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The C Standard, 6.3.2.3, paragraph 7 [ISO/IEC 9899:20112024], states

A pointer to an object or incomplete type may be converted to a pointer to a different object or incomplete type. If the resulting pointer is not correctly aligned for the referenced the referenced type, the behavior is undefined.

See undefined behavior 2524.

If the misaligned pointer is dereferenced, the program may terminate abnormally. On some architectures, the cast alone may cause a loss of information even if the value is not dereferenced if the types involved have differing alignment requirements.

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Assigning an unaligned value to a pointer that references a type that needs to be aligned is undefined behavior 24. An implementation may notice, for example, that tmp and header must be aligned and use an inline memcpy() that uses instructions that assume aligned data.

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EXP36-C-EX2: If a pointer is known to be correctly aligned to the target type, then a cast to that type is permitted. There are several cases where a pointer is known to be correctly aligned to the target type. The pointer could point to an object declared with a suitable alignment specifier. It could point to an object returned by aligned_alloc(), calloc(), malloc(), or realloc(), as per the C standard, section 7.2224.3, paragraph 1 1  [ISO/IEC 9899:20112024].

This compliant solution uses the alignment specifier, which is new to C11, to declare the char object c with the same alignment as that of an object of type int. As a result, the two pointers reference equally aligned pointer types:

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Accessing a pointer or an object that is not properly aligned can cause a program to crash or give erroneous information, or it can cause slow pointer accesses (if the architecture allows misaligned accesses).

Automated Detection

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

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