man-pages/man2/write.2

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.\" This manpage is Copyright (C) 1992 Drew Eckhardt;
.\"                               1993 Michael Haardt, Ian Jackson.
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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
.\"
.TH WRITE 2 2001-12-13 "Linux 2.0.32" "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 to the file referenced by the file descriptor
.I fd
from the buffer starting at
.IR buf .
POSIX requires that a \fBread\fP() which can be proved to occur after a
\fBwrite\fP() 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 are returned (zero indicates
nothing was written).  On error, \-1 is returned, and \fIerrno\fP is set
appropriately.  If \fIcount\fP is zero and the file descriptor refers to
a regular file, 0 will be returned without causing any other effect.
For a special file, the results are not portable.
.SH ERRORS
.TP
.B EAGAIN
Non-blocking I/O has been selected using
.B 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' 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 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.
.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)