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.
(Assuming that the sizeof(double) = 8 and sizeof(float) = 4.)

size_t size = 16;
size_t alignment = 1<<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.
(Assuming that the sizeof(double) = 8 and sizeof(float) = 4.)

size_t size = 16;
size_t alignment = 1<<12;
float *ptr;
double *ptr1;

ptr = aligned_alloc(align , size);

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

Implementation Details

The non-compliant example produces the following (unexpected) output on the x86_64-redhat-linux platform that was compiled with gcc version 4.1.2.
(ptr\[0\] is initialized to 12.5 and ptr\[1\] is initialized to 25.5)

ptr\[0\] (0x2b7000000000) = 12.500000
ptr\[1\] (0x2b7000000004) = 25.500000
ptr1\[0\] (0x2b7000000000) = 12.500000
ptr1\[1\] (0x2b7000000008) = 0.000000

Risk Assessment

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

References

\[[ISO/IEC 9899:201x|http://www.open-std.org/Jtc1/sc22/wg14/www/docs/n1401.pdf]\] Section 7.21.3