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       write - write to a file descriptor


       #include <unistd.h>

       ssize_t write(int fd, const void *buf, size_t count);


       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.


       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


       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.

              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.

              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

       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.

       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
              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.


       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.


       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
       actually  transferred.   (This  is  true  on  both  32-bit  and  64-bit


       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


       close(2),  fcntl(2),  fsync(2), ioctl(2), lseek(2), open(2), pwrite(2),
       read(2), select(2), writev(2), fwrite(3)


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