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 filesystems 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 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 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 3.54 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/.