How to avoid holistic promotion in C?

Question

It's unclear how to write portable C code using the widescreen API. Consider this example:

#include <locale.h>
#include <wchar.h>
#include <wctype.h>
int main(void)
{
  setlocale(LC_CTYPE, "C.UTF-8");
  wchar_t wc = L'ÿ';
  if (iswlower(wc)) return 0;
  return 1;
}

Compiling with gcc-6.3.0 using the -Wconversion option gives the following warning:

test.c: In function 'main':
test.c:9:16: warning: conversion to 'wint_t {aka unsigned int}' from 'wchar_t {aka int}' may change the sign of the result [-Wsign-conversion]
if (iswlower(wc)) return 0;
             ^

To get rid of this warning, we will add (wint_t), for example iswlower((wint_t)wc), but this is not possible. The following example shows why it is incapable.

#include <stdio.h>

/* this is our hypothetical implementation */
typedef signed int wint_t;
typedef signed short wchar_t;
#define WEOF ((wint_t)0xffffffff)

void f(wint_t wc)
{
    if (wc==WEOF)
      printf("BUG. Valid character recognized as WEOF. This is due to integer promotion. How to avoid it?\n");
}
int main(void)
{
    wchar_t wc = (wchar_t)0xffff;
    f((wint_t)wc);
    return 0;
}

My question is: how to make this example portable and at the same time avoid the gcc warning.

Answer 1

To keep things simple, I'm going to assume that the platform / implementation under discussion has the following characteristics:

• int - 32
• short - 16

C99 , .

, / :

• wint_t , (7.24.1/2)
• WEOF , (7.24.1/3)
• wchar_t (7.17/2)

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