man2/write.2

.\" This manpage is Copyright (C) 1992 Drew Eckhardt;
.\"             and Copyright (C) 1993 Michael Haardt, Ian Jackson.
.\" and Copyright (C) 2007 Michael Kerrisk <mtk.manpages@gmail.com>
.\"
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.\" Modified Sat Jul 24 13:35:59 1993 by Rik Faith <faith@cs.unc.edu>
.\" Modified Sun Nov 28 17:19:01 1993 by Rik Faith <faith@cs.unc.edu>
.\" Modified Sat Jan 13 12:58:08 1996 by Michael Haardt
.\"   <michael@cantor.informatik.rwth-aachen.de>
.\" Modified Sun Jul 21 18:59:33 1996 by Andries Brouwer <aeb@cwi.nl>
.\" 2001-12-13 added remark by Zack Weinberg
.\" 2007-06-18 mtk:
.\"    	Added details about seekable files and file offset.
.\"	Noted that write() may write less than 'count' bytes, and
.\"	gave some examples of why this might occur.
.\"	Noted what happens if write() is interrupted by a signal.
.\"
.TH WRITE 2 2021-03-22 "Linux" "Linux Programmer's Manual"
.SH NAME
write \- write to a file descriptor
.SH SYNOPSIS
.nf
.B #include <unistd.h>
.PP
.BI "ssize_t write(int " fd ", const void *" buf ", size_t " count );
.fi
.SH DESCRIPTION
.BR write ()
writes up to
.I count
bytes from the buffer starting at
.I buf
to the file referred to by the file descriptor
.IR fd .
.PP
The number of bytes written may be less than
.I count
if, for example,
there is insufficient space on the underlying physical medium, or the
.B RLIMIT_FSIZE
resource limit is encountered (see
.BR setrlimit (2)),
or the call was interrupted by a signal
handler after having written less than
.I count
bytes.
(See also
.BR pipe (7).)
.PP
For a seekable file (i.e., one to which
.BR 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
.BR open (2)ed
with
.BR 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 atomic step.
.PP
POSIX requires that a
.BR read (2)
that can be proved to occur after a
.BR write ()
has returned will return the new data.
Note that not all filesystems are POSIX conforming.
.PP
According to POSIX.1, if
.I count
is greater than
.BR SSIZE_MAX ,
the result is implementation-defined;
see NOTES for the upper limit on Linux.
.SH RETURN VALUE
On success, the number of bytes written is returned.
On error, \-1 is returned, and \fIerrno\fP is set
to indicate the error.
.PP
Note that a successful
.BR write ()
may transfer fewer than
.I 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
.BR 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 another
.BR 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).
.PP
If \fIcount\fP is zero and
.I fd
refers to a regular file, then
.BR write ()
may return a failure status if one of the errors below is detected.
If no errors are detected, or error detection is not performed,
0 is returned without causing any other effect.
If
\fIcount\fP is zero and
.I fd
refers to a file other than a regular file,
the results are not specified.
.SH ERRORS
.TP
.B EAGAIN
The file descriptor
.I fd
refers to a file other than a socket and has been marked nonblocking
.RB ( O_NONBLOCK ),
and the write would block.
See
.BR open (2)
for further details on the
.BR O_NONBLOCK
flag.
.TP
.BR EAGAIN " or " EWOULDBLOCK
.\" Actually EAGAIN on Linux
The file descriptor
.I fd
refers to a socket and has been marked nonblocking
.RB ( 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.
.TP
.B EBADF
.I fd
is not a valid file descriptor or is not open for writing.
.TP
.B EDESTADDRREQ
.I fd
refers to a datagram socket for which a peer address has not been set using
.BR connect (2).
.TP
.B EDQUOT
The user's quota of disk blocks on the filesystem containing the file
referred to by
.I fd
has been exhausted.
.TP
.B EFAULT
.I buf
is outside your accessible address space.
.TP
.B EFBIG
An attempt was made to write a file that exceeds the implementation-defined
maximum file size or the process's file size limit,
or to write at a position past the maximum allowed offset.
.TP
.B EINTR
The call was interrupted by a signal before any data was written; see
.BR signal (7).
.TP
.B EINVAL
.I fd
is attached to an object which is unsuitable for writing;
or the file was opened with the
.B O_DIRECT
flag, and either the address specified in
.IR buf ,
the value specified in
.IR count ,
or the file offset is not suitably aligned.
.TP
.B EIO
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
.BR 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
.I may
be reported by subsequent.
.BR write ()
requests, and
.I will
be reported by a subsequent
.BR fsync (2)
(whether or not they were also reported by
.BR write ()).
.\" commit 088737f44bbf6378745f5b57b035e57ee3dc4750
An alternate cause of
.B 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
.I "Lost locks"
section of
.BR fcntl (2)
for further details.
.TP
.B ENOSPC
The device containing the file referred to by
.I fd
has no room for the data.
.TP
.B EPERM
The operation was prevented by a file seal; see
.BR fcntl (2).
.TP
.B EPIPE
.I fd
is connected to a pipe or socket whose reading end is closed.
When this happens the writing process will also receive a
.B SIGPIPE
signal.
(Thus, the write return value is seen only if the program
catches, blocks or ignores this signal.)
.PP
Other errors may occur, depending on the object connected to
.IR fd .
.SH CONFORMING TO
SVr4, 4.3BSD, POSIX.1-2001.
.\" SVr4 documents additional error
.\" conditions EDEADLK, ENOLCK, ENOLNK, ENOSR, ENXIO, or ERANGE.
.PP
Under SVr4 a write may be interrupted and return
.B EINTR
at any point,
not just before any data is written.
.SH NOTES
The types
.I size_t
and
.I ssize_t
are, respectively,
unsigned and signed integer data types specified by POSIX.1.
.PP
A successful return from
.BR 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
.BR write (),
.BR fsync (2),
or even
.BR close (2).
The only way to be sure is to call
.BR fsync (2)
after you are done writing all your data.
.PP
If a
.BR write ()
is interrupted by a signal handler before any bytes are written,
then the call fails with the error
.BR EINTR ;
if it is interrupted after at least one byte has been written,
the call succeeds, and returns the number of bytes written.
.PP
On Linux,
.BR write ()
(and similar system calls) will transfer at most
0x7ffff000 (2,147,479,552) bytes,
returning the number of bytes actually transferred.
.\" commit e28cc71572da38a5a12c1cfe4d7032017adccf69
(This is true on both 32-bit and 64-bit systems.)
.PP
An error return value while performing
.BR 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
.BR write ()
was attempted should be considered inconsistent.
.SH BUGS
According to POSIX.1-2008/SUSv4 Section XSI 2.9.7
("Thread Interactions with Regular File Operations"):
.PP
.RS 4
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: ...
.RE
.PP
Among the APIs subsequently listed are
.BR write ()
and
.BR 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
.BR open (2))
perform a
.BR write ()
(or
.BR 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.
.\" http://thread.gmane.org/gmane.linux.kernel/1649458
.\"    From: Michael Kerrisk (man-pages <mtk.manpages <at> gmail.com>
.\"    Subject: Update of file offset on write() etc. is non-atomic with I/O
.\"    Date: 2014-02-17 15:41:37 GMT
.\"    Newsgroups: gmane.linux.kernel, gmane.linux.file-systems
.\" commit 9c225f2655e36a470c4f58dbbc99244c5fc7f2d4
.\"    Author: Linus Torvalds <torvalds@linux-foundation.org>
.\"    Date:   Mon Mar 3 09:36:58 2014 -0800
.\"
.\"        vfs: atomic f_pos accesses as per POSIX
.SH SEE ALSO
.BR close (2),
.BR fcntl (2),
.BR fsync (2),
.BR ioctl (2),
.BR lseek (2),
.BR open (2),
.BR pwrite (2),
.BR read (2),
.BR select (2),
.BR writev (2),
.BR fwrite (3)