34 Chapter 3. Macros
3.10.5. Self-Referential Macros
A self-referential macro is one whose name appears in its definition. Recall that all macro definitions are rescanned for more macros to replace. If the self-reference were considered a use of the macro, it would produce an infinitely large expansion. To prevent this, the self-reference is not considered a macro call. It is passed into the preprocessor output unchanged. Consider an example:
#define foo (4 + foo) where foo is also a variable in your program.
Following the ordinary rules, each reference to foo will expand into (4 + foo) ; then this will be rescanned and will expand into (4 + (4 + foo)) ; and so on until the computer runs out of memory.
The self-reference rule cuts this process short after one step, at (4 + foo) . Therefore, this macro definition has the possibly useful effect of causing the program to add 4 to the value of foo wherever foo is referred to.
In most cases, it is a bad idea to take advantage of this feature. A person reading the program who sees that foo is a variable will not expect that it is a macro as well. The reader will come across the identifier foo in the program and think its value should be that of the variable foo , whereas in fact the value is four greater.
One common, useful use of self-reference is to create a macro which expands to itself. If you write
#define EPERM EPERM then the macro EPERM expands to EPERM . Effectively, it is left alone by the preprocessor whenever it’s used in running text. You can tell that it’s a macro with #ifdef . You might do this if you want to define numeric constants with an enum , but have #ifdef be true for each constant.
If a macro x expands to use a macro y , and the expansion of y refers to the macro x , that is an indirect self-reference of x .
x is not expanded in this case either. Thus, if we have
#define x (4 + y)
#define y (2 * x) then x and y expand as follows: x y
==> (4 + y)
==> (4 + (2 * x))
==> (2 * x)
==> (2 * (4 + y))
Each macro is expanded when it appears in the definition of the other macro, but not when it indirectly appears in its own definition.
