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compare-and-swap

An atomic step that writes a new value only if memory still holds the value you expected, and tells you whether it did.

Compare-and-swap atomically compares *p with an expected value and, if equal, stores a desired value; it reports success and the value observed (x86-64 lock cmpxchg with the expected value in rax; ARM64 LSE cas; C++ compare_exchange_strong/weak).

Examples

  • CAS(&x, 5, 6) sets x to 6 only if it still holds 5, and reports whether it did.

See also

Large System Extensions atomic read-modify-write contention CAS loop
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