write — write to a file descriptor
#include <unistd.h>
ssize_t write( |
int fd, |
| const void *buf, | |
size_t count); |
write() writes up to
count bytes from the
buffer pointed buf to
the file referred to by the file descriptor fd.
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
than count bytes.
(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
current 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
atomic step.
POSIX requires that a read(2) which can be proved
to occur after a write() has
returned returns the new data. Note that not all file systems
are POSIX conforming.
On success, the number of bytes written is returned (zero
indicates nothing was written). On error, −1 is
returned, and errno is set
appropriately.
If count is zero
and fd refers to a
regular file, then 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 count is zero and fd refers to a file other than
a regular file, the results are not specified.
The file descriptor fd refers to a file other
than a socket and has been marked nonblocking
(O_NONBLOCK), and the
write would block.
The file descriptor fd refers 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.
fd is not a
valid file descriptor or is not open for writing.
buf is
outside your accessible address space.
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.
The call was interrupted by a signal before any data was written; see signal(7).
fd is
attached to an object which is unsuitable for writing;
or the file was opened with the O_DIRECT flag, and either the address
specified in buf, the value specified
in count, or
the current file offset is not suitably aligned.
A low-level I/O error occurred while modifying the inode.
The device containing the file referred to by
fd has no room
for the data.
fd is
connected to a pipe or socket whose reading end is
closed. When this happens the writing process will also
receive a SIGPIPE signal.
(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
to fd.
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.
A successful return from 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 fsync(2) after you are done
writing all your data.
If a 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.
close(2), fcntl(2), fsync(2), ioctl(2), lseek(2), open(2), pwrite(2), read(2), select(2), writev(2), fwrite(3)
This page is part of release 3.24 of the Linux man-pages project. A
description of the project, and information about reporting
bugs, can be found at
http://www.kernel.org/doc/man-pages/.
|
This manpage is Copyright (C) 1992 Drew Eckhardt; 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 professionally. 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 <michaelcantor.informatik.rwth-aachen.de> Modified Sun Jul 21 18:59:33 1996 by Andries Brouwer <aebcwi.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. |