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Concept
Aliasing (computing)
Résumé
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 there is an array of size 2 (for this example's sake, calling it ), next to another variable (call it ), (i.e., the 3rd element) would be aliased to if they are adjacent in memory.
include
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.Source officielle
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