FreeBSD Developers' Handbook
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# 3.7 Race Conditions

A race condition is anomalous behavior caused by the unexpected dependence on the relative timing of events. In other words, a programmer incorrectly assumed that a particular event would always happen before another.

Some of the common causes of race conditions are signals, access checks, and file opens. Signals are asynchronous events by nature so special care must be taken in dealing with them. Checking access with `access(2)` then `open(2)` is clearly non-atomic. Users can move files in between the two calls. Instead, privileged applications should `seteuid()` and then call `open()` directly. Along the same lines, an application should always set a proper umask before `open()` to obviate the need for spurious `chmod()` calls.

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