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Function calls, nested assignment statements, and increment and decrement

operators cause side effects'' - some variable is changed as a by-product

of the evaluation of an expression. In any expression involving side effects,

there can be subtle dependencies on the order in which variables taking part

in the expression are updated. One unhappy situation is typified by the

statement

a[i] = i++;

The question is whether the subscript is the old value of

i

or the

new. Compilers can interpret this in different ways, and generate different

answers depending on their interpretation. The standard intentionally leaves

most such matters unspecified. When side effects (assignment to variables)

take place within an expression is left to the discretion of the compiler,

since the best order depends strongly on machine architecture. (The standard

does specify that all side effects on arguments take effect before a function

is called, but that would not help in the call to

printf

above.)

The moral is that writing code that depends on order of evaluation is a bad programming practice in any language. Naturally, it is necessary to know what things to avoid, but if you don't know how they are done on various machines, you won't be tempted to take advantage of a particular implementation.

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