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What is atomic_compare_exchange_strong in C?

Adnan Abbas

atomic_compare_exchange_strong is a function that compares memory contents pointed to by two different objects.


bool atomic_compare_exchange_strong( volatile A* obj, C* expected, C desired );

The function compares the value of obj with expected, and if they are bitwise equal, it replaces the value contained in obj with desired value. Otherwise, it replaces the value pointed by expected with the one contained in obj by performing a load operation.

The operation of the function described above, where it reads and replaces data is atomic. This means that other threads cannot change the value between the time it is read and replaced.

As the name suggests, this function makes strong comparisons which means that the actual contents on the disk are compared. This comparison may fail for some values that compare using the == operator.

Unlike its counterpart, atomic_compare_exchange_weak, this function only returns true when both the values pointed by obj and expected are indeed equal to each other. It does not allow any spurious failures. But, in some scenarios where loops are used, atomic_compare_exchange_weak is a better option performance-wise.


  • obj is a pointer to an atomic object to be compared.
  • expected is a pointer to the value which is to be compared with the one contained in obj.
  • desired is the value to store in obj if the comparison is successful.

Return value

The function returns true if the values pointed by obj and expected are equal and false if otherwise.




Adnan Abbas
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