Re: [PATCH] inttostr.h: add compile-time buffer overrun checks

[Ben Pfaff]

Paul Eggert <address@hidden> writes:

> On 10/18/10 04:53, Pádraig Brady wrote:
>
>> /* Like sizeof, except that it treats a variable sized array
>>    as a pointer rather than determining the size at runtime.  */
>> #define CT_SIZEOF(x) (__builtin_constant_p(sizeof x) ? sizeof x: sizeof 
>> (void*))
>
> I don't see how this would work.  If x is a variable-sized
> array, CT_SIZEOF(x) is not a constant expression, according
> to the C rules, because for A?B:C to be a constant expression,
> B and C both have to be constant expressions.

One could use __builtin_choose_expr() to avoid that particular problem:

 -- Built-in Function: TYPE __builtin_choose_expr (CONST_EXP, EXP1,
          EXP2)
     You can use the built-in function `__builtin_choose_expr' to
     evaluate code depending on the value of a constant expression.
     This built-in function returns EXP1 if CONST_EXP, which is a
     constant expression that must be able to be determined at compile
     time, is nonzero.  Otherwise it returns 0.

     This built-in function is analogous to the `? :' operator in C,
     except that the expression returned has its type unaltered by
     promotion rules.  Also, the built-in function does not evaluate
     the expression that was not chosen.  For example, if CONST_EXP
     evaluates to true, EXP2 is not evaluated even if it has
     side-effects.

-- 
Ben Pfaff 
http://benpfaff.org