If ptr was allocated with an alignment returned from aligned_alloc() and if realloc() reallocates memory with a different alignment then, the behavior is undefined.

This rule is specifically for C1X standards.

Non- Compliant Code

This non-compliant example shows that ptr is aligned to an alignment of 4096 bytes where as the realloc() function aligns the memory to a different alignment.

size_t size = 16;
size_t alignment = 2<<12;
float *ptr;
double *ptr1;

ptr = aligned_alloc(align , size);
ptr1 = realloc(ptr, size);

The realloc function has an undefined behavior as the alignment that realloc() enforces is different from aligned_alloc() function's alignment.

Compliant Solution

This compliant example checks that aligned_alloc() has the same alignment as the alignment realloc() function enforces on the memory pointed to by ptr.

size_t size = 16;
size_t alignment = 2 << 12;
float *ptr;
double *ptr1;

ptr = aligned_alloc(align , size);

if(align == alignof(ptr1)) {
ptr1 = realloc(ptr, size);
}

Risk Assessment

Improper alignment could lead to accessing arbitrary memory locations and write into it.

References

ISO/IEC 9899:201x Section 7.21.3