The order of evaluation of subexpressions and the order in which side effects take place are frequently defined as unspecified behavior by the C standardStandard. Counterintuitively, unspecified behavior is where the standard provides two or more possibilities and imposes no further requirements on which is chosen in any instance. Consequently, unspecified behavior can be a portability issue because different implementations can make different choices. If dynamic scheduling is used, however, there may not be a fixed-code execution sequence over the life of a process. Operations that can be executed in different sequences may in fact be executed in a different order.
According to the C standardStandard, Section 6.5 [ISO/IEC 9899:2011],
Except as specified later, side effects and value computations of subexpressions are unsequenced.
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- The order in which the arguments to a function are evaluated (C11C Standard, Section 6.5.2.2, "Function callsCalls")
- The order of evaluation of the operands in an assignment statement (C11C Standard, Section 6.5.16, "Assignment operatorsOperators")
- The order in which any side effects occur among the initialization list expressions is unspecified. In particular, the evaluation order need not be the same as the order of subobject initialization (C11C Standard, Section 6.7.9, "Initialization")
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Tool | Version | Checker | Description | ||||||
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Compass/ROSE |
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| Could detect violations of this recommendation by searching for the following pattern:
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| EVALUATION_ORDER | Can detect the specific instance where a statement contains multiple side effects on the same value with an undefined evaluation order because the statement may behave differently with different compiler flags or different compilers or platforms. | |||||||
| 35 D | Fully implemented. | |||||||
| PRQA QA-C |
| 3226 | Partially implemented. |
A programmer could also violate the recommendation using dynamic memory passed to both functions, but that would be extremely difficult to detect using static analysis.
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Search for vulnerabilities resulting from the violation of this rule on the CERT website.
Related Guidelines
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| EXP10-CPP. Do not depend on the order of evaluation of subexpressions or the order in which side effects take place | |
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| ISO/IEC TR 24772 | Operator precedence/order of evaluation [JCW] and Side-effects and order of evaluation [SAM] |
| MISRA-C | Rule 12.2 |
Bibliography
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| ] | Section 6.5, "Expressions," Section 6.5.16, "Assignment |
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| Operators," Section 6.5.2.2, "Function |
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| Calls," and Section 6.7.9, "Initialization" |
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ISO/IEC PDTR 24772 "JCW Operator precedence/order of evaluation" and "SAM Side-effects and order of evaluation"
MISRA Rule 12.2
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