Re: why the difference of two global pointers is not a constant?

[Andrew Haley <aph at redhat dot com>]

Gabriel Dos Reis writes:
 > "Rafael Espíndola" <rafael.espindola@gmail.com> writes:
 > 
 > | I am trying to build a table with offsets of global pointers from a
 > | given pointer:
 > | 
 > | void *fs[] = {f1 - f1, f2 - f1};
 > | 
 > | where f1 and f2 are functions.
 > | 
 > | GCC is able to figure out that (f1 - f1) is 0, but says "initializer
 > | element is not constant" when trying to compute (f2 - f1).
 > 
 > because that is what the language standard says.
 > 
 > In general, the difference between two global pointers is something
 > known only to the linker -- too late to evaluate as constant
 > expression.

And in any case it's not legal C: see Section 6.5.6 para 9.

Andrew.