Token pasting in #include directive

[Chris Torek]

>In article <18672 at watdragon.waterloo.edu> afscian at violet.waterloo.edu
>(Anthony Scian) writes:
>>What about the library prototypes that are coded "int foo( int x, int y )"
>>when they should be "int foo( int __x, int __y )"?

In article <970 at sdrc.UUCP> scjones at sdrc.UUCP (Larry Jones) writes:
>Eh?  Why is the second any more correct than the first?  Since
>the argument names in a prototype only have prototype scope, they
>can't conflict with any other names in the program and therefor
>do not need leading underscores.

The following is (apparently---anyone who can supply text proving otherwise
is welcome to follow up) legal:

	#define x point.xpart
	#define	y point.ypart

	#include <math.h>

Thus, if <math.h> includes the line

	double pow(double x, double y);

the compiler will attempt to parse the expansion

	double pow(double point.xpart, double point.ypart);

which will give a syntax error.

However, the following is (apparently) illegal:

	#define _x point.x	/* this has file scope and is thus illegal */
	#define _y point.y	/* (at least, at this point) */

	#include <math.h>

hence <math.h> *could* include the line

	double pow(double _x, double _y);

Names such as `_a' (apparently) cannot exist with file scope% at the
time of a `#include some_standard_header', and names such as `_A'
(underscore followed by an uppercase letter or a second underscore)
are completely off-limits to users.

-----
% Maybe they can, provided they are not `#define' symbols, since
	static double _x() {
		return 3.14159265358979323/2.718281828459045235);
	}
	#include <math.h>
  seems unlikely to cause trouble.  But `#define x some,long,expr'
  is otherwise quite all right, but will discombobulate prototypes.
-- 
In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 454 7163)
Domain:	chris at cs.umd.edu	Path:	uunet!mimsy!chris