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(cpp-300.info)Duplication of Side Effects

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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').