收到一堆GCC警告

在编译时,一个简单的程序可以发现一个字符串中是否有相同的字母,从而产生一系列C警告。我做错什么了?(程序工作正常)

    using Gee;
void main(string[] args) {
var s="♜♝♞♟♠♞";
unichar c;
var records = new HashMap<unichar, bool> ();

for (int i = 0; s.get_next_char (ref i, out c);) {
    stdout.printf (@"$i, $c\t");
    if (records[c]==true){
        stdout.printf("буква найдена: true\n");break;
    }else{
        records[c]=true;stdout.printf("буква не найдена: false\n");
    } 
}
}

gavr@archlabs ~/D/c/V/T / Task51> vala console.vala --pkg gee-0.8
/tmp / console.vala.BF33WZ.c: in the function " g_unichar_to_string»:
/tmp / console.vala.BF33WZ.c: 65: 27: warning: passing argument 2 " g_unichar_to_utf8 "cancels the" const " qualifier of the type [- Wdiscarded-qualifiers]
  g_unichar_to_utf8 (self, _tmp1_);
                           ^~~~~~
In file included from/usr/include/glib-2.0/glib / gstring.h: 33,
                 from / usr/include/glib-2.0/glib / giochannel.h: 34,
                 from / usr/include/glib-2.0 / glib.h: 54,
                 from / tmp / console.vala.BF33WZ.c: 6.:
/usr/include/glib-2.0/glib / gunicode.h:844:42: note: type "gchar *" {aka "char*"} was expected, but the argument is of type "const gchar *" {aka " const char *»}
                              gchar *outbuf);