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The C language does not necessarily carry out operations within an expression in the order they appear in the code. For instance, in this expression,
foo () + bar ()
foo might be called first or
bar might be called first.
foo updates a datum and
bar uses that datum, the
results can be unpredictable.
The unpredictable order of computation of subexpressions also makes a difference when one of them contains an assignment. We already saw this example of bad code,
x = 20; printf ("%d %d\n", x, x = 4);
in which the second argument,
x, has a different value
depending on whether it is computed before or after the assignment in
the third argument.