The order of evaluation of subexpressions and the order in which side effects take place are frequently defined as unspecified behavior by C99. Counter-intuitively, 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, as 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 orderings may in fact be executed in a different order.
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This recommendation is related to guideline rule EXP30-C. Do not depend on order of evaluation between sequence points, but this recommendation focuses on behavior that is nonportable or potentially confusing.
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Search for vulnerabilities resulting from the violation of this rule on the CERT website.
Related Guidelines
CERT C++ Secure Coding Standard: EXP10-CPP. Do not depend on the order of evaluation of subexpressions or the order in which side effects take place
Bibliography
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\[[ISO/IEC 9899:1999|AA. Bibliography#ISO/IEC 9899-1999]\] Section 6.5, "Expressions," Section 6.5.16, "Assignment operators," Section 6.5.2.2, "Function calls," and Section 6.7.8, "Initialization" \[[
ISO/IEC PDTR 24772|AA. Bibliography#ISO/IEC PDTR 24772]\] "JCW Operator precedence/Order of Evaluation" and "SAM Side-effects and Order of Evaluation" \[[MISRA 2004|AA. Bibliography#MISRA 04]\] Rule
MISRA Rule 12.2
Bibliography
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03. Expressions (EXP) EXP11-C. Do not apply operators expecting one type to data of an incompatible type