An infinite loop is a sequence of instructions in a computer program which loops endlessly. Looping is repeating an instruction set until a specific condition is met. An infinite loop occurs when the condition will never be met, due to some inherent characteristic of the loop. There are a few situations when this is desired behavior. For example, many server programs such as Internet or database servers loop forever waiting for and servicing requests. More often, though, the term is used for those situations when this is not the intended result; that is, when this is a bug. Such errors are made by experienced programmers as well as novices, and their causes can be quite subtle.

One common cause, for example, is that the programmer intends to iterate over a collection of items such as a linked list, executing the loop code once for each item, but improperly formed links which create a reference loop in the list, causing the code to continue forever.

Unexpected behavior of a terminating condition can also cause this problem. Here is an example (in C):

 float x = 0.1;
 while (x != 1.1) {
   x = x + 0.1;
   printf("x = %f\n", x);
 }

On some systems, this loop will run ten times as expected; but on some systems it will never terminate. The problem is that the loop terminating condition (x != 1.1) tests for exact equality of two floating point values, and the way floating point values are represented in many computers will make this test fail, because 1.1 cannot be represented exactly. The best way to fix this would be to use an integer as the loop index . A loop index is a variable that stores the point at which the computer is at in the loop. This is necessary so the computer can know when to leave the loop. In addition to using integer array indices, it is best to avoid using != as a loop condition as well. Instead one should use <, <=, >, or >= in most situations.

A similar problem occurs frequently in numerical analysis: in order to compute a certain result, an iteration is intended to be carried out until the error is smaller than a chosen tolerance. However, because of rounding errors during the iteration, the tolerance can never be reached, resulting in an infinite loop.

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.

Pseudo-infinite loops

A pseudo-infinite loop is a loop that appears infinite but is really just a very long loop. An example in C:

unsigned int i;
for (i=1;i>0;i++)
{ loop code }

This appears like it will go on for ever, but due to memory limitations, the value of i will roll back to 0, breaking a the loop. The actual limit of i depends on the details of the system and compiler used. In a perfect, albeit unrealistic implementation, though, this would be an infiinite loop.

See also

• Infinite Loop - a road around Apple Computer, Inc headquarters in Cupertino, California