write

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).  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
              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.

       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
       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 4.04 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                             2015-07-23                          WRITE(2)
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