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    ==> (4 + y)
          ==> (4 + (2 * x))
     
     y    ==> (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.