StringIO#putc

static VALUE
strio_putc(VALUE self, VALUE ch)
{
    struct StringIO *ptr = writable(self);
    VALUE str;

    check_modifiable(ptr);
    if (RB_TYPE_P(ch, T_STRING)) {
        if (NIL_P(ptr->string)) return ch;
        str = rb_str_substr(ch, 0, 1);
    }
    else {
        char c = NUM2CHR(ch);
        if (NIL_P(ptr->string)) return ch;
        str = rb_str_new(&c, 1);
    }
    strio_write(self, str);
    return ch;
}