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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 refers to a file other than a socket  and
              has  been  marked non-blocking (O_NONBLOCK), and the write would
              block.

       EAGAIN or EWOULDBLOCK
              The file descriptor fd refers to a socket and  has  been  marked
              non-blocking   (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.

       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 3.23 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/.