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WRITE(2)		   Linux Programmer's Manual		      WRITE(2)

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 off-
       set, 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  filesys-
       tems 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  ap-
       propriately.

       If  count is zero and fd	refers to a regular file, then write() may re-
       turn a failure status if	one of the errors below	is  detected.	If  no
       errors  are  detected, 0	will be	returned without causing any other ef-
       fect.  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.

       EDESTADDRREQ
	      fd  refers to a datagram socket for which	a peer address has not
	      been set using connect(2).

       EDQUOT The user's quota of disk blocks on the filesystem	containing the
	      file referred to by fd has been exhausted.

       EFAULT buf is outside your accessible address space.

       EFBIG  An attempt was made to write a file that exceeds the implementa-
	      tion-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 writ-
	      ten; 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 ad-
	      dress 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 SIG-
	      PIPE 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  re-
       turns the number	of bytes written.

BUGS
       According to POSIX.1-2008/SUSv4 Section XSI 2.9.7 ("Thread Interactions
       with Regular File Operations"):

	   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: ...

       Among the APIs subsequently listed  are	write()	 and  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 de-
       scription (see open(2)) perform a write() (or 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.

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 3.74 of the Linux man-pages project.  A
       description of the project, information about reporting bugs,  and  the
       latest	  version     of     this    page,    can    be	   found    at
       http://www.kernel.org/doc/man-pages/.

Linux				  2014-05-04			      WRITE(2)

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | CONFORMING TO | NOTES | BUGS | SEE ALSO | COLOPHON

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