firstprivate

Firstprivate is a data-sharing clause in OpenMP used in parallel regions. It specifies that for each thread, there is a private copy of the listed variables, and that each private copy is initialized with the value that the corresponding variable had immediately before entering the parallel region.

For scalar variables, each thread’s private copy is initialized to the current value of that variable. For arrays, each thread receives its own private array whose elements are initialized from the corresponding elements of the original array. For objects of a class type, the per-thread copies are constructed using the object’s copy constructor. The initialization occurs once at the start of the parallel region; after that, modifications in one thread affect only its own copy and do not change the original variable.

Because the private copies are independent, updates performed inside the parallel region do not affect the original variable outside the region. After the region ends, the original value remains unchanged unless the program assigns to it through other means or uses other data-sharing constructs to propagate results. Firstprivate is commonly used to provide each thread with an initial value that should be preserved for the duration of the parallel work while avoiding data races on subsequent updates to that same variable.