Is it standard for #if to assume undefined symbolic constants as 0?

Question

In this c program

#include
int main()
{

        #if UnDefinedSymbolicConstant==0
                printf(\"UnDefinedSymbolicCon         


        

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

Answer 2

It is the default, but you might at least generate warnings for this using the -Wundef option to gcc.

`-Wundef': Warn whenever an identifier which is not a macro is encountered in an `#if' directive, outside of `defined'. Such identifiers are replaced with zero.