#include <uchar.h>

mbstate_t state;
int main() {
    char in[] = u8"zß水🍌"; // or "z\u00df\u6c34\U0001F34C"
    size_t in_sz = sizeof in / sizeof *in;

    char16_t out[in_sz];
    char *p_in = in, *end = in + in_sz;
    char16_t *p_out = out;
    size_t rc;
    while((rc = mbrtoc16(p_out, p_in, end - p_in, &state)))
    {
        if(rc == (size_t)-1)      // invalid input
            break;
        else if(rc == (size_t)-2) // truncated input
            break;
        else if(rc == (size_t)-3) // UTF-16 high surrogate
            p_out += 1;
        else {
            p_in += rc;
            p_out += 1;
        };
    }

    size_t out_sz = p_out - out + 1;
}