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data race

Two threads touching the same memory at the same time, at least one of them writing, with nothing to order them. In C++ that is undefined behaviour.

In C++ a data race is two conflicting accesses (same location, at least one a write) from different threads, not both atomic, with neither happening before the other; a program containing one has undefined behaviour.

Examples

  • Two threads each running count++ 1,000 times on a plain int can end with less than 2,000.

See also

race condition undefined behaviour std::atomic
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