Defibrillator for stalled software

It's happened to everyone: You're using a familiar piece of software to do something you've done a thousand times before - say, find a particular word in a document - and all of a sudden the program just stops working. You click the cursor and move the mouse, but nothing changes on-screen, and finally you just quit the program, losing whatever work you'd done since the last time you saved.

Graphic: Christine Daniloff

Often, a stalled program has gone into what computer scientists call an infinite loop, where it keeps executing a single block of code over and over. At the 25th European Conference on Object-Oriented Programming in Lancaster, England, in July, researchers from MIT's Computer Science and Artificial Intelligence Laboratory (CSAIL) presented a new tool that automatically interrupts infinite loops and moves on to the next line of code in the computer program. In tests, their system restored five different programs to stable enough states that data could be saved and the programs exited safely; in the majority of cases, the programs also provided at least a partial solution to the computations they were trying to perform when they got hung up.

Loops are among the most basic building blocks of computer programs. They allow a programmer to specify, in a single step, a procedure that has to be performed on many pieces of data in sequence. For instance, the search function in a word processor might have to look at thousands of individual letters in even a fairly short document, comparing each of them to the letters in a search term. If it doesn't find a match, it will move onto the next letter and ''loop'' back to re-execute the code that does the comparing.

Wheel spinning

A commercial program might contain tens of thousands of loops, and a slight error in the code for any one of them could lead to an infinite loop, in which the computer doesn't know when to stop repeating the same operation. Computer science professor Martin Rinard and his graduate students Michael Carbin, Sasa Misailovic and Michael Kling developed a software tool that they call Jolt, which recognizes infinite loops by monitoring the program's use of memory. A computer user who's worried that his or her computer has entered an infinite loop could activate Jolt, which would take a series of ''snapshots'' of the computer's memory after each iteration of a loop.

''The snapshots could be completely different,'' explains Carbin, who is first author on the paper. ''That can be an indicator that your program is computing. It may be doing something useful for you, so maybe you don't want to break out of this. But if it's not, and it has exactly the same state, then clearly it's stuck in an infinite loop.''