Different alignments are possible for different types of objects. If the type checking system is overridden by an explicit cast, or the pointer is cast into and out of void *, the alignment of an object may be changed. As a result, if a pointer to one object is converted to a pointer to a different object, the objects must have the same alignment.

Non-Compliant Code Example

C99 allows a pointer may be cast into and out of void *. As a result, it is possible to silently convert from one pointer type to another without the compiler diagnosing the problem by storing or casting a pointer to void * and then storing or casting it to the final type. In this non-compliant code example, the type checking system is circumvented due to the caveats of void pointers.

char *loop_ptr;
int *int_ptr;

int *loop_function(void *v_pointer){
  return v_pointer;
}
int_ptr = loop_function(loop_ptr);

This example compiles without warning. However, v_pointer may be aligned on a one byte boundary. Once it is cast to an int, some architectures will require that the object is aligned on a four byte boundary. If int_ptr is later dereferenced, the program may termination abnormally.

Compliant Solution

Because the input parameter directly influences the return value, and loopFunction() returns an int *, the formal parameter v_pointer is redeclared to only accept int *.

int *loop_ptr;
int *int_ptr;

int *loopFunction(int *v_pointer) {
  return v_pointer;
}
int_ptr = loopFunction(loop_ptr);

Implementation Details

The following list shows common alignments for Microsoft, Borland, and GNU compilers for the IA-32 architecture.

Risk Assessment

Accessing a pointer or an object that is no longer on the correct access boundary can cause a program to crash, give wrong information, or may cause slow pointer accesses (if the architecture does not care about alignment).

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

References

\[[Bryant 03|AA. C References#Bryant 03]\] 
\[[ISO/IEC 9899-1999|AA. C References#ISO/IEC 9899-1999]\] Section 6.2.5, "Types"