Interlocked Variable Access

The functions InterlockedCompareExchange, InterlockedExchangeAdd, InterlockedDecrement, InterlockedExchange, and InterlockedIncrement provide a simple mechanism for synchronizing access to a variable that is shared by multiple threads. The threads of different processes can use this mechanism if the variable is in shared memory.

The InterlockedIncrement and InterlockedDecrement functions combine the operations of incrementing or decrementing the variable and checking the resulting value. This atomic operation is useful in a multitasking operating system, in which the system can interrupt one thread's execution to grant a slice of processor time to another thread. Without such synchronization, one thread could increment a variable but be interrupted by the system before it can check the resulting value of the variable. A second thread could then increment the same variable. When the first thread receives its next timeslice, it will check the value of the variable, which has now been incremented not once but twice. The interlocked variable-access functions protect against this kind of error.

The InterlockedExchange function atomically exchanges the values of the specified variables. The InterlockedExchangeAdd function combines two operations: adding two variables together and storing the result in one of the variables.

The InterlockedCompareExchange function combines two operations: comparing two values and storing a third value in one of the variables, based on the outcome of the comparison.

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