Re: [PING] Re: [PATCH] c/66516 - missing diagnostic on taking the address of a builtin function

[Martin Sebor <msebor at gmail dot com>]

On 08/28/2015 02:09 PM, Joseph Myers wrote:
> On Fri, 28 Aug 2015, Martin Sebor wrote:
>
> > I ran into one regression in the gcc.dg/lto/pr54702_1.c test.
> > The file takes the address of malloc without declaring it, and
> > after calling it first. The code is invalid but GCC compiles it
> > due to a bug. I raised it in c/67386 -- missing diagnostic on
> > a use of an undeclared function, and suppressed the error by
>
> But that PR isn't a bug - the code is working exactly as it's meant to (an
> implicit declaration acts exactly like an explicit declaration "int func
> ();" in the nearest containing scope).  The declaration has an
> incompatible type, it's true, but GCC deliberately allows that with a
> warning.
> What if (a) you use a built-in function that returns int, instead of
> malloc, and (b) use -std=gnu89, so the implicit declaration isn't even an
> extension?  Then you have something that's completely valid, including
> taking the address of the implicitly declared function.

In that case the patched GCC issues an error for taking the address
of the undeclared function as the test case below shows.

I was aware of the C90 implicit declaration rule but I interpreted
it as saying that the injected declaration is only in effect for
the call expression. Since no other tests broke, I assumed the one
that did was buggy. Anyway, after testing a few other compilers it
looks like they all also extend the implicit declaration through
the rest of the scope, so the patch will need further tweaking to
allow this corner case.

The problem is that DECL_IS_BUILTIN(expr) returns true for
an implicitly declared builtin function with a library fallback
but false for one that's been declared explicitly. I'll either
have to find some other test to determine that the implicitly
declared function has a fallback or fix whatever is causing
the macro to return the wrong value.

Martin

$ cat t.c && /build/gcc-66516/gcc/xgcc -B /build/gcc-66516/gcc 
-std=gnu89 t.cint (*p)(int);

void foo (void) {
    p = abs;
}

int bar (void) {
    int n = abs (0);
    p = abs;
    return n;
}

t.c: In function ‘foo’:
t.c:4:9: error: ‘abs’ undeclared (first use in this function)
     p = abs;
         ^
t.c:4:9: note: each undeclared identifier is reported only once for each 
function it appears in
t.c: In function ‘bar’:
t.c:9:5: error: builtin function ‘abs’ must be directly called
     p = abs;
     ^