Problem - Kevin Tafuro

BOOL UnlockFile(HANDLE hFile, DWORD dwFileOffsetLow,
DWORD dwFileOffsetHigh, DWORD nNumberOfBytesToUnlockLow,
DWORD nNumberOfBytesToUnlockHigh);
2.9 Synchronizing Resource Access Across
Processes on Unix
Problem
You want to ensure that two processes cannot simultaneously access the same
resource, such as a segment of shared memory.
Solution
Use a lock file to signal that you are accessing the resource.
Discussion
Using a lock file to synchronize access to shared resources is not as simple as it
sounds. Suppose that your program creates a lock file and then crashes. If this happens,
the lock file will remain, and your program (as well as any other program that
attempted to obtain the lock) will fail until someone manually removes the lock file.
Obviously, this is undesirable. The solution is to store the process ID of the process
holding the lock in the lock file. Other processes attempting to obtain the lock can
then test to see whether the process holding the lock still exists. If it does not, the
lock file is stale, it is safe to remove, and you can make another attempt to obtain the
lock.
Unfortunately, this solution is still not a perfect one. What happens if another process
is assigned the same ID as the one stored in the stale lock file? The answer to
this question is simply that no process can obtain the lock until the process with the
stale ID terminates or someone manually removes the lock file. Fortunately, this case
should not be encountered frequently.
As a result of solving the stale lock problem, a new problem arises: there is now a
race condition between the time the check for the existence of the process holding
the lock is performed and the time the lock file is removed. The solution to this problem
is to attempt to reopen the lock file after writing the new one to make sure that
the process ID in the lock file is the same as the locking process’s ID. If it is, the lock
is successfully obtained.
The function presented below, spc_lock_file( ), requires a single argument: the
name of the file to be used as the lock file. You must store the lock file in a “safe”
directory (see Recipe 2.4) on a local filesystem. Network filesystems—versions of
60 | Chapter 2: Access Control
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