Semaphore Files

• Semaphore files solve many problems associated with locking

• Example: How to lock a Berkeley DB file?

        tie %db => 'DB_File', $filename, ... or die ...;
        open FH, "<&=".tied(%db)->fd or die ...;
        flock FH, LOCK_EX or die ...;

• Complicated and doesn't work

  • Berkeley DB reads and caches data immediately after the open

  • By then it's too late to get the lock

• Solution: Semaphore file

         open S, "> $SEMAPHORE" or die ...;
         flock S, LOCK_EX or die...;
         tie %db => 'DB_File', $filename, ... or die ...;
         ...
         untie %db;
         close S;

• Problem solved

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Copyright © 2003 M. J. Dominus