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Evaluation of an expression may produce side effects. At specific points during execution, known as sequence points, all side effects of previous evaluations have completed, and no side effects of subsequent evaluations have yet taken place.

The C Standard, Section section 6.5 [ISO/IEC 9899:2011], states:

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Programs cannot safely rely on the order of evaluation of operands between sequence points. In this noncompliant code example, the order of evaluation of the operands to the + operator is unspecified.:

a = i + b[++i];

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These examples are independent of the order of evaluation of the operands and can be interpreted in only one way.:

++i;
a = i + b[i];

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The order of evaluation for function arguments is unspecified.:

func(i++, i);

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This solution is appropriate when the programmer intends for both arguments to func() to be equivalent.:

i++;
func(i, i);

This solution is appropriate when the programmer intends for the second argument to be one greater than the first.:

j = i++;
func(j, i);

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Bibliography

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