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According to C99, Section 6.5 \[[ISO/IEC 9899:1999|AA. Bibliography#ISO/IEC 9899-1999]\] (see also [undefined behavior 32 | CC. Undefined Behavior#ub_32] of Annex J):

Between the previous and next sequence point an object can only have its stored value modified once by the evaluation of an expression. Additionally, the prior value can be read only to determine the value to be stored.

(See also undefined behavior 32 of Annex J.)

This requirement must be met for each allowable ordering of the subexpressions of a full expression; otherwise, the behavior is undefined.

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Automated Detection

Splint Version 3.1.1 can detect violations of this rule.

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Tool	Version	Checker	Description
Section Splint	Include Page c:Splint_V c:Splint_V
Section GCC	Include Page c:GCC_V c:GCC_V		Section can detect violations of this rule when the -Wsequence-point flag is used.
Section Compass/ROSE			Section can detect simple violations of this rule. It needs to examine each expression and make sure that no variable is modified twice in the expression. Also no variable is modified once, and read elsewhere, with the single exception that a variable may appear on both the left and right of an assignment operator.

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Section Coverity Prevent

Include Page c:Coverity_V c:Coverity_V

Section EVALUATION_ORDER

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Section can detect the specific instance where Statement contains multiple side-effects on the same value with an undefined evaluation order because with different compiler flags or different compilers or platforms, the statement may behave differently.

Related Vulnerabilities

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

Other Languages

Related Guidelines

This rule appears in the C++ Secure Coding Standard as : EXP30-CPP. Do not depend on order of evaluation between sequence points.

This rule appears in the Java Secure Coding Standard as : EXP09-J. Do not depend on operator precedence while using expressions containing side-effects.

Bibliography

\[[ISO/IEC 9899:1999|AA. Bibliography#ISO/IEC 9899-1999]\] Section 5.1.2.3, "Program execution," Section 6.5, "Expressions," and Annex C, "Sequence points"
\[[ISO/IEC PDTR 24772|AA. Bibliography#ISO/IEC PDTR 24772]\] "JCW Operator precedence/Order of Evaluation" and "SAM Side-effects and order of evaluation"
\[[MISRA 042004|AA. Bibliography#MISRA 04]\] Rule 12.1
\[[Summit 052005|AA. Bibliography#Summit 05]\] Questions 3.1, 3.2, 3.3, 3.3b, 3.7, 3.8, 3.9, 3.10a, 3.10b, and 3.11
\[[Saks 072007|AA. Bibliography#Saks 07]\]

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