Re: Elimination of (more or less) all compilation warnings on OSX

On 03/06/2014 06:50 PM, Tom Lane wrote:
> Heikki Linnakangas <hlinnakangas@vmware.com> writes:
>> It's worth noting that isspace, isdigit etc. take an "unsigned char"
>> argument. The compiler won't warn if you pass a "char", so you have to
>> be careful with that. In practice, I don't think it will lead to actual
>> bugs on any real platform and locale, but still..
>
> It definitely used to lead to bugs on some old platforms.  It may be
> that everybody is more careful nowadays, but the POSIX spec is still
> perfectly clear about it:
>
>       The c argument is an int, the value of which the application
>       shall ensure is a character representable as an unsigned char or
>       equal to the value of the macro EOF. If the argument has any
>       other value, the behavior is undefined.
>
> I had my old HPUX box hacked up so that gcc would produce "subscript
> is a char" warnings for unsafe usage.  It seems a bit more difficult to
> get modern platforms to do that, though; people stopped exposing the
> underlying flag array in the system headers ...

A-ha, clever. I bet we could do something similar with modern compilers
too. Searching around, I found this trick (from
https://stackoverflow.com/questions/4712720/typechecking-macro-arguments-in-c):

> #define CHECK_TYPE(var, type)                                        \
>     {                                                                \
>         __typeof(var) *__tmp;                                        \
>         __tmp = (type *) NULL;                                        \
>         (void) __tmp; /* to avoid "set but not used" warnings */    \
>     }

The idea is to create a pointer variable that has the same type as the
argument, and assign an unsigned char value to it. With -Wpointer-sign,
gcc will complain. That's not portable, but that's OK. If we can get
some compilers to complain, that's enough.

With that, we can do:

#ifdef isalnum
#undef isalnum
#endif
#define isalnum(x) ((CHECK_TYPE(x, unsigned char)), isalnum(x))

I did that for all the is* routines, and unsurprisingly there were a
bunch of places were we got that wrong in psqlodbc. I'll fix those after
we get the minor release out.

I'll also try that on the PostgreSQL sources.

- Heikki