or reset. In the Apollo Guidance Computer, for example, this outer loop was contained in the Exec program, and if the computer had absolutely no other work to do it would loop run a dummy job that would simply turn off the "computer activity" indicator light. Modern computers also typically do not halt the processor or motherboard circuit-driving clocks when they crash. Instead they fall back to an error condition displaying messages to the operator, and enter an infinite loop waiting for the user to either respond to a prompt to continue, or to reset the device. Multi-threading In multi-threaded programs some threads can be executing inside infinite loops without causing the entire program to be stuck in an infinite loop. If the main thread exits all threads of the process are forcefully stopped thus all execution ends and the process/program terminates. The threads inside the infinite loops can perform "housekeeping" tasks or they can be in a blocked state waiting for input (from socket/queue) and resume execution every time input is received. Unintentional looping Most often, the term is used for those situations when this is not the intended result; that is, when this is a bug. Such errors are most common among novice programmers, but can be made by experienced programmers as well, because their causes can be quite subtle. One common cause, for example, is that the programmer intends to iterate over sequence of nodes in a data structure such as a linked list or tree, executing the loop code once for each node. Improperly formed links can create a reference loop in the data structure, where one node links to another that occurs earlier in the sequence. This makes part of the data structure into a ring, causing naive code to loop forever. While most infinite loops can be found by close inspection of the code, there is no general method to determine whether a given program will ever halt or will run forever; this is the undecidability of the halting problem. Interruption As long as the system is responsive, infinite loops can often be interrupted by sending a signal to the process (such as SIGINT in Unix), or an interrupt to the processor, causing the current process to be aborted. This can be done in a task manager, in a terminal with the Control-C command, or by using the kill command or system call. However,