#include <stdio.h>
#include <ctype.h>

int main(void) {
    char s[] = "rust to c";
    for (size_t i = 0; s[i] != '\0'; i++) {
        s[i] = (char)toupper((unsigned char)s[i]);
    }
    printf("%s\n", s);
    return 0;
}

toupper each byte