i want to understand how dynamic memory allocation done for result of sqlite3_get_table with malloc ?

JonB

recordset is a 2D array/table. It holds all the rows, and is indexed by row number. There are a total of nrows allocated.
recordset[row] is one row. It holds an array of the columns in the row. There are a total of ncols allocated.
recorset[row][col] is one cell/value, in one column of one row. The column values are char *, i.e. C-strings. For each one the length of the string plus 1 byte for '\0' is alloacted.

rows -> columns -> cells/values of type C-string.

Qt embedded developer

@JonB Hi can you give example for deallocate memory for recordset ?

JonB

@Qt-embedded-developer
This is absolutely basic C! If you malloc(), what function do you have to call to free the allocated memory?

Qt embedded developer

@JonB i have to call free() to deallocate memory.

void free(void *ptr);

i have to free recordset memory inside below part else condition

if (ret != SQLITE_OK) {
       printf("SQL error: %s\n", err_messg);
       sprintf(RET, "%d", ret);
       // InsertErrorLog
//        if (strcmp(strErrorLogDescription, "") != 0) {
//            InsertErrorlog("", 0, 0, err_messg, strErrorLogDescription);
//        }
       success = 0;
   } else {
       resultSet_Table.rows = nrow;
       resultSet_Table.cols = ncol;
       resultSet_Table.recordset = recordset;
       success = 1;
   }

JonB

@Qt-embedded-developer
So write the code to call free()s corresponding to where you did your malloc()s. What else is there to ask/say?

Qt embedded developer

@JonB

i have allocated memory like below so to where and how to write free() for this all allocated memory.

recordset = (char ***) malloc(nrow * sizeof (char **));
    for (count = ncol; count < ((nrow + 1) * ncol); count++) {

        printf("Enter in LOOP\n");
        recordset[i] = (char **) malloc(ncol * sizeof (char *));
        for (j = 0; j < ncol; j++) {
            printf("%s ", result[count]);
            recordset[i][j] = (char *) malloc((strlen(result[count]) + 1));
            strcpy(recordset[i][j], result[count]);

            if (j != (ncol - 1))
                count++;
        }
        i++;
        printf("\n");
    }

in else part if i wrote code like below. is it right ?
```
free(recordset);
for (count = ncol; count < ((nrow + 1) * ncol); count++) {

    printf("Enter in LOOP\n");
    free(recordset[i]); 
    for (j = 0; j < ncol; j++) {
        free(recordset[i][j]);
    }
    i++;
    printf("\n");
}

JonB

@Qt-embedded-developer
No, that is not right.

You are freeing the items in the same order as you allocated them. Once you have done free(recordset) you must not then do free(recordset[i]), and once you have done free(recordset[i]) you must not then do free(recordset[i][j]).

Please stop and think about it/why. This is basic C. free() things in the opposite order to how you built your structure with malloc(). Do not free() something if you then go into it to access/free() something else.

Qt embedded developer

@JonB means i need to write code like below:

int i = 0;

for (count = ncol; count < ((nrow + 1) * ncol); count++) {

    printf("Enter in LOOP\n");
  
    for (j = 0; j < ncol; j++) {
        free(recordset[i][j]);
    }
  free(recordset[i]); 
    i++;
    printf("\n");
}

free(recordset);

And can you elaborate below sentence:
Do not free() something if you then go into it to access/free() something else.

JonB

@Qt-embedded-developer
In principle you at least now have the free()s in the right order.

I don't know why you have an i and a count, whether you go through the right items, and why you use (nrow + 1) (the + 1). But that's a coding issue for you to sort out. I have answered your questions, over to you to implement now.

Do not free() something if you then go into it to access/free() something else.

Exactly what I wrote; If you go e.g.

free(recordset);
free(recordset[i]);  // or anything to `recordset[i]`

think about what state recordset[i] might be in after you have free()d recordset.

Qt embedded developer

@JonB said in i want to understand how dynamic memory allocation done for result of sqlite3_get_table with malloc ?:

over to you to implement now

Hi

here below i allocated memory for string element in array index
recordset[i][j] = (char *) malloc((strlen(result[count]) + 1));

mpergand

Yeah good C exercise :)

       int rows=2, cols=3;
       const char* R1[]={"r1 c1","r1 c2","r1,c3"};
       const char* R2[]={"r2 c1","r2 c2","r2,c3"};
       const char** data[]={ R1,R2};
           
       char*** recordset;  // array of pointers (rows) to array of pointers (cols)
       recordset=(char***) malloc(sizeof(char***)*rows);
       
   	for (int r = 0; r < rows; ++r) 
   		{
   		recordset[r]=(char**) malloc(sizeof(char**)*cols);
   		for (int c = 0; c < cols; ++c) 
   			{
   			recordset[r][c]=(char*) malloc(sizeof(char)*(strlen(data[r][c])+1));
   			strcpy(recordset[r][c],data[r][c]);
   			}
   		}
   
   	for(int r=0 ; r<rows; r++)
   		std::cout<<recordset[r][0]<<", "<<recordset[r][1]<<", "<<recordset[r][2]<<", "<<std::endl;
   
   	// delete
   	for (int r = 0; r < rows; ++r) 
   		{
   		for (int c = 0; c < cols; ++c) 
   			{
   			free(recordset[r][c]);
   			}
   			
   		free(recordset[r]);
   		}
   		
   	free(recordset);

JonB

@mpergand said in i want to understand how dynamic memory allocation done for result of sqlite3_get_table with malloc ?:

  	recordset[r][c]=(char*) malloc(sizeof(char*));
  	strcpy(recordset[r][c],data[r][c]);

Ummm, really...? ;-)

mpergand

@JonB
Bien vu ;)