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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 has been marked non-blocking (O_NONBLOCK)
              and the write would block.

       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 3.15 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/.