Type-based aliasing

The compiler follows the type-based aliasing rule in the C and C++ standards when the -qalias=ansi option is in effect (which it is by default). This rule, also known as the ANSI aliasing rule, states that a pointer can only be dereferenced to an object of the same type or a compatible type. 1

The common coding practice of casting a pointer to an incompatible type and then dereferencing it violates this rule. (Note that char pointers are an exception to this rule.)

The compiler uses the type-based aliasing information to perform optimizations to the generated code. Contravening the type-based aliasing rule can lead to unexpected behavior, as demonstrated in the following example:

   int *p;
   double d = 0.0;

   int *faa(double *g);         /* cast operator inside the function   */

   void foo(double f) {
       p = faa(&f);       /* turning &f into a int ptr     */
       f += 1.0;                /* compiler may discard this statement */
       printf("f=%x\n", *p);

   int *faa(double *g) { return (int*) g; }   /* questionable cast;       */
                                              /* the function can be in   */
                                              /* another translation unit */

   int main() {

In the above printf statement, *p cannot be dereferenced to a double under the ANSI aliasing rule. The compiler determines that the result of f += 1.0; is never used subsequently. Thus, the optimizer may discard the statement from the generated code. If you compile the above example with optimization enabled, the printf statement may output 0 (zero).

Related information

The C Standard states that an object shall have its stored value accessed only by an lvalue that has one of the following types:

The C++ standard states that if a program attempts to access the stored value of an object through an lvalue of other than one of the following types, the behavior is undefined: