Aliasing (computing)

From Academic Kids

In computing, aliasing is a term that generally means that one variable or some reference, when changed, has an indirect (usually unexpected) effect on some other data.

For example, the C programming language does not perform array bounds checking. One can then exploit the implementation of the programming language by the compiler, plus the computer architecture's assembly language conventions, to achieve aliasing effects.

If an array is created on the stack, with a variable laid out in memory directly beside that array, one could index outside that array and then directly change that variable by changing the relevant array element. For example, if we have a int array of size ten (for this example's sake, calling it vector), next to another int variable (call it i), vector[11] would be aliased to i if they are adjacent in memory.

This is possible in some implementations of C because an array is in reality a pointer to some location in memory, and array elements are merely offsets off that memory location. Since C has no bounds checking, indexing and addressing outside of the array is possible. Note that the aforementioned aliasing behaviour is implementation specific. Some implementations may leave space between arrays and variables on the stack, for instance, to minimize possible aliasing effects. C programming language specifications do not specify how data is to be laid out in memory.

Another variety of aliasing can occur in any language that can refer to one location in memory with more than one name (for example, with pointers). See the C example of the xor swap algorithm that is a function; it assumes the two pointers passed to it are distinct, but if they are in fact equal (or aliases of each other), the function fails. This is a common problem with functions that accept pointer arguments, and their tolerance (or the lack thereof) for aliasing must be carefully documented, particularly for functions that perform complex manipulations on memory areas passed to them.

Controlled aliasing behaviour may be desirable in some cases (that is, aliasing behaviour that is specified, unlike that relevant to memory layout in C). The Perl programming language specifies, in some constructs, aliasing behaviour, such as in foreach loops. This allows certain data structures to be modified directly with less code. For example,

@x = (1, 2, 3);

foreach $e (@x)
{
   $e++
}

print "@x\n";

will print out "2 3 4" as a result. If one would want to bypass aliasing effects, one would copy the contents of the index variable into another and change the copy.

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