Question

gcc does not like the following code:

inline const plus(unsigned x,unsigned y) __attribute__((pure));
inline const plus(unsigned x,unsigned y) { return x+y; }

int arr[plus(1,1)];

it throws the following error:

error: variably modified ‘arr’ at file scope

The only thing is, I have done everything I can think of to tell gcc that it can optimize a call to plus(a,b) to “a+b” and I have only passed constants, so the result should be constant!

Am I missing something to make this work? Or is gcc just not that smart?

By the way, the reason for using plus(1,1) instead of 1+1 is that it makes for more generic construction of the array size using macros.

Answer 1

There may be gcc extensions that allow something like this, but at least in standard C a function call is never counted as a constant expression, no matter how many const you add, or how constant it really is. You may have to use a macro instead:

#define plus(x, y) ((x)+(y))