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This aligned_alloc()function was introduced in the C11 standard [ISO/IEC 9899:2011].

Noncompliant Code Example

This noncompliant code example aligns ptr to a 4096-byte boundary, whereas the realloc() function aligns the memory to a different alignment.

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The resulting program has undefined behavior because the alignment that realloc() enforces is different from that of aligned_alloc().

Implementation Details

When compiled with GCC Version 4.1.2 and run on the x86_64 Red Hat Linux platform, the following code produces the following output:

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Unfortunately, ptr1 is no longer aligned to 4096 bytes.

Compliant Solution

This compliant solution implements an aligned realloc() function. It allocates resize bytes of new memory with the same alignment as the old memory and then moves the old memory there, consequently, freeing up the old memory.

size_t resize = 1024;
size_t alignment = 1 << 12;
int *ptr;
int *ptr1;

if ((ptr = aligned_alloc(alignment, sizeof(int))) == NULL) {
  /* handle error */
}

/* ... */

if ((ptr1 = aligned_alloc(alignment, resize)) == NULL) {
  /* handle error */
}

if ((memcpy(ptr1, ptr, sizeof(int)) == NULL) {
  /* handle error */
}

free(ptr);

Risk Assessment

Improper alignment can lead to arbitrary memory locations being accessed and written to.

Bibliography

[ISO/IEC 9899:2011] Section 7.22.3.1

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