It Bq Er EAGAIN
The file is marked for non-blocking I/O,
and no data could be written immediately.
.\" ===========
.It Bq Er EBADF
.Fa fildes
is not a valid file descriptor open for writing.
.\" ===========
.It Bq Er ECONNRESET
A write is attempted on a socket that is not connected.
.\" ===========
.It Bq Er EFBIG
An attempt is made to write a file that exceeds the process's
file size limit or the maximum file size.
.\" ===========
.It Bq Er EFBIG
The file is a regular file,
.Fa nbyte
is greater than 0,
and the starting position is greater than or equal
to the offset maximum established in the open file description
associated with
.Fa fildes .
.\" ===========
.It Bq Er EINTR
A signal interrupts the write before it could be completed.
.\" ===========
.It Bq Er EINVAL
The value provided for
.Fa nbyte
exceeds
.Dv INT_MAX .
.\" ===========
.It Bq Er EIO
An I/O error occurs while reading from or writing to the file system.
.\" ===========
.It Bq Er ENETDOWN
A write is attempted on a socket
and the local network interface used to reach the destination is down.
.\" ===========
.It Bq Er ENETUNREACH
A write is attempted on a socket and no route to the network is present.
.\" ===========
.It Bq Er ENOSPC
There is no free space remaining on the file system containing the file.
.\" ===========
.It Bq Er ENXIO
A request is made of a nonexistent device,
or the request is outside the capabilities of the device.
.\" ===========
.It Bq Er EPIPE
An attempt is made to write to a pipe that is not open
for reading by any process.
.\" ===========
.It Bq Er EPIPE
An attempt is made to write to a socket of type
.Dv SOCK_STREAM
that is not connected to a peer socket.
.\" ===========
.It Bq Er EDEADLK
The file is a
.Dq dataless
file that requires materialization and the I/O policy of the current thread
or process disallows dataless file materialization
.Po see
.Xr getiopolicy_np 3
.Pc .
.El
.Pp
The
.Fn write
and
.Fn writev
calls may also return the following errors:
.Bl -tag -width Er
.\" ===========
.It Bq Er EAGAIN
See EWOULDBLOCK, below.
.\" ===========
.It Bq Er EWOULDBLOCK
The file descriptor is for a socket, is marked O_NONBLOCK,
and write would block.
The exact error code depends on the protocol,
but EWOULDBLOCK is more common.
.El
.Pp
In addition,
.Fn writev
may return one of the following errors:
.Bl -tag -width Er
.\" ===========
.It Bq Er EDESTADDRREQ
The destination is no longer available when writing to a
.Ux
domain datagram socket on which
.Xr connect 2
or
.Xr connectx 2
had been used to set a destination address.
.\" ===========
.It Bq Er ENOBUFS
The mbuf pool has been completely exhausted when writing to a socket.
.El
.Pp
The
.Fn writev
and
.Fn pwritev
calls may also return the following errors:
.Bl -tag -width Er
.\" ===========
.It Bq Er EINVAL
.Fa Iovcnt
is less than or equal to 0, or greater than
.Dv UIO_MAXIOV .
.\" ===========
.It Bq Er EINVAL
One of the
.Fa iov_len
values in the
.Fa iov
array is negative.
.\" ===========
.It Bq Er EINVAL
The sum of the
.Fa iov_len
values in the
.Fa iov
array overflows a 32-bit integer.
.El
.Pp
The
.Fn pwrite
and
.Fn pwritev
calls may also return the following errors:
.Bl -tag -width Er
.\" ===========
.It Bq Er EINVAL
The specified file offset is invalid.
.\" ===========
.It Bq Er ESPIPE
The file descriptor is associated with a pipe, socket, or FIFO.
.El
.Sh LEGACY SYNOPSIS
.Fd #include <sys/types.h>
.Fd #include <sys/uio.h>
.Fd #include <unistd.h>
.Pp
These include files are needed for all three functions.
.Sh SEE ALSO
.Xr fcntl 2 ,
.Xr lseek 2 ,
.Xr open 2 ,
.Xr pipe 2 ,
.Xr select 2 ,
.Xr compat 5
.Sh STANDARDS
The
.Fn write
function call is expected to conform to
.St -p1003.1-90 .
The
.Fn writev
and
.Fn pwrite
functions are expected to conform to
.St -xpg4.2 .
.Fn pwritev
is nonstandard.
.Sh HISTORY
The
.Fn pwrite
function call
appeared in
.At V.4 .
The
.Fn writev
function call
appeared in
.Bx 4.2 .
A
.Fn write
function call appeared in
.At v6 .