Implementation Notes

Function-like Macros

In general, function-like macros must behave exactly like regular functions. This means that all parameters to the macro must be evaluated as if they were normal function parameters even though they may be given as complex expressions from the caller.

Example: Bad implementation of set_bit()

 #define set_bit(nr, bitmap)    (bitmap[nr >> 5] |= 1 << (nr & 31))

 void foo(int a, int b)
 {
        set_bit(a | b, &foo_bitmap);
        set_bit(a++, &bar_bitmap);
 }

The preceding example has two problems:

• The macros parameters aren't parenthesized, so the standard C precedence rules will cause unexpected results. The first call to set_bit() will be expanded as (&foo_bitmap[a | b >> 5] |= 1 << (a | b & 31). Both the >> and & operators have higher precedence than the | operator, so the macro will not behave in the same way as a function.
• The nr parameter is evaluated twice. In the second call, this will cause a to be incremented twice.

For an example of a correct implementation, please see the set_bit() macro as it appears in the framework.