CODEWITHSUNDEEP
cmacrosarmredefinition
corning
corning

Reputation: 401

Incompatible redefinition of macro

When I compile my c project with make command, there is the error

Error: #47-D: incompatible redefinition of macro "MACRO_NAME"

It looks like MACRO_NAME is already defined in one of the header files, but I want to redefine or hardcode new value for MACRO_NAME.

How to remove this error?

Upvotes: 2

Views: 6025

Answers (2)

Yunnosch
Yunnosch

Reputation: 26703

There is no clean way to define this macro so that it realiably and predictably is not used in the meaning of one of the definitions, when the other one is meant.

If you use a mechanism with undef, then you run the risk to undefine the other meaning, define it to your meaning and then end up with code which expects the other meaning seeing and using your meaning.

The only way to achieve reliability and predictability is to make sure that code which expects one meaning does not include (neither directly nor indirectly) the header which defines the other meaning.

You can do so by

a) defining in a way that neither definition can be done when the other one is already defined. To do so, in both cases

#ifdef MACRO_NAME
#error Separation of the two meanings of MACRO_NAME failed!
/* the other definition of MACRO_NAME is alreay visible */
#endif

#define MACRO_NAME MyMeaning

b) make sure that no code includes both definitions

Actually a) is only a technical help to make sure b). If you say that you will not ever include both definitions into one code file, then you have no problem. In that case, you do not get the #error from a). In that case you do not have a problem with using the wrong definition. Good. How do you know? How can you be sure that you do not have the problem, even if you change code, even if your colleague changes code? Use a), then you will be clearly told when you get caught by the redefinition trap. If you use #undef instead, then you have not prevented the problem, just hidden it and made it harder to debug.

c) in the case that you can only influence one of the two definitions, i.e. the other one is by another supplier, the best way is to change the name of your own definition. Whatever effort that causes in your code, it will be less than getting caught be unintended redefinition problems.

d) in the case that you cannot influence any of the two definitions (which is of course NOT the case you are asking about) you have to separate the code files into two groups, those who use one definition and not the other and the group which uses the other definition and only that one.

Upvotes: 3

corning
corning

Reputation: 401

Use #undef to redefine MACRO in the header file when it is needed

#ifdef MACRO_NAME
#undef MACRO_NAME
#endif
#define MACRO_NAME 100

Upvotes: 0

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