The C Preprocessor: Self-Referential Macros

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#### 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.

Next: Argument Prescan, Previous: Duplication of Side Effects, Up: Macro Pitfalls [Contents][Index]