为什么mysql函数在c ++中返回值-1？

Question

bool MySql_Register(const char* id, const char* pw) {
  MYSQL* connect_ptr;
  connect_ptr = mysql_init(NULL);
  if(!mysql_real_connect(connect_ptr, HOST, USER, PW, NAME, 3306, NULL, 0)) {
    fprintf(stderr, "%s ",mysql_error(connect_ptr));
    exit(1);
  }

  char sql[256] = {0};
  sprintf(sql, "select * from user where id='%s'", id);
  mysql_query(connect_ptr, sql);
  int len = mysql_affected_rows(connect_ptr);

  if(len == 1)
    return PK_ID_OVERLAP;
  sprintf(sql, "insert into user (id, pw) values ('%s', '%s')", id, pw);
  mysql_query(connect_ptr, sql);
  len = mysql_affected_rows(connect_ptr);

  if(len == 1)
    return true;
  else
    return false;
}

mysql_connect and mysql_query running is very well.
but mysql_affected_rows returned value -1.
I don't know where is the problem....

Answer 1

From the documentation :

-1 indicates that the query returned an error or that, for a SELECT query, mysql_affected_rows() was called prior to calling mysql_store_result()

Moreover, I suggest you to use prepared statements instead of sprintf to avoid SQL injections.