Question

In this c program

#include<stdio.h>
int main()
{

        #if UnDefinedSymbolicConstant==0
                printf("UnDefinedSymbolicConstant is equal to 0\n ");
        #else
                printf("UnDefinedSymbolicConstant is not equal to 0\n");

        #endif
        return 0;
}

UnDefinedSymbolicConstant has not been #define anywhere, still it is being assumed as 0 and gives the output on gcc-4.3.4 as:

UnDefinedSymbolicConstant is equal to 0

so,
Is this the standard behaviour or it works like this only in gcc?

Answer 1

Yes, this is specified by the standard in 6.10.1:

After all replacements due to macro expansion and the defined unary operator have been performed, all remaining identifiers (including those lexically identical to keywords) are replaced with the pp-number 0