Question

During one review I came across a piece of code like the following :

#if defined(x) || y

What does the above statement mean ? Will the condition execute properly ?

Answer 1

Yes it is valid.

Here is what the Standard (C99) says in 6.10p1:

if-group:
# if constant-expression new-line groupopt
# ifdef identifier new-line groupopt
# ifndef identifier new-line groupopt

the defined operator is seen as unary operator part of a constant expression (6.10.1p1).

In your example, the condition is evaluated as true if the macro x is defined OR if y is defined and different than 0