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File: libc.info,  Node: Synchronizing I/O,  Next: Asynchronous I/O,  Prev: Waiting for I/O,  Up: Low-Level I/O

13.9 Synchronizing I/O operations
=================================

In most modern operating systems, the normal I/O operations are not
executed synchronously.  I.e., even if a `write' system call returns,
this does not mean the data is actually written to the media, e.g., the
disk.

   In situations where synchronization points are necessary, you can use
special functions which ensure that all operations finish before they
return.

 -- Function: int sync (void)
     A call to this function will not return as long as there is data
     which has not been written to the device.  All dirty buffers in
     the kernel will be written and so an overall consistent system can
     be achieved (if no other process in parallel writes data).

     A prototype for `sync' can be found in `unistd.h'.

     The return value is zero to indicate no error.

   Programs more often want to ensure that data written to a given file
is committed, rather than all data in the system.  For this, `sync' is
overkill.

 -- Function: int fsync (int FILDES)
     The `fsync' function can be used to make sure all data associated
     with the open file FILDES is written to the device associated with
     the descriptor.  The function call does not return unless all
     actions have finished.

     A prototype for `fsync' can be found in `unistd.h'.

     This function is a cancellation point in multi-threaded programs.
     This is a problem if the thread allocates some resources (like
     memory, file descriptors, semaphores or whatever) at the time
     `fsync' is called.  If the thread gets canceled these resources
     stay allocated until the program ends.  To avoid this, calls to
     `fsync' should be protected using cancellation handlers.

     The return value of the function is zero if no error occurred.
     Otherwise it is -1 and the global variable ERRNO is set to the
     following values:
    `EBADF'
          The descriptor FILDES is not valid.

    `EINVAL'
          No synchronization is possible since the system does not
          implement this.

   Sometimes it is not even necessary to write all data associated with
a file descriptor.  E.g., in database files which do not change in size
it is enough to write all the file content data to the device.
Meta-information, like the modification time etc., are not that
important and leaving such information uncommitted does not prevent a
successful recovering of the file in case of a problem.

 -- Function: int fdatasync (int FILDES)
     When a call to the `fdatasync' function returns, it is ensured
     that all of the file data is written to the device.  For all
     pending I/O operations, the parts guaranteeing data integrity
     finished.

     Not all systems implement the `fdatasync' operation.  On systems
     missing this functionality `fdatasync' is emulated by a call to
     `fsync' since the performed actions are a superset of those
     required by `fdatasync'.

     The prototype for `fdatasync' is in `unistd.h'.

     The return value of the function is zero if no error occurred.
     Otherwise it is -1 and the global variable ERRNO is set to the
     following values:
    `EBADF'
          The descriptor FILDES is not valid.

    `EINVAL'
          No synchronization is possible since the system does not
          implement this.