Re: Portable check for unportable macro usage

On 2016-10-19 12:14:50 -0400, Tom Lane wrote:
> A portability hazard that we fight constantly is that the <ctype.h>
> macros such as isalpha() can't safely be called on "char" values.
> Per POSIX, you have to make sure the input is cast to "unsigned char".
> That's far too easy to forget, but on most machines you won't get any
> warning about it.

> We have a couple of buildfarm machines that will generate "char value
> used as array subscript" warnings for this, because their implementations
> of these macros are indeed just array lookups.  But I think gaur is
> the only one that's still active, and it's not going to last forever.
> 
> Ralph Corderoy, one of the nmh hackers, was looking at this problem
> and came up with this cute solution [1]:
> 
>     #ifndef NDEBUG
>     #if EOF != -1
>     #error "Please report this to nmh's authors."
>     #endif
> 
>     extern int ctype_identity[257]; /* [n] = n-1 */
>     #define isupper(c) ((isupper)((ctype_identity + 1)[c]))
>     ...
>     #endif

Hm. I'd be kind of inclined to instead do something akin to

#include <ctype.h>
#define system_isupper(c) isupper(c)
#undef isupper
#define isupper(c) (AssertVariableIsOfTypeMacro(c, unsigned char), isupper(c))
=>
/home/andres/src/postgresql/src/include/c.h:745:7: error: static assertion failed: "c does not have type unsigned
char"

that would probably result in better error messages,and we can leave it
enabled regardless of debug mode.

That requires one usage adoption in wparser_def.c's p_iswhat()

Greetings,

Andres Freund