Problem - Kevin Tafuro

a loop until all the data is read. A timeout could be applied to the read operation to
cause the incomplete lock to be treated as stale. This race condition will only occur if
a process creates the lock file without writing any data to it. This could be caused by
an attacker, or it could occur because the process is terminated at precisely the right
time so that it doesn’t get the chance to write its ID to the lock file.
Once the process ID is read from the lock file, an attempt to send the process a signal
of 0 is made. If the signal cannot be sent because the process does not exist, the
call to kill( ) will return failure, and errno will be set to ESRCH. If this happens, the
lock file is stale, and it can be removed. This is where the race condition discussed
earlier occurs. The lock file is removed, the attempt counter is decremented, and the
loop is restarted.
Between the time that kill( ) returns failure with an ESRCH error code and the time
that unlink( ) is called to remove the lock file, another process could successfully
delete the lock file and begin creating a new one. If this happens, the process will
successfully write its process ID to the now deleted lock file and assume that it has
the lock. It will not have the lock, though, because this process will have deleted the
lock file the other process was creating. For this reason, after the lock file is created,
the process must attempt to read the lock file and compare process IDs. If the process
ID in the lock file is the same as the process making the comparison, the lock
was successfully obtained.
See Also
Recipe 2.4
2.10 Synchronizing Resource Access Across
Processes on Windows
Problem
You want to ensure that two processes cannot simultaneously access the same
resource.
Solution
Use a named mutex (mutually exclusive lock) to synchronize access to the resource.
Discussion
Coordinating access to a shared resource between multiple processes on Windows is
much simpler and much more elegant than it is on Unix. For maximum portability
Synchronizing Resource Access Across Processes on Windows | 63
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