Print UTF-32 string with wprintf - c11

Question

I am migrating some code from using wchar_t to char32_t, and when compiling with the -Werror=pointer-sign flag set, I am getting the following issue:
// main.c
#include <uchar.h>
#include <wchar.h>
int main(void) {
wprintf(U"some data\n");
}
Compiling: gcc -std=c11 -Werror=pointer-sign main.c
Output:
main.c: In function ‘main’:
main.c:5:10: error: pointer targets in passing argument 1 of ‘wprintf’ differ in signedness [-Werror=pointer-sign]
wprintf(U"some data\n");
^~~~~~~~~~~~~~
In file included from main.c:2:
/usr/include/wchar.h:587:12: note: expected ‘const wchar_t * restrict’ {aka ‘const int * restrict’} but argument is of type ‘unsigned int *’
extern int wprintf (const wchar_t *__restrict __format, ...)
^~~~~~~
To remedy this, I can do:
wprintf((const int *)U"some data\n");
//or
printf("%ls\n", U"some data");
Although this is quite a pain. Is there a nice and easy way to do this? What is the real difference between const unsigned int* vs const signed int*, aside from the data type it points to? Is this possibly dangerous, or should I just disable the flag altogether?

Answer 1

char32_t is an unsigned type.
wchar_t is either signed or unsigned, depending on implementation. In your case, it is signed.
You can't pass a pointer-to-unsigned where a pointer-to-signed is expected. So yes, you need a type-cast, however you should be casting to const wchar_t *, since that is what wprintf() actually expects (wchar_t just happens to be implemented as an int on your compiler, but don't cast to that directly):
wprintf((const wchar_t *)U"some data\n");
It doesn't get much cleaner than that, unless you wrap it in your own function, eg:
int wprintf32(const char32_t *str, ...)
{
va_list args;
va_start(args, str);
int result = vwprintf((const wchar_t *)str, args);
va_end(args);
return result;
}
wprintf32(U"some data\n");
Note that this code will not work properly at all on platforms where sizeof(wchar_t) < sizeof(char32_t), such as Windows. On those platforms, where sizeof(wchar_t) is 2, you will have to actually convert your string data from UTF-32 to UTF-16 instead, eg:
int wprintf32(const char32_t *str, ...)
{
va_list args;
int result;
va_start(args, str);
if (sizeof(wchar_t) < sizeof(char32_t))
{
wchar_t *str = convert_to_utf16(str); // <-- for you to implement
result = vwprintf(str, args);
free(str);
}
else
result = vwprintf((const wchar_t *)str, args);
va_end(args);
return result;
}
wprintf32(U"some data\n");