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 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 off-
       set and the write operation are performed as an atomic step.

       POSIX  requires	that  a	 read(2)  that	can be proved to occur after a
       write() has returned will return the  new  data.	  Note	that  not  all
       filesystems are POSIX conforming.

       According to POSIX.1, if count is greater than SSIZE_MAX, the result is
       implementation-defined; see NOTES for the upper limit on Linux.

RETURN VALUE
       On success, the number of bytes written	is  returned  (zero  indicates
       nothing	was  written).	 It  is not an error if this number is smaller
       than the number of bytes requested; this may happen for example because
       the disk device was filled.  See also NOTES.

       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,  or  error detection is not performed, 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 speci-
       fied.

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.  See open(2) for further details on the O_NONBLOCK flag.

       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
	      address  specified  in buf, the value specified in count, or the
	      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.

       EPERM  The operation was prevented by a file seal; see fcntl(2).

       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
       The  types  size_t  and	ssize_t are, respectively, unsigned and signed
       integer data types specified by POSIX.1.

       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.

       On  Linux,  write()  (and  similar  system calls) will transfer at most
       0x7ffff000 (2,147,479,552) bytes, returning the number of  bytes	 actu-
       ally transferred.  (This is true on both 32-bit and 64-bit systems.)

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
       description (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 4.10 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
       https://www.kernel.org/doc/man-pages/.



Linux				  2017-03-13			      WRITE(2)