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race condition

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Anomalous behavior due to unexpected critical dependence on the relative timing of events.

For example, if one process writes to a file while another is reading from the same location then the data read may be the old contents, the new contents or some mixture of the two depending on the relative timing of the read and write operations.

A common remedy in this kind of race condition is file locking; a more cumbersome remedy is to reorganize the system such that a certain processes (running a daemon or the like) is the only process that has access to the file, and all other processes that need to access the data in that file do so only via interprocess communication with that one process.

As an example of a more subtle kind of race condition, consider a distributed chat network like IRC, where a user is granted channel-operator privileges in any channel he starts. If two users on different servers, on different ends of the same network, try to start the same-named channel at the same time, each user's respective server will grant channel-operator privileges to each user, since neither will yet have received the other's signal that that channel has been started.

In this case of a race condition, the "shared resource" is the conception of the state of the network (what channels exist, as well as what users started them and therefore have what privileges), which each server is free to change as long as it signals the other servers on the network about the changes so that they can update their conception of the state of the network. However, the latency across the network makes possible the kind of race condition described. In this case, heading off race conditions by imposing a form of control over access to the shared resource -- say, appointing one server to be in charge of who holds what privileges -- would mean turning the distributed network into a centralized one (at least for that one part of the network operation). Where this is not acceptable, the more pragmatic solution is to have the system recognize when a race condition has occurred and to repair the ill effects.

Race conditions also affect electronic circuits where the value output by a logic gate depends on the exact timing of two or more input signals. For example, consider a two input AND gate fed with a logic signal X on input A and its negation, NOT X, on input B. In theory, the output (X AND NOT X) should never be high. However, if changes in the value of X take longer to propagate to input B than to input A then when X changes from false to true, there will be a brief period during which both inputs are true, and so the gate's output will also be true. If this output is fed to an edge-sensitive component such as a counter or flip-flop then the temporary effect ("glitch") will become permanent.


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