Name

write — write to a file descriptor

Synopsis

#include <unistd.h>
ssize_t write( int fd,
  const void *buf,
  size_t count);
 

DESCRIPTION

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.

RETURN VALUE

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.

ERRORS

EAGAIN

The file descriptor fd refers to a file other than a socket and has been marked nonblocking (O_NONBLOCK), and the write would block.

EAGAIN or EWOULDBLOCK

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.

EBADF

fd is not a valid file descriptor or is not open for writing.

EFAULT

buf is outside your accessible address space.

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.

EINTR

The call was interrupted by a signal before any data was written; see signal(7).

EINVAL

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.

EIO

A low-level I/O error occurred while modifying the inode.

ENOSPC

The device containing the file referred to by fd has no room for the data.

EPIPE

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.

CONFORMING TO

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.

NOTES

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.

SEE ALSO

close(2), fcntl(2), fsync(2), ioctl(2), lseek(2), open(2), pwrite(2), read(2), select(2), writev(2), fwrite(3)

COLOPHON

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.