Duplication of Side Effects

3.10.4 Duplication of Side Effects

Many C programs define a macro min, for “minimum”, like this:

#define min(X, Y)  ((X) < (Y) ? (X) : (Y))

When you use this macro with an argument containing a side effect, as shown here,

next = min (x + y, foo (z));

it expands as follows:

next = ((x + y) < (foo (z)) ? (x + y) : (foo (z)));

where x + y has been substituted for X and foo (z) for Y.

The function foo is used only once in the statement as it appears in the program, but the expression foo (z) has been substituted twice into the macro expansion. As a result, foo might be called two times when the statement is executed. If it has side effects or if it takes a lon