Section (1) write
write — send a message to another user
write user [ttyname]
write allows you to communicate with other users, by copying lines from your terminal to theirs.
When you run the write command, the user you are writing to gets a message of the form:
Message from [email protected] on yourtty at hh:mm ...
Any further lines you enter will be copied to the specified user_zsingle_quotesz_s terminal. If the other user wants to reply, they must run write as well.
When you are done, type an end-of-file or interrupt
character. The other user will see the message
EOF indicating that the conversation is
You can prevent people (other than the superuser) from writing to you with the mesg(1) command. Some commands, for example nroff(1) and pr(1), may automatically disallow writing, so that the output they produce isn_zsingle_quotesz_t overwritten.
If the user you want to write to is logged in on more than one terminal, you can specify which terminal to write to by giving the terminal name as the second operand to the write command. Alternatively, you can let write select one of the terminals − it will pick the one with the shortest idle time. This is so that if the user is logged in at work and also dialed up from home, the message will go to the right place.
The traditional protocol for writing to someone is that the string `−o_zsingle_quotesz_, either at the end of a line or on a line by itself, means that it_zsingle_quotesz_s the other person_zsingle_quotesz_s turn to talk. The string `oo_zsingle_quotesz_ means that the person believes the conversation to be over.
The write command is part of the util-linux package and is available from https://www.kernel.org/pub/linux/utils/util-linux/.
Copyright (c) 1989, 1993
The Regents of the University of California. All rights reserved.
This code is derived from software contributed to Berkeley by
Jef Poskanzer and Craig Leres of the Lawrence Berkeley Laboratory.
Redistribution and use in source and binary forms, with or without
modification, are permitted provided that the following conditions
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_zsingle_quotesz__zsingle_quotesz_ 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
(#)write.1 8.1 (Berkeley) 6/6/93
Section (2) write
write — write to a file descriptor
|const void *buf,|
write() writes up to
count bytes from the
buffer starting at
buf to the file referred to by
the file descriptor
The number of bytes written may be less than
count if, for example, there is
insufficient space on the underlying physical medium, or the
RLIMIT_FSIZE resource limit is
encountered (see setrlimit(2)), or the call
was interrupted by a signal handler after having written less
(See also pipe(7).)
For a seekable file (i.e., one to which lseek(2) may be applied,
for example, a regular file) writing takes place at the file
offset, and the file offset is incremented by the number of
bytes actually written. If the file was open(2)ed with
O_APPEND, the file offset is first set to
the end of the file before writing. The adjustment of the
file offset and the write operation are performed as an
POSIX requires that a read(2) that can be proved
to occur after a
returned will return the new data. Note that not all
filesystems are POSIX conforming.
According to POSIX.1, if
count is greater than
SSIZE_MAX, the result is
implementation-defined; see NOTES for the upper limit on
On success, the number of bytes written is returned. On
error, −1 is returned, and
errno is set to indicate the cause of the
Note that a successful
write() may transfer fewer than
count bytes. Such partial
writes can occur for various reasons; for example, because
there was insufficient space on the disk device to write all
of the requested bytes, or because a blocked
write() to a socket, pipe, or similar was
interrupted by a signal handler after it had transferred
some, but before it had transferred all of the requested
bytes. In the event of a partial write, the caller can make
write() call to
transfer the remaining bytes. The subsequent call will either
transfer further bytes or may result in an error (e.g., if
the disk is now full).
count is zero
fd refers to a
regular file, then
return a failure status if one of the errors below is
detected. If no errors are detected, or error detection is
not performed, 0 will be returned without causing any other
fd refers to
a file other than a regular file, the results are not
The file descriptor
fdrefers to a file other than a socket and has been marked nonblocking (
O_NONBLOCK), and the write would block. See open(2) for further details on the
- EAGAIN or EWOULDBLOCK
The file descriptor
fdrefers to a socket and has been marked nonblocking (
O_NONBLOCK), and the write would block. POSIX.1-2001 allows either error to be returned for this case, and does not require these constants to have the same value, so a portable application should check for both possibilities.
fdis not a valid file descriptor or is not open for writing.
fdrefers to a datagram socket for which a peer address has not been set using connect(2).
The user_zsingle_quotesz_s quota of disk blocks on the filesystem containing the file referred to by
fdhas been exhausted.
bufis outside your accessible address space.
An attempt was made to write a file that exceeds the implementation-defined maximum file size or the process_zsingle_quotesz_s file size limit, or to write at a position past the maximum allowed offset.
The call was interrupted by a signal before any data was written; see signal(7).
fdis attached to an object which is unsuitable for writing; or the file was opened with the
O_DIRECTflag, and either the address specified in
buf, the value specified in
count, or the file offset is not suitably aligned.
A low-level I/O error occurred while modifying the inode. This error may relate to the write-back of data written by an earlier
write(), which may have been issued to a different file descriptor on the same file. Since Linux 4.13, errors from write-back come with a promise that they
maybe reported by subsequent.
write() requests, and
willbe reported by a subsequent fsync(2) (whether or not they were also reported by
write()). An alternate cause of EIO on networked filesystems is when an advisory lock had been taken out on the file descriptor and this lock has been lost. See the Lost locks section of fcntl(2) for further details.
The device containing the file referred to by
fdhas no room for the data.
The operation was prevented by a file seal; see fcntl(2).
fdis connected to a pipe or socket whose reading end is closed. When this happens the writing process will also receive a
SIGPIPEsignal. (Thus, the write return value is seen only if the program catches, blocks or ignores this signal.)
Other errors may occur, depending on the object connected
SVr4, 4.3BSD, POSIX.1-2001.
Under SVr4 a write may be interrupted and return EINTR at any point, not just before any data is written.
The types size_t and ssize_t are, respectively, unsigned and signed integer data types specified by POSIX.1.
A successful return from
write() does not make any guarantee that
data has been committed to disk. On some filesystems,
including NFS, it does not even guarantee that space has
successfully been reserved for the data. In this case, some
errors might be delayed until a future
write(), fsync(2), or even close(2). The only way to
be sure is to call fsync(2) after you are done
writing all your data.
write() is interrupted
by a signal handler before any bytes are written, then the
call fails with the error EINTR; if it is interrupted after at least
one byte has been written, the call succeeds, and returns the
number of bytes written.
similar system calls) will transfer at most 0x7ffff000
(2,147,479,552) bytes, returning the number of bytes actually
transferred. (This is true on both 32-bit and 64-bit
An error return value while performing
write() using direct I/O does not mean the
entire write has failed. Partial data may be written and the
data at the file offset on which the
write() was attempted should be considered
According to POSIX.1-2008/SUSv4 Section XSI 2.9.7 (Thread Interactions with Regular File Operations):
All of the following functions shall be atomic with respect to each other in the effects specified in POSIX.1-2008 when they operate on regular files or symbolic links: ...
Among the APIs subsequently listed are
write() and writev(2). And among the
effects that should be atomic across threads (and processes)
are updates of the file offset. However, on Linux before
version 3.14, this was not the case: if two processes that
share an open file description (see open(2)) perform a
write() (or writev(2)) at the same
time, then the I/O operations were not atomic with respect
updating the file offset, with the result that the blocks of
data output by the two processes might (incorrectly) overlap.
This problem was fixed in Linux 3.14.
This page is part of release 5.04 of the Linux
man-pages project. A
description of the project, information about reporting bugs,
and the latest version of this page, can be found at
This manpage is Copyright (C) 1992 Drew Eckhardt;
and Copyright (C) 1993 Michael Haardt, Ian Jackson.
and Copyright (C) 2007 Michael Kerrisk <mtk.manpagesgmail.com>
Permission is granted to make and distribute verbatim copies of this
manual provided the copyright notice and this permission notice are
preserved on all copies.
Permission is granted to copy and distribute modified versions of this
manual under the conditions for verbatim copying, provided that the
entire resulting derived work is distributed under the terms of a
permission notice identical to this one.
Since the Linux kernel and libraries are constantly changing, this
manual page may be incorrect or out-of-date. The author(s) assume no
responsibility for errors or omissions, or for damages resulting from
the use of the information contained herein. The author(s) may not
have taken the same level of care in the production of this manual,
which is licensed free of charge, as they might when working
Formatted or processed versions of this manual, if unaccompanied by
the source, must acknowledge the copyright and authors of this work.
Modified Sat Jul 24 13:35:59 1993 by Rik Faith <faithcs.unc.edu>
Modified Sun Nov 28 17:19:01 1993 by Rik Faith <faithcs.unc.edu>
Modified Sat Jan 13 12:58:08 1996 by Michael Haardt
Modified Sun Jul 21 18:59:33 1996 by Andries Brouwer <aebcwi.nl>
2001-12-13 added remark by Zack Weinberg
Added details about seekable files and file offset.
Noted that write() may write less than _zsingle_quotesz_count_zsingle_quotesz_ bytes, and
gave some examples of why this might occur.
Noted what happens if write() is interrupted by a signal.