CERT C++: EXP36-C

The issue Source buffer misaligned with destination buffer occurs when the source pointer and the target pointer in a pointer-to-pointer conversion do not align. The misalignment occurs for reasons such as:

Polyspace^® does not report violations of this rule if:

If the alignment of a pointer changes in a pointer-to-pointer conversion, dereferencing the converted pointer causes abnormal program termination.

Avoid changing the alignment of a pointer in a pointer-to-pointer conversion.

#include <string.h>
struct record {
  int len;
  /* ... */
};

int copyBuffer (char *data, int offset)
{
  struct record *tmp;
  struct record dest;
  tmp = (struct record *) (data + offset);   //Noncompliant
  memcpy (&dest, tmp, sizeof (dest));
  /* ... */

  return dest.len;
}

In this example, the function copyBuffer converts a char* pointer into a struct record* pointer, followed by a memcpy operation. The memcpy operation assumes a struct record* alignment of tmp, which leads to undefined behavior. The rule checker reports a violation of this rule.

To avoid the issue, use data + offset as the source argument of the memcpy operation instead of using an intermediate struct record* pointer.

This example aligns the char pointer char_p as an int by using the alignas specifier. Because char_p is aligned as an int, converting it to an int* does not violate this rule.

#include <stdalign.h>
#include <assert.h>
  
void foo(void) {
  /* Align char_p to the alignment of an int */
  alignas(int) char char_p = 'a';
  int *int_p = (int *)&char_p; //Compliant
  char *char_new = (char *)int_p;
  /* Both char_new and &char_p point to equally aligned objects */
  
}