The C Preprocessor: Duplication of Side Effects

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#### 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 long time to compute, the results might not be what you intended. We say that `min` is an unsafe macro.

The best solution to this problem is to define `min` in a way that computes the value of `foo (z)` only once. The C language offers no standard way to do this, but it can be done with GNU extensions as follows:

    #define min(X, Y)                \
    ({ typeof (X) x_ = (X);          \
       typeof (Y) y_ = (Y);          \
       (x_ < y_) ? x_ : y_; })

The ‘`({ … })`’ notation produces a compound statement that acts as an expression. Its value is the value of its last statement. This permits us to define local variables and assign each argument to one. The local variables have underscores after their names to reduce the risk of conflict with an identifier of wider scope (it is impossible to avoid this entirely). Now each argument is evaluated exactly once.

If you do not wish to use GNU C extensions, the only solution is to be careful when using the macro `min`. For example, you can calculate the value of `foo (z)`, save it in a variable, and use that variable in `min`:

    #define min(X, Y)  ((X) < (Y) ? (X) : (Y))
    …
    {
      int tem = foo (z);
      next = min (x + y, tem);
    }

(where we assume that `foo` returns type `int`).

Next: Self-Referential Macros, Previous: Swallowing the Semicolon, Up: Macro Pitfalls [Contents][Index]