man-pages/man2/write.2

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.\" This manpage is Copyright (C) 1992 Drew Eckhardt;
.\"                               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 2007-06-18 "Linux" "Linux Programmer's Manual"
.SH NAME
write \- write to a file descriptor
.SH SYNOPSIS
.B #include <unistd.h>
.sp
.BI "ssize_t write(int " fd ", const void *" buf ", size_t " count );
.SH DESCRIPTION
.BR write ()
writes up to
.I count
bytes from the buffer pointed
.I buf
to the file referred to by the file descriptor
.IR fd .

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).)

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 current 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.

POSIX requires that a
.BR read (2)
which can be proved to occur after a
.BR write ()
has returned returns the new data.
Note that not all file systems are POSIX conforming.
.SH "RETURN VALUE"
On success, the number of bytes written is returned (zero indicates
nothing was written).
On error, \-1 is returned, and \fIerrno\fP is set
appropriately.

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,
0 will be 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
has been marked non-blocking
.RB ( O_NONBLOCK )
and the write would block.
.TP
.B EBADF
.I fd
is not a valid file descriptor or is not open for writing.
.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.
.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 current file offset is not suitably aligned.
.TP
.B EIO
A low-level I/O error occurred while modifying the inode.
.TP
.B ENOSPC
The device containing the file referred to by
.I fd
has no room for the data.
.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.

Under SVr4 a write may be interrupted and return
.B EINTR
at any point,
not just before any data is written.
.SH NOTES
A successful return from
.BR write ()
does not make any guarantee that data has been committed to disk.
In fact, on some buggy implementations, it does not even guarantee
that space has successfully been reserved for the data.
The only way to be sure is to call
.BR fsync (2)
after you are done writing all your data.

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.
.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)