Question

How is a circular #define resolved?

Asked by: riceman0

Can someone explain why the code below returns the error:

'SAM1': undeclared identifier

On the 'x=' line?

I would think that the first #define would be fine, then the second #define would be expanded to

#define SAM2 SAM2

which essentially has no effect, so I would expect the error to be

'SAM2': undeclared identifier

after the " int x = SAM1;" is expanded to  " int x = SAM2;

but apparently I misunderstand.  What does happen in this case?

Thanks.

#define SAM1 SAM2
#define SAM2 SAM1
 
int main()
{
       int x = SAM1;
	return 0;
}

                                  
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Asked On
2009-10-28 at 18:16:28ID24853276
Topic

C Programming Language

Participating Experts
3
Points
500
Comments
5

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Answers

 

by: mrjoltcolaPosted on 2009-10-28 at 18:50:47ID: 25689977

The ISO preprocessor is not recursive. It simple expands the text once. If there are recursive macros, they are not expanded by the preprocessor, but instead the program is passed to the compiler. If you try GCC with the traditional-cpp flag you get different results than default.


msmith@dmz ~]$ gcc -traditional-cpp t.c
t.c:10: error: detected recursion whilst expanding macro "SAM1"
t.c:10: error: detected recursion whilst expanding macro "SAM2"

http://gcc.gnu.org/onlinedocs/cpp/Traditional-macros.html

So what happens in ISO mode is the preprocessor makes one pass and translates to:


int main()
{
       int x = SAM2;
	return 0;
}
                                              
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by: ozoPosted on 2009-10-28 at 19:54:56ID: 25690303

what version of cc are you using, with what switches?
what do you get with
cc -E

 

by: riceman0Posted on 2009-10-29 at 04:34:26ID: 25692476

Not sure I understand the answer, my "model" involves one pass, but assumes that earlier #defines can affect subsequent #defines, and would work like this:

#define SAM1 SAM2  ... store macro "SAM1->SAM2"
#define SAM2 SAM1  ... expand to "#define SAM2 SAM2" and store macro  "SAM2->SAM2"
int x = SAM2;  .. expand to "int x = SAM2;" and register error 'undefined identifier SAM2'

instead I got the error 'undefined identifier SAM1', as if the first #define didn't affect the second.

my "model" works with the code below, as follows

#define SAM2 SAM3  ... store macro "SAM1->SAM2"
#define SAM1 SAM2  ... expand to "#define SAM1 SAM3" and store macro  "SAM1->SAM3"
int x = SAM1;  .. expand to "int x = SAM3;" and register error 'undefined identifier SAM3'

which is the error I got.  So it looks like the first #define affected the second #define.

So obviously my "model" is wrong, can you tell me what's wrong with it?

Thanks.


#define SAM2 SAM3
#define SAM1 SAM2
 
 
int main()
{
int x = SAM1;   //error C2065: 'SAM3' : undeclared identifier
 
	return 0;
}
                                              
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by: mrjoltcolaPosted on 2009-10-29 at 07:12:33ID: 25693842

I told you wrong. By one pass, I meant it will not recursively evaluate (at least the ISO standard says so) macros, in the sense of self-referential.

However, I am looking for a good explanation, obviously I cannot explain it well enough myself as I do not fully understand the semantics.

 

by: Infinity08Posted on 2009-10-29 at 08:42:22ID: 25694931

A preprocessor macro (#define) simply causes a textual replacement of the macro identifier with the macro replacement list for any occurrence of the macro identifier that follows the macro, with these exceptions :

(a) if the macro identifier occurs inside a string literal, character constant or comment, it is not replaced (string literals and character constants are seen as single pre-processing tokens at this point)

(b) other macro's are not touched.


Second point to keep in mind, is that after replacing a macro identifier with the macro's replacement list, the replaced text is scanned again for more macro identifiers to be replaced. If more are found, they are also replaced, except if a recursive replacement is found (the same macro identifier is found again), in which case it's not replaced. This avoids infinite recursion loops.


So, for your example :

        #define SAM1 SAM2
        #define SAM2 SAM1
       
        int x = SAM1;

First, the source is scanned for macro identifiers. One is found (SAM1), and replaced, so we get :

        int x = SAM2;

This replaced text (SAM2) is then re-scanned, and we again find a macro identifier (SAM2), that is replaced :

        int x = SAM1;

This replaced text (SAM1) is then re-scanned, and we again find a macro identifier (SAM1), which is not replaced, since it is a recursive/nested replacement.

We continue scanning the rest of the source, but find no more macro identifiers, so this is the final result :

        int x = SAM1;

which is then passed to the compiler, which doesn't recognize SAM1 as a valid identifier, and throws an error about that.



Similarly for your other example :

        #define SAM2 SAM3
        #define SAM1 SAM2
       
        int x = SAM1;

First, the source is scanned for macro identifiers. One is found (SAM1), and replaced, so we get :

        int x = SAM2;

This replaced text (SAM2) is then re-scanned, and we again find a macro identifier (SAM2), that is replaced :

        int x = SAM3;

This replaced text (SAM3) is then re-scanned, but no more macro identifiers are found in it.

We continue scanning the rest of the source, but find no more macro identifiers, so this is the final result :

        int x = SAM3;

which is then passed to the compiler, which doesn't recognize SAM3 as a valid identifier, and throws an error about that.

20120131-EE-VQP-002

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