write


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 file sys-
       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
       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.

       EDESTADDRREQ
              fd  refers to a datagram socket for which a peer address has not
              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  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.

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 3.35 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at http://man7.org/linux/man-pages/.



Linux                             2010-08-29                          WRITE(2)
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