a is a function or a variable?
If you have following line, it is just a declaration. It needs to be defined with "int a;"
extern int a;
Alternatively if you have function with no return type, it would be treated as int like given below:
add(int a, int b); // watch no return type specified in this case.
Make sure you have proper return type or at least "void" to avoid this warning.,
wat does it mean , if we get this type of warning
'a' undefined; assuming extern returning int