Bounds checking

In computer programming, bounds checking is any method of detecting whether a variable is within some bounds before it is used. It is usually used to ensure that a number fits into a given type (range checking), or that a variable being used as an array index is within the bounds of the array (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 is a compiler optimization technique that eliminates unneeded bounds checking.

A range check is a check to make sure a number is within a certain range; for example, to ensure that a value about to be assigned to a 16-bit integer is within the capacity of a 16-bit integer (i.e. checking against wrap-around). This is not quite the same as type checking. Other range checks may be more restrictive; for example, a variable to hold the number of a calendar month may be declared to accept only the range 1 to 12.

Index checking means that, in all expressions indexing an array, the index value is checked against the bounds of the array (which were established when the array was defined), and if the index is out-of-bounds, further execution is suspended via some sort of error. Because using a number outside of the upper range in an array may cause the program to crash, or may introduce security vulnerabilities (see buffer overflow), index checking is a part of many high-level languages.

Pascal, Fortran, Java have index checking ability. The VAX computer has an INDEX assembly instruction for array index checking which takes six operands, all of which can use any VAX addressing mode. The B6500 and similar Burroughs computers performed bound checking via hardware, irrespective of which computer language had been compiled to produce the machine code. A limited number of later CPUs have specialised instructions for checking bounds, e.g., the CHK2 instruction on the Motorola 68000 series.

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