Question

I’m working on a system using lots of MySQL queries and I’m running into some memory problems I’m pretty sure have to do with me not handling pointers right…

Basically, I’ve got something like this:

MYSQL_ROW function1() {
   string query="SELECT * FROM table limit 1;";
   MYSQL_ROW return_row;

   mysql_init(&connection); // "connection" is a global variable
   if (mysql_real_connect(&connection,HOST,USER,PASS,DB,0,NULL,0)){
      if (mysql_query(&connection,query.c_str()))
         cout << "Error: " << mysql_error(&connection);
      else{
         resp = mysql_store_result(&connection); //"resp" is also global
        if (resp) return_row = mysql_fetch_row(resp);
        mysql_free_result(resp);
      }
      mysql_close(&connection);
   }else{
      cout << "connection failed\n";
      if (mysql_errno(&connection))
         cout << "Error: " << mysql_errno(&connection) << " " << mysql_error(&connection);
   }
   return return_row;
}

And function2():

MYSQL_ROW function2(MYSQL_ROW row) {
    string query = "select * from table2 where code = '" + string(row[2]) + "'";
    MYSQL_ROW retorno;

    mysql_init(&connection);
    if (mysql_real_connect(&connection,HOST,USER,PASS,DB,0,NULL,0)){
        if (mysql_query(&connection,query.c_str()))
            cout << "Error: " << mysql_error(&conexao);
        else{
            // My "debugging" shows me at this point `row[2]` is already fubar
            resp = mysql_store_result(&connection);
            if (resp) return_row = mysql_fetch_row(resp);
            mysql_free_result(resp);
        }
        mysql_close(&connection);
    }else{
        cout << "connection failed\n";
        if (mysql_errno(&connection))
        cout << "Error : " << mysql_errno(&connection) << " " << mysql_error(&connection);
    }
    return return_row;
}

And main() is an infinite loop basically like this:

int main( int argc, char* args[] ){
    MYSQL_ROW row = NULL;
    while (1) {
        row = function1();
        if(row != NULL) function2(row);
   }
}

(variable and function names have been generalized to protect the innocent)

But after the 3rd or 4th call to function2, that only uses row for reading, row starts losing its value coming to a segfault error…

Anyone’s got any ideas why? I’m not sure the amount of global variables in this code is any good, but I didn’t design it and only got until tomorrow to fix and finish it, so workarounds are welcome!

Thanks!

Answer 1

Update: I misunderstood how mysql results are used. It looks like the row pointer array points to the results array which you free in function1() and then use it in function2() after it has been returned to the heap.

What you need to do is copy return_row[2] to a persistent string before freeing the results. Then pass that on to function2(). I see you doing something similar in function2() so you need to fix it there as well (though in your example you aren’t doing anything with its return value).

Also, you are correct that free(row); is not the correct thing to do.