Interlocked Variable Access

The interlocked functions 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.

Simple reads and writes to properly-aligned 32-bit variables are atomic. In other words, when one thread is updating a 32-bit variable, you will not end up with only one portion of the variable updated; all 32 bits are updated in an atomic fashion. However, access is not guaranteed to be synchronized. If two threads are reading and writing from the same variable, you cannot determine if one thread will perform its read operation before the other performs its write operation.

The interlocked functions should be used to perform complex operations in an atomic manner. 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 time slice, 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 InterlockedExchangePointer 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 InterlockedCompareExchangePointer function combines two operations: comparing two values and storing a third value in one of the variables, based on the outcome of the comparison.