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This problem is often caused by a violation of aliasing rules. The C Standard, 6.5, paragraph 7 [ ISO/IEC 9899:20112024 ], specifies those circumstances in which an object may or may not be aliased.
An object shall have its stored value accessed only by an lvalue expression that has one of the following types:
- a type compatible with the effective type of the object,
- a qualified version of a type compatible with the effective type of the object,
- a type that is the signed or unsigned type corresponding to compatible with the underlying type of the effective type of the object, a type that is
- the signed or unsigned type corresponding to compatible with a qualified version of the underlying type of the effective type of the object,
- an aggregate or union type that includes one of the aforementioned types among its members (including, recursively, a member of a subaggregate or contained union), oror
- a character type.
Accessing an object by means of any other lvalue expression (other than unsigned char) is undefined behavior 37.
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If the member used to read the contents of a union object is not the same as the member last used to store a value in the object, the appropriate part of the object representation of the value is reinterpreted as an object representation in the new type as described in 6.2.6 (a process sometimes called “type punning”). This might be a trap representation.
The call to printf() typically outputs "2222 2222". However, there is no guarantee that this will be true; the object representations of a and i are unspecified and need not be compatible in this way, despite this operation being commonly accepted as an implementation extension. (See unspecified behavior 11.)
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According to the C Standard, 6.7.67.2 3 [ISO/IEC 9899:20112024], using two or more incompatible arrays in an expression is undefined behavior. (See also undefined behavior 76.)
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Tool | Version | Checker | Description | ||||||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| LDRA tool suiteCppcheck Premium |
| LDRA
| LDRA
| 94 S, 554 Spremium-cert-exp39-c | Partially implemented | ||||||||||||||||||||
| Helix QAC |
| C0310, C0751, C3305 C++3017, C++3030, | Parasoft C/C++ | test3033 | |||||||||||||||||||||
| Klocwork |
| Parasoft
| Parasoft
| CERT_C-EXP39-a | There shall be no implicit conversions from integral to floating type | Polyspace Bug Finder | |||||||||||||||||||
| Include Page | Polyspace Bug Finder_V | Polyspace Bug Finder_V | Checks for cast to pointer pointing to object of different type (rule partially covered) | MISRA.CAST.FUNC_PTR.2012 | |||||||||||||||||||||
| LDRA tool suite |
| 94 S, 554 S | Partially implemented | ||||||||||||||||||||||
| Parasoft C/C++test |
| CERT_C-EXP39-a | There shall be no implicit conversions from integral to floating type | ||||||||||||||||||||||
| Polyspace Bug Finder |
| Checks for cast to pointer pointing to object of different type (rule partially covered) | |||||||||||||||||||||||
| PRQA QA-C | |||||||||||||||||||||||||
| Include Page | PRQA QA-C_v | PRQA QA-C_v | 0310, 0751, 3305 | Partially implemented | PRQA QA-C++ | ||||||||||||||||||||
| Include Page | cplusplus:PRQA QA-C++_V | cplusplus:PRQA QA-C++_V | 3017, 3030, 3033 | PVS-Studio |
| V580 | Helix QAC | ||||||||||||||||||
| Include Page | Helix QAC_V | Helix QAC_VC: 0310, 0751, 3305 C++: 3017, 3030, 3033 |
Related Vulnerabilities
Search for vulnerabilities resulting from the violation of this rule on the CERT website.
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| [Acton 2006] | "Understanding Strict Aliasing" |
| GCC Known Bugs | "C Bugs, Aliasing Issues while Casting to Incompatible Types" |
| [ISO/IEC 9899:20112024] | 6.5, "Expressions" 6.7.67.23, "Array Declarators" |
| [Walfridsson 2003] | Aliasing, Pointer Casts and GCC 3.3 |
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