CODEWITHSUNDEEP
cc-preprocessor
Jin
Jin

Reputation: 1948

Confusion about redefining constant using define directive(#define) in C

As far as I know, standard allows redefinition only if the new definition duplicates the old one. So for example,

If,

#define X 100

Then, 

#define X 200 // Not allowed
#define X 100 // Allowed

But what confuses me is when the header file includes redefintion which doesn't duplicate the old defintion. For example,

There is a header file, header.h such that

#ifndef X
#define X 100
#endif

and source code main.c such that 

#define X 10
#include "header.h"

Since #define X 100 is below #define X 10 in the main file, I thought this would occur error. But surprisingly, the compiler does not complain! Why is such behaviour allowed in C?

Upvotes: 1

Views: 50

Answers (1)

Iharob Al Asimi
Iharob Al Asimi

Reputation: 53006

Of course it doesn't, because #define X 100 is never reached.

Think about this, what does #ifndef do?

Put the #define X 10 below the #include, what happens now?

Upvotes: 3

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