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To bypass the restrictions of POSIX's file locks other advisory locking mechanisms, based only on standard file operations, where invented. One of them is the so-called dot-locking scheme where the lock of file-name is represented by the file file-name.lock. Care is taken that only one process may generate the lock for a given file.
obtain-dot-lock[procedure] (obtain-dot-lock file-name [interval retry-number stale-time])
Tries to obtain the lock for file-name. If the file is already locked, the thread sleeps for interval seconds (default is 1) before it retries. If the lock cannot be obtained after retry-number attempts, the procedure returns #f, otherwise #t. The default value of retry-number is #f which corresponds to an infinite number of retries.
If stale-time is non-#f, it specifies the minimum age a lock may have (in seconds) before it is considered stale. Obtain-dot-lock attempts to delete stale locks. If it was successful obtaining a lock after breaking it, obtain-dot-lock returns 'broken. If stale-time is #f, obtain-dot-lock never considers a lock stale. The default for stale-time is 300.
Note that it is possible that obtain-dot-lock breaks a lock but nevertheless fails to obtain it otherwise. If it is necessary to handle this case specially, use break-dot-lock directly (see below) rather than specifying a non-#f stale-time
release-dot-lock[procedure] (release-dot-lock file-name)
Releases the lock for file-name. On success, release-dot-lock returns #t, otherwise #f. Note that this procedure can also be used to break the lock for file-name.
break-dot-lock[procedure] (break-dot-lock file-name)
Breaks the lock for file-name if one exists. Note that breaking a lock does not imply a subsequent obtain-dot-lock will succeed, as another party may have acquired the lock between break-dot-lock and obtain-dot-lock.
with-dot-lock*[syntax] (with-dot-lock file-name body ...)
[procedure] (with-dot-lock* file-name thunk)
The procedure with-dot-lock* obtains the requested lock, and then calls (thunk). When thunk returns, the lock is released. A non-local exit (e.g., throwing to a saved continuation or raising an exception) also causes the lock to be released.
After a normal return from thunk, its return values are returned by with-dot-lock*. The with-dot-lock special form is equivalent syntactic sugar.
(ported to CHICKEN by felix winkelmann)
Copyright (c) Olin Shivers All rights reserved.
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