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Architectures are free to define the byte alignment of the fundamental types of C. Typically, there are several different possible alignments used by for the fundamental types of C. If the C type checking system is overridden by an explicit cast, it is possible the alignment of the underlying object or type may not match up with the object to which it was cast. Therefore, the alignment must always be the same if a pointer is to be cast into another.

Non-compliant Code Example

(proportional to their size). By definition of C99, a pointer may be cast into and out of void* without warning validly. Thus it is possible to silently switch from one type of pointer to another without flagging a compiler warning by first storing or casting the initial pointer to void* and then storing or casting it to the final type. Because of all of these things, it is possible to silently switch the pointer type, which means that if that pointer is dereferenced and the alignment is different, it might cause the program to terminate abnormally.

...

In the following non-compliant code, 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 should compile without warning. However, but v_pointer might be aligned on a 1 byte boundary. Once it is cast to an int, some architectures will require it to be on 4 byte boundaries. If int_ptr is then later dereferenced, abnormal termination of the program might be killedmay result.

Compliant Solution

In this compliant solution, the parameter is changed to only accept other int* pointers since the input parameter directly influences the output parameter.

int *loop_ptr;
int * int_ptr;

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

Implementation Details

List of common alignments for Microsoft, Borland, and GNU compilers to x86

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 have may cause slow pointer accesses (if the architecture does not care about alignment).

References

\[[Bryant 03|AA. C References#Bryant 03]\] 
\[[ISO/IEC 9899-1999:TC2|AA. C References#ISO/IEC 9899-1999TC2]\] Section 6.2.5, "Types"