race condition

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 filelocking; 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.

(2002-08-03)