Synchronously locks the file or part of the file.
By default an exclusive lock will be obtained, but that can be overridden
by the mode argument.
Locks the byte range from start to end of the file ,with the
byte at position end not included. If no arguments are
specified, the full file is locked, If only start is specified
the file is locked from byte position start to the end of the
file, no matter how large it grows. It is possible to specify an
explicit value of end which is past the current length of the file.
To obtain an exclusive lock on a file it must be opened for writing.
If mode is FileLock.EXCLUSIVE or FileLock.SHARED, an exception is
thrown if the lock cannot be obtained. If mode is
FileLock.BLOCKING_EXCLUSIVE or FileLock.BLOCKING_SHARED, the
call returns only after the lock has been obtained.
NOTE file locking does have slight differences in behavior across platforms:
On Linux and OS X this uses advisory locks, which have the surprising semantics that all locks associated with a given file are removed when any file descriptor for that file is closed by the process. Note that this does not actually lock the file for access. Also note that advisory locks are on a process level. This means that several isolates in the same process can obtain an exclusive lock on the same file.
On Windows the regions used for lock and unlock needs to match. If that is not the case unlocking will result in the OS error "The segment is already unlocked".
Source
void lockSync(
[FileLock mode = FileLock.EXCLUSIVE, int start = 0, int end = -1]);