Race condition

A race condition or race hazard is the condition of an electronics, software, or other system where the system's substantive behavior is dependent on the sequence or timing of uncontrollable events, leading to unexpected or inconsistent results. It becomes a bug when one or more of the possible behaviors is undesirable.

The term race condition was in use by 1954, for example in David A. Huffman's doctoral thesis "The synthesis of sequential switching circuits".

Race conditions can occur especially in logic circuits or concurrent or distributed software programs. Using mutual exclusion can prevent race conditions.

In electronics

A typical example of a race condition may occur when a logic gate combines signals that have traveled along different paths from the same source. The inputs to the gate can change at slightly different times in response to a change in the source signal. The output may, for a brief period, change to an unwanted state before settling back to the designed state. Certain systems can tolerate such glitches, but if this output functions as a clock signal for further systems that contain memory, for example, the system can rapidly depart from its designed behaviour (in effect, the temporary glitch becomes a permanent glitch).

Consider, for example, a two-input AND gate fed with the following logic:

output

=

A

∧

A

¯

{\displaystyle {\text{output}}=A\wedge {\overline {A}}}

A logic signal

A

{\displaystyle A}

on one input and its Boolean negation

A

¯

{\displaystyle {\bar {A}}}

on another input can never, in theory, output a true value as

A

∧

A

¯

≠

1

{\displaystyle A\wedge {\overline {A}}\neq 1}

.