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. Let's 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.