别名 (计算)

例子

缓冲区溢出

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.

# include <stdio.h>

int main()
{
 int arr[2] = { 1, 2 };
 int i=10;

 /* Write beyond the end of arr. Undefined behaviour in standard C, will write to i in some implementations. */
 arr[2] = 20;

 printf("element 0: %d \t", arr[0]); // outputs 1
 printf("element 1: %d \t", arr[1]); // outputs 2
 printf("element 2: %d \t", arr[2]); // outputs 20, if aliasing occurred
 printf("i: %d \t\t", i); // might also output 20, not 10, because of aliasing, but the compiler might have i stored in a register and print 10
 /* arr size is still 2. */
 printf("arr size: %d \n", (sizeof(arr) / sizeof(int)));
}

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).

It is not erroneous for a compiler to omit aliasing effects for accesses that fall outside the bounds of an array.

别名指针

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.

Specified aliasing

Controlled aliasing behaviour may be desirable in some cases (that is, aliasing behaviour that is specified, unlike that enabled by memory layout in C). It is common practice in Fortran. 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,

my @array = (1, 2, 3);

foreach my $element (@array) {
   # Increment $element, thus automatically
   # modifying @array, since $element is ''aliased''
   # to each of @array's elements in turn.
   $element++;
}

 print "@array \n";

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