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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: void sync (void)
Preliminary: | MT-Safe | AS-Safe | AC-Safe | *Note POSIX Safety
Concepts::.
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'.
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)
Preliminary: | MT-Safe | AS-Safe | AC-Safe | *Note POSIX Safety
Concepts::.
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)
Preliminary: | MT-Safe | AS-Safe | AC-Safe | *Note POSIX Safety
Concepts::.
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.
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