Automatic Variables and Stack Frames

The stack frames have another important responsibility. Most functions have automatic variables—parameters and any local variables the function declares. Automatic variables need to exist while a function is executing. They need to remain active if the function makes calls to other functions. But when a called function returns to its caller, the called function’s automatic variables need to “go away.” The called function’s stack frame is a perfect place to reserve the memory for the called function’s automatic variables. That stack frame exists as long as the called function is active. When that function returns—and no longer needs its local automatic variables—its stack frame is popped from the stack, and those local automatic variables are no longer known to the program.