287 lines
6.8 KiB
Groff
287 lines
6.8 KiB
Groff
.\" 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.
|
|
.\" 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
|
|
.\" $FreeBSD$
|
|
.\"
|
|
.Dd January 22, 2012
|
|
.Dt WRITE 2
|
|
.Os
|
|
.Sh NAME
|
|
.Nm write ,
|
|
.Nm writev ,
|
|
.Nm pwrite ,
|
|
.Nm pwritev
|
|
.Nd write output
|
|
.Sh LIBRARY
|
|
.Lb libc
|
|
.Sh SYNOPSIS
|
|
.In sys/types.h
|
|
.In unistd.h
|
|
.Ft ssize_t
|
|
.Fn write "int d" "const void *buf" "size_t nbytes"
|
|
.Ft ssize_t
|
|
.Fn pwrite "int d" "const void *buf" "size_t nbytes" "off_t offset"
|
|
.In sys/uio.h
|
|
.Ft ssize_t
|
|
.Fn writev "int d" "const struct iovec *iov" "int iovcnt"
|
|
.Ft ssize_t
|
|
.Fn pwritev "int d" "const struct iovec *iov" "int iovcnt" "off_t offset"
|
|
.Sh DESCRIPTION
|
|
The
|
|
.Fn write
|
|
system call
|
|
attempts to write
|
|
.Fa nbytes
|
|
of data to the object referenced by the descriptor
|
|
.Fa d
|
|
from the buffer pointed to by
|
|
.Fa buf .
|
|
The
|
|
.Fn writev
|
|
system call
|
|
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].
|
|
The
|
|
.Fn pwrite
|
|
and
|
|
.Fn pwritev
|
|
system calls
|
|
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 {
|
|
void *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.
|
|
The
|
|
.Fn writev
|
|
system call
|
|
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 d ,
|
|
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.
|
|
.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 EBADF
|
|
The
|
|
.Fa d
|
|
argument
|
|
is not a valid descriptor open for writing.
|
|
.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 EFBIG
|
|
An attempt was made to write a file that exceeds the process's
|
|
file size limit or the maximum file size.
|
|
.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 d
|
|
was negative.
|
|
.It Bq Er ENOSPC
|
|
There is no free space remaining on the file system
|
|
containing the file.
|
|
.It Bq Er EDQUOT
|
|
The user's quota of disk blocks on the file system
|
|
containing the file has been exhausted.
|
|
.It Bq Er EIO
|
|
An I/O error occurred while reading from or writing to the file system.
|
|
.It Bq Er EINTR
|
|
A signal interrupted the write before it could be completed.
|
|
.It Bq Er EAGAIN
|
|
The file was marked for non-blocking I/O,
|
|
and no data could be written immediately.
|
|
.It Bq Er EROFS
|
|
An attempt was made to write over a disk label area at the beginning
|
|
of a slice.
|
|
Use
|
|
.Xr disklabel 8
|
|
.Fl W
|
|
to enable writing on the disk label area.
|
|
.It Bq Er EINVAL
|
|
The value
|
|
.Fa nbytes
|
|
is greater than
|
|
.Dv INT_MAX .
|
|
.El
|
|
.Pp
|
|
In addition,
|
|
.Fn writev
|
|
and
|
|
.Fn pwritev
|
|
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
|
|
had been used to set a destination address.
|
|
.It Bq Er EINVAL
|
|
The
|
|
.Fa iovcnt
|
|
argument
|
|
was less than or equal to 0, or greater than
|
|
.Dv IOV_MAX .
|
|
.It Bq Er EINVAL
|
|
One of the
|
|
.Fa iov_len
|
|
values in the
|
|
.Fa iov
|
|
array was negative.
|
|
.It Bq Er EINVAL
|
|
The sum of the
|
|
.Fa iov_len
|
|
values in the
|
|
.Fa iov
|
|
array overflowed a 32-bit integer.
|
|
.It Bq Er ENOBUFS
|
|
The mbuf pool has been completely exhausted when writing to a socket.
|
|
.El
|
|
.Pp
|
|
The
|
|
.Fn pwrite
|
|
and
|
|
.Fn pwritev
|
|
system calls may also return the following errors:
|
|
.Bl -tag -width Er
|
|
.It Bq Er EINVAL
|
|
The
|
|
.Fa offset
|
|
value was negative.
|
|
.It Bq Er ESPIPE
|
|
The file descriptor is associated with a pipe, socket, or FIFO.
|
|
.El
|
|
.Sh SEE ALSO
|
|
.Xr fcntl 2 ,
|
|
.Xr lseek 2 ,
|
|
.Xr open 2 ,
|
|
.Xr pipe 2 ,
|
|
.Xr select 2
|
|
.Sh STANDARDS
|
|
The
|
|
.Fn write
|
|
system call is expected to conform to
|
|
.St -p1003.1-90 .
|
|
The
|
|
.Fn writev
|
|
and
|
|
.Fn pwrite
|
|
system calls are expected to conform to
|
|
.St -xpg4.2 .
|
|
.Sh HISTORY
|
|
The
|
|
.Fn pwritev
|
|
system call appeared in
|
|
.Fx 6.0 .
|
|
The
|
|
.Fn pwrite
|
|
function appeared in
|
|
.At V.4 .
|
|
The
|
|
.Fn writev
|
|
system call appeared in
|
|
.Bx 4.2 .
|
|
The
|
|
.Fn write
|
|
function appeared in
|
|
.At v6 .
|