Aliasing (computing)

In computing, aliasing describes a situation in which a data location in memory can be accessed through different symbolic names in the program. Thus, modifying the data through one name implicitly modifies the values associated with all aliased names, which may not be expected by the programmer. As a result, aliasing makes it particularly difficult to understand, analyze and optimize programs. Aliasing analysers intend to make and compute useful information for understanding aliasing in programs.

For example, most implementations of the C programming language do not perform array bounds checking. One can then exploit the implementation of the programming language by the compiler and the computer architecture's assembly language conventions, to achieve aliasing effects by writing outside of the array (a type of buffer overflow). This invokes undefined behaviour according to the C language specification; however many implementations of C will show the aliasing effects described here.

If an array is created on the stack, with a variable laid out in memory directly beside that array, one could index outside the array and directly change the variable by changing the relevant array element. For example, if we have an int array of size 2 (for this example's sake, calling it arr), next to another int variable (call it i), arr[2] (i.e. the 3rd element) would be aliased to i if they are adjacent in memory.

This is possible in some implementations of C because an array is a block of contiguous memory, and array elements are merely referenced by offsets off the address of the beginning of that block multiplied by the size of a single element. Since C has no bounds checking, indexing and addressing outside of the array is possible. Note that the aforementioned aliasing behaviour is undefined behaviour. Some implementations may leave space between arrays and variables on the stack, for instance, to align variables to memory locations that are a multiple of the architecture's native word size. The C standard does not generally specify how data is to be laid out in memory. (ISO/IEC 9899:1999, section 6.2.6.1).

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