Bounds Checking

In computer programming, bounds checking is any method of detecting whether a variable is within some bounds before its use. It is particularly relevant to a variable used as an index into an array to ensure its value lies within the bounds of the array. For example: a value of 32768 about to be assigned to a sixteen-bit signed integer variable (whose bounds are −32768 to +32767), or accessing element 25 on an array with index range 0 through 9 only. The first is also known as range checking, the second as index checking.

A failed bounds check usually results in the generation of some sort of exception signal.

Because performing bounds checking during every usage is time-consuming it is not always done. Bounds-checking elimination are compiler technologies that eliminate unneeded bounds checking in many common cases.

Many programming languages, such as C, never perform automatic bounds checking, to raise speed. However, this leaves uncaught many off-by-one errors and buffer overflows. Many programmers believe these languages sacrifice too much for rapid execution. In his 1980 Turing Award lecture, C. A. R. Hoare described his experience in the design of ALGOL 60, a language that included bounds checking, saying:

A consequence of this principle is that every occurrence of every subscript of every subscripted variable was on every occasion checked at run time against both the upper and the lower declared bounds of the array. Many years later we asked our customers whether they wished us to provide an option to switch off these checks in the interest of efficiency on production runs. Unanimously, they urged us not to—they already knew how frequently subscript errors occur on production runs where failure to detect them could be disastrous. I note with fear and horror that even in 1980, language designers and users have not learned this lesson. In any respectable branch of engineering, failure to observe such elementary precautions would have long been against the law.

Mainstream languages that enforce run time checking include Ada, C#, Haskell, Java, JavaScript, Lisp, PHP, Python, Ruby, and Visual Basic. The D and OCaml languages have run time bounds checking that is enabled or disabled with a compiler switch. C# also supports unsafe regions: sections of code that (among other things) temporarily suspend bounds checking to raise efficiency. These are useful for speeding up small time-critical bottlenecks without sacrificing the safety of a whole program.