Re: [PATCH] Wrap a macro in do {} while (0) (PR sanitizer/80063)

[Martin Sebor <msebor at gmail dot com>]

On 03/20/2017 07:42 AM, Marek Polacek wrote:
> On Mon, Mar 20, 2017 at 09:37:10AM -0400, Jason Merrill wrote:
> > On Mon, Mar 20, 2017 at 9:28 AM, Marek Polacek <polacek@redhat.com> wrote:
> > >  #define DEF_SANITIZER_BUILTIN(ENUM, NAME, TYPE, ATTRS) \
> > > +  do {                                                                 \
> > > +    decl = add_builtin_function ("__builtin_" NAME, TYPE, ENUM,                \
> > > +                                BUILT_IN_NORMAL, NAME, NULL_TREE);     \
> > > +    set_call_expr_flags (decl, ATTRS);                                 \
> > > +    set_builtin_decl (ENUM, decl, true);                               \
> > > +  } while (0);
> >
> > We leave out the trailing ; in a macro like this.
>
> True, but that wasn't possible in this case, otherwise I got tons of
>
> /home/marek/src/gcc/gcc/asan.c:2570:3: error: expected ‘;’ before ‘do’
>    do {         \
>    ^
> /home/marek/src/gcc/gcc/sanitizer.def:459:1: note: in expansion of macro ‘DEF_SANITIZER_BUILTIN’
>  DEF_SANITIZER_BUILTIN(BUILT_IN_UBSAN_HANDLE_TYPE_MISMATCH_ABORT

I would view these as helpful errors and expect them to be fixed
by terminating the macro invocations with a semicolon rather than
by adding it to the macro definition itself.  Is there a problem
with doing that that I'm not considering?

Martin

PS The GCC manual documents this problem in the section titled
Swallowing the Semicolon:
https://gcc.gnu.org/onlinedocs/cpp/Swallowing-the-Semicolon.html

This problem is also the subject of the CERT C Coding Standard
rule PRE11-C. Do not conclude macro definitions with a semicolon
(although the examples there are contrived).

Leaving the semicolon out also tends to confuse formatting tools.

It would be nice to be able to add a warning to help detect these
kinds of problems in macros (and others), and enable it for GCC
itself.