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#include <assert.h>
 
void func(void) {
  char c = 'x';
  int i = c;
  int *ip = &i;

  assert(ip == &i);
}

Compliant Solution (C11, alignas())

This compliant solution uses the alignment specifier to declare the char object c with the same alignment as that of an object of type int. As a result, the two pointers reference equally aligned pointer types:

#include <stdalign.h>
#include <assert.h>
 
void func(void) {
  /* Align c to the alignment of an int */
  alignas(int) char c = 'x';
  int *ip = (int *)&c; 
  char *cp = (char *)ip;
  /* Both cp and &c point to equally aligned objects */
  assert(cp == &c);
}

Noncompliant Code Example

The C Standard allows any object pointer to be cast to and from 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 noncompliant code example, loop_function() is passed the char pointer loop_ptr but returns an object of type int pointer:

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

  /* ... */
}

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Noncompliant Code Example

Some architectures require that pointers are correctly aligned when accessing objects larger than a byte. However, it is common in system code that unaligned data (for example, the network stacks) must be copied to a properly aligned memory location, such as in this noncompliant code example:

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EXP36-EX1: Some hardware architectures have relaxed requirements with regard to pointer alignment. Using a pointer that is not properly aligned is correctly handled by the architecture, although there might be a performance penalty. On such an architecture, improper pointer alignment is permitted but remains an efficiency problem.

EXP36-EX2: If a pointer is known to be correctly aligned to the target type, then a cast to that type is permitted. There are several cases where a pointer is known to be correctly aligned to the target type. The pointer could point to an object declared with a suitable alignas() qualifier. It could point to an object returned by aligned_alloc(), calloc(), malloc(), or realloc(), as per the C standard, section 7.22.3, paragraph 1  [ISO/IEC 9899:2011].

This compliant solution uses the alignment specifier to declare the char object c with the same alignment as that of an object of type int. As a result, the two pointers reference equally aligned pointer types:

#include <stdalign.h>
#include <assert.h>
 
void func(void) {
  /* Align c to the alignment of an int */
  alignas(int) char c = 'x';
  int *ip = (int *)&c; 
  char *cp = (char *)ip;
  /* Both cp and &c point to equally aligned objects */
  assert(cp == &c);
}

Another solution is to ensure that loop_ptr points to an object returned by malloc() because this object is guaranteed to be suitably aligned for the storage of any type of object. However, this subtlety is easily missed when the program is modified in the future. It is easier and safer to let the type system document the alignment needs.

Risk Assessment

Accessing a pointer or an object that is not properly aligned can cause a program to crash or give erroneous information, or it can cause slow pointer accesses (if the architecture allows misaligned accesses).

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