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
only applies to compilers that conform to the (emerging) C1X standard add ref.
Noncompliant Code Example
This noncompliant code example aligns ptr to a 4096 byte boundary whereas 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 assuming that the sizeof(double) = 8 and sizeof(float) = 4.)
| Code Block | ||
|---|---|---|
| ||
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 resulting program has an undefined behavior as the alignment that realloc() enforces is different from aligned_alloc() function's alignment.
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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 again assuming that the sizeof(double) = 8 and sizeof(float) = 4).)
| Code Block | ||
|---|---|---|
| ||
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);
}
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Implementation Details
| Wiki Markup |
|---|
The non-compliantnoncompliant examplecodexample 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) |
| Code Block |
|---|
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.
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