fsync, fdatasync - synchronize a file's in-memory state with that on the
int fsync(int fildes);
int fdatasync(int fildes);
fsync moves all modified data and attributes of fildes to a storage
device. When fsync returns, all in-memory modified copies of buffers
associated with fildes have been written to the physical medium. fsync
is different from sync, which schedules disk I/O for all files but
returns before the I/O completes.
fsync should be used by programs that require that a file be in a known
state. For example, a program that contains a simple transaction
facility might use fsync to ensure that all changes to a file or files
caused by a given transaction were recorded on a storage medium.
fdatasync is the same as fsync, except that it only moves all the
modified data, and not the attributes of fildes to a storage device.
fsync and fdatasync fail if one or more of the following are true:
EBADF fildes is not a valid file descriptor open for writing.
ENOLINK fildes is on a remote machine and the link on that machine
is no longer active.
EINTR A signal was caught during execution of the system call.
EIO An I/O error occurred while reading from or writing to the
EINVAL fildes is not a valid descriptor Synchronized I/O.
Upon successful completion, a value of 0 is returned. Otherwise, a value
of -1 is returned and errno is set to indicate the error.
The way the data reach the physical medium depends on both implementation
and hardware. fsync returns when the device driver tells it that the
write has taken place.
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