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Aucbvax.4163 fa.unix-wizards utzoo!decvax!ucbvax!unix-wizards Fri Oct 2 00:51:53 1981 process interlocking problems in unix >From ucsfcgl!sdcarl!dgl@Berkeley Fri Oct 2 00:46:53 1981 The lack of a proper process interlock under unix is very lamentable. I have dealt with this problem trying to implement a bullit-proof interlock for a multi-process database (a file system for sound waveform data) and have come to the realization that there is no way to completely eliminate race conditions short of adding an explicit locking mechanism of some sort to the kernel.

Two solutions have proved to be good enough for most applications: 1) it is possible to use multiplexed files, using a daemon process to watch references for a particular file, and manipulate which processes are allowed to complete the open as a way to selectively control the sequence of access requests among asynchronous processes. 2) For systems where multiplex files are not available, the following strategy works fairly well: open a lock file, write in it your own process id (checking first to see that there is no file there to begin with). Close the file, and reopen it, trying to read the contents. If at any step you fail, go back to the first step. If you are able to read, and it is the same process id you wrote, close the file and READ IT AGAIN. If it STILL agrees, you've locked the system. (If you fail reading, or another number comes back, you've been locked out. Enter a loop that alternately sleeps and does an access() on the lock file until it dissapears, then do the entire sequence again.) As a final check, just before deleting the lockfile to free the system, read the file again to see if it is still your pid. If not, report a synchronization error. Using this scheme, I still occasionally get synchronization errors.

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