Stored struct member data via char array pointer return garbage value

Question

I am trying to display my database data on an OLED LCD using MySQL_Connection.h and MySQL_Cursor.h by ChuckBell.

The link to this library is https://github.com/ChuckBell/MySQL_Connector_Arduino>

I was able to fetch data from mysql database successfully. However, I wish to store the data in a char array so that I can later display them on the OLED LCD. The problem is the value stored always returns garbage value. I know its something to do with char array pointer but after searching for so long, I still couldn't find the correct syntax. Below is the snippet of my code.

Start by setting up Wifi connection and mysql connection.

#include <MySQL_Connection.h>
#include <MySQL_Cursor.h>
char query[] = "SELECT * FROM test.assetdemo WHERE RFID = \"048EB25A\"";  //sql query
char* sqldata[11];               //array of char pointer to store the 11 data in the database 

void setup(){
    Serial.begin(115200);
    internetConnect(ssid,pw);                             //connect to Wifi
    conn.connect(server_addr, 3306, user, password);      //connect to mysqldatabase
}

Then begin looping function to store and display the database data.

void loop(){
    Serial.println("\nRunning SELECT and printing results\n");

    MySQL_Cursor *cur_mem = new MySQL_Cursor(&conn);   // Initiate the query class instance
    cur_mem->execute(query);                           // Execute the query

    row_values *row = NULL;                            // Read the rows and print them
    do {
        row = cur_mem->get_next_row();
        if (row != NULL) {
            for (int f = 0; f < cols->num_fields; f++) {
                sqldata[f] = row->values[f];
                Serial.print(f);
                Serial.print("    ");
                Serial.println(sqldata[f]);    /*This works*/
              }
            Serial.println();
          }
     } while (row != NULL);
     Serial.println(sqldata[0]);      /*This return garbage value*/
     delete cur_mem;                   // frees up memory used  
     delay(5000);
}

The output is shown as below Click here or view below. As you can see, the values are displayed correctly (except 8th which is of boolean type which i will change it later) in the do while loop. However, when I exit the loop and print the value again, it returns garbage value ⸮. sqldata[0] supposes to return 048EB25A.

Running SELECT and printing results


0    048EB25A
1    Blood Pressure Monitor
2    NA
3    WelchAllyn 503-0054-03
4    010720
5    NA
6    NA
7    Blood Pressure Cuff
8    
9    Yes
10    1

⸮

The code snippet below shows the struct declaration in MySQL_Cursor.h

typedef struct {
  char *db;
  char *table;
  char *name;
} field_struct;

// Structure for storing result set metadata.
typedef struct {
  int num_fields;     // actual number of fields
  field_struct *fields[MAX_FIELDS];
} column_names;

// Structure for storing row data.
typedef struct {
  char *values[MAX_FIELDS];
} row_values;

Part of me knew that sqldata[f] = row->values[f]; is causing the garbage value. The value will change as the pointer only points to the address. How can I store the variable statically so that after the do while, the value will persist? Kind sirs and madams please explain this mystery.

PS: I am confused at array and pointer, even more when they involve struct.

Try changing "char* sqldata[11]" as "static char* sqldata[11]" and see. — P.W
– P.W, Commented Aug 23, 2018 at 9:41
Probbaly row = cur_mem->get_next_row(); invalidates the memory filled by the previous row = cur_mem->get_next_row();. Don't copy the pointers but copy the strings. — Jabberwocky
– Jabberwocky, Commented Aug 23, 2018 at 9:54
@user846834 that doesn't work =( Still return garbage value. — TK Ooi
– TK Ooi, Commented Aug 24, 2018 at 4:18
@Jabberwocky Thanks for the advice. After searching for a long time, I finally able to store the strings instead of the pointers. Will update the solution later. Thanks for the help! — TK Ooi
– TK Ooi, Commented Aug 24, 2018 at 4:19
Don't put an answer into the question and don't put "[SOLVE]" into the title. This site isn't a forum. — gre_gor
– gre_gor, Commented Aug 24, 2018 at 14:54

rantan pan · Accepted Answer · 2018-08-23 09:58:48Z

The explanation is that the memory is freed inside get_next_row function https://github.com/ChuckBell/MySQL_Connector_Arduino/blob/master/src/MySQL_Cursor.cpp

/*
  get_next_row - Iterator for reading rows from a result set
  This method returns an instance of a structure (row_values)
  that contains an array of strings representing the row
  values returned from the server.
  The caller can use the values however needed - by first
  converting them to a specific type or as a string.
*/
row_values *MySQL_Cursor::get_next_row() {
  int res = 0;

  free_row_buffer();

  // Read the rows
  res = get_row_values();
  if (res != MYSQL_EOF_PACKET) {
    return &row;
  }
  return NULL;
}

At the second attempt to get rows free_row_buffer is called.

Thanks for pointing out the free_row_buffer. Guess I had to figure out how to store the values before the free_row_buffer() is called. Anyway thanks for the help!

TK Ooi · Accepted Answer · 2018-08-24 16:46:12Z

I was able to find the solution myself thanks to the advices given by @Jabberwocky and @rantan pan. They provided me a direction on where to look for the solution. By editting sqldata as shown below, I was able to store the strings instead of pointer into sqldata.

Re-declare sqldata as shown below.

char **sqldata = new char*[11];     //to store 11 data for each query executed
for ( int i = 0; i < 11; i++ )
{
    sqldata[i] = new char[11];
}

Then replace sqldata[f] = row->values[f]; with strcpy(sqldata[f], (*row).values[f]);

All is working right now. Time to print this crap on an OLED LCD. Thanks again folks!

Paul Efford · Accepted Answer · 2019-04-05 07:22:20Z

Solution from @TK Ooi is ok, but as an improvement to it I would go safer with the size of the array where we want to store the data, and thus can be done instead of using

int numberRows = 11;
char **sqldata = new char*[numberRows];
for(int i = 0; i < numberRows; i++ ) {
    sqldata[i] = new char[numberChars];
}
strcpy(sqldata[f], (*row).values[f]);`

better declaring your array of char* as global but doing inside your query function the following:

int numberChars = strlen((*row).values[f]);
sqldata[f] = new char[numberChars]; // implies already extra place for null terminator
snprintf(sqldata[f], strlen(sqldata[f]), "%s", (*row).values[f]);`

This way, we are assigning always the exact length to our array to store the recovered data. Note that using numberChars instead of strlen(sqldata[f]) will fail because you need to pass the length according to the type stored in sqldata: it's not the same for int as for char...

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