Question

I have found some preprocessing directives in which macro name and replacement-list are same. For example, some preprocessing directives in stdbool.h provided by gcc.

#else /* __cplusplus */

/* Supporting <stdbool.h> in C++ is a GCC extension.  */
#define _Bool   bool
#define bool    bool
#define false   false
#define true    true

#endif /* __cplusplus */

I don’t understand why the programmer wrote these preprocessing directives. They are useless, and the replacements will waste time. I know that this don’t cause infinite recursion. How to avoid infinite recursion? What’s the related provisions in C standard?

Answer 1

Infinite recursion during macro replacement is prevented by C++11 standard section 16.3.4 paragraph 2:

If the name of the macro being replaced is found during this scan of the replacement list (not including the
rest of the source file’s preprocessing tokens), it is not replaced. Furthermore, if any nested replacements
encounter the name of the macro being replaced, it is not replaced. These nonreplaced macro name preprocessing tokens are no longer available for further replacement even if they are later (re)examined in contexts
in which that macro name preprocessing token would otherwise have been replaced.

Basically, this means that a macro which appears inside its own expansion won’t be replaced again.

The reason that true, bool, and false are defined as macros by the GCC extension is to make C++ code more compatible with C99. In C99, these are defined as macros in stdbool.h, so code may check if they are defined using e.g. #ifdef bool.